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----------------------------------------------------------------------------- This file contains a concatenation of the PCRE man pages, converted to plain text format for ease of searching with a text editor, or for use on systems that do not have a man page processor. The small individual files that give synopses of each function in the library have not been included. Neither has the pcredemo program. There are separate text files for the pcregrep and pcretest commands. ----------------------------------------------------------------------------- PCRE(3) Library Functions Manual PCRE(3) NAME PCRE - Perl-compatible regular expressions (original API) PLEASE TAKE NOTE This document relates to PCRE releases that use the original API, with library names libpcre, libpcre16, and libpcre32. January 2015 saw the first release of a new API, known as PCRE2, with release numbers start- ing at 10.00 and library names libpcre2-8, libpcre2-16, and libpcre2-32. The old libraries (now called PCRE1) are still being main- tained for bug fixes, but there will be no new development. New projects are advised to use the new PCRE2 libraries. INTRODUCTION The PCRE library is a set of functions that implement regular expres- sion pattern matching using the same syntax and semantics as Perl, with just a few differences. Some features that appeared in Python and PCRE before they appeared in Perl are also available using the Python syn- tax, there is some support for one or two .NET and Oniguruma syntax items, and there is an option for requesting some minor changes that give better JavaScript compatibility. Starting with release 8.30, it is possible to compile two separate PCRE libraries: the original, which supports 8-bit character strings (including UTF-8 strings), and a second library that supports 16-bit character strings (including UTF-16 strings). The build process allows either one or both to be built. The majority of the work to make this possible was done by Zoltan Herczeg. Starting with release 8.32 it is possible to compile a third separate PCRE library that supports 32-bit character strings (including UTF-32 strings). The build process allows any combination of the 8-, 16- and 32-bit libraries. The work to make this possible was done by Christian Persch. The three libraries contain identical sets of functions, except that the names in the 16-bit library start with pcre16_ instead of pcre_, and the names in the 32-bit library start with pcre32_ instead of pcre_. To avoid over-complication and reduce the documentation mainte- nance load, most of the documentation describes the 8-bit library, with the differences for the 16-bit and 32-bit libraries described sepa- rately in the pcre16 and pcre32 pages. References to functions or structures of the form pcre[16|32]_xxx should be read as meaning "pcre_xxx when using the 8-bit library, pcre16_xxx when using the 16-bit library, or pcre32_xxx when using the 32-bit library". The current implementation of PCRE corresponds approximately with Perl 5.12, including support for UTF-8/16/32 encoded strings and Unicode general category properties. However, UTF-8/16/32 and Unicode support has to be explicitly enabled; it is not the default. The Unicode tables correspond to Unicode release 6.3.0. In addition to the Perl-compatible matching function, PCRE contains an alternative function that matches the same compiled patterns in a dif- ferent way. In certain circumstances, the alternative function has some advantages. For a discussion of the two matching algorithms, see the pcrematching page. PCRE is written in C and released as a C library. A number of people have written wrappers and interfaces of various kinds. In particular, Google Inc. have provided a comprehensive C++ wrapper for the 8-bit library. This is now included as part of the PCRE distribution. The pcrecpp page has details of this interface. Other people's contribu- tions can be found in the Contrib directory at the primary FTP site, which is: ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre Details of exactly which Perl regular expression features are and are not supported by PCRE are given in separate documents. See the pcrepat- tern and pcrecompat pages. There is a syntax summary in the pcresyntax page. Some features of PCRE can be included, excluded, or changed when the library is built. The pcre_config() function makes it possible for a client to discover which features are available. The features them- selves are described in the pcrebuild page. Documentation about build- ing PCRE for various operating systems can be found in the README and NON-AUTOTOOLS_BUILD files in the source distribution. The libraries contains a number of undocumented internal functions and data tables that are used by more than one of the exported external functions, but which are not intended for use by external callers. Their names all begin with "_pcre_" or "_pcre16_" or "_pcre32_", which hopefully will not provoke any name clashes. In some environments, it is possible to control which external symbols are exported when a shared library is built, and in these cases the undocumented symbols are not exported. SECURITY CONSIDERATIONS If you are using PCRE in a non-UTF application that permits users to supply arbitrary patterns for compilation, you should be aware of a feature that allows users to turn on UTF support from within a pattern, provided that PCRE was built with UTF support. For example, an 8-bit pattern that begins with "(*UTF8)" or "(*UTF)" turns on UTF-8 mode, which interprets patterns and subjects as strings of UTF-8 characters instead of individual 8-bit characters. This causes both the pattern and any data against which it is matched to be checked for UTF-8 valid- ity. If the data string is very long, such a check might use suffi- ciently many resources as to cause your application to lose perfor- mance. One way of guarding against this possibility is to use the pcre_fullinfo() function to check the compiled pattern's options for UTF. Alternatively, from release 8.33, you can set the PCRE_NEVER_UTF option at compile time. This causes an compile time error if a pattern contains a UTF-setting sequence. If your application is one that supports UTF, be aware that validity checking can take time. If the same data string is to be matched many times, you can use the PCRE_NO_UTF[8|16|32]_CHECK option for the second and subsequent matches to save redundant checks. Another way that performance can be hit is by running a pattern that has a very large search tree against a string that will never match. Nested unlimited repeats in a pattern are a common example. PCRE pro- vides some protection against this: see the PCRE_EXTRA_MATCH_LIMIT fea- ture in the pcreapi page. USER DOCUMENTATION The user documentation for PCRE comprises a number of different sec- tions. In the "man" format, each of these is a separate "man page". In the HTML format, each is a separate page, linked from the index page. In the plain text format, the descriptions of the pcregrep and pcretest programs are in files called pcregrep.txt and pcretest.txt, respec- tively. The remaining sections, except for the pcredemo section (which is a program listing), are concatenated in pcre.txt, for ease of searching. The sections are as follows: pcre this document pcre-config show PCRE installation configuration information pcre16 details of the 16-bit library pcre32 details of the 32-bit library pcreapi details of PCRE's native C API pcrebuild building PCRE pcrecallout details of the callout feature pcrecompat discussion of Perl compatibility pcrecpp details of the C++ wrapper for the 8-bit library pcredemo a demonstration C program that uses PCRE pcregrep description of the pcregrep command (8-bit only) pcrejit discussion of the just-in-time optimization support pcrelimits details of size and other limits pcrematching discussion of the two matching algorithms pcrepartial details of the partial matching facility pcrepattern syntax and semantics of supported regular expressions pcreperform discussion of performance issues pcreposix the POSIX-compatible C API for the 8-bit library pcreprecompile details of saving and re-using precompiled patterns pcresample discussion of the pcredemo program pcrestack discussion of stack usage pcresyntax quick syntax reference pcretest description of the pcretest testing command pcreunicode discussion of Unicode and UTF-8/16/32 support In the "man" and HTML formats, there is also a short page for each C library function, listing its arguments and results. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. Putting an actual email address here seems to have been a spam magnet, so I've taken it away. If you want to email me, use my two initials, followed by the two digits 10, at the domain cam.ac.uk. REVISION Last updated: 10 February 2015 Copyright (c) 1997-2015 University of Cambridge. ------------------------------------------------------------------------------ PCRE(3) Library Functions Manual PCRE(3) NAME PCRE - Perl-compatible regular expressions #include <pcre.h> PCRE 16-BIT API BASIC FUNCTIONS pcre16 *pcre16_compile(PCRE_SPTR16 pattern, int options, const char **errptr, int *erroffset, const unsigned char *tableptr); pcre16 *pcre16_compile2(PCRE_SPTR16 pattern, int options, int *errorcodeptr, const char **errptr, int *erroffset, const unsigned char *tableptr); pcre16_extra *pcre16_study(const pcre16 *code, int options, const char **errptr); void pcre16_free_study(pcre16_extra *extra); int pcre16_exec(const pcre16 *code, const pcre16_extra *extra, PCRE_SPTR16 subject, int length, int startoffset, int options, int *ovector, int ovecsize); int pcre16_dfa_exec(const pcre16 *code, const pcre16_extra *extra, PCRE_SPTR16 subject, int length, int startoffset, int options, int *ovector, int ovecsize, int *workspace, int wscount); PCRE 16-BIT API STRING EXTRACTION FUNCTIONS int pcre16_copy_named_substring(const pcre16 *code, PCRE_SPTR16 subject, int *ovector, int stringcount, PCRE_SPTR16 stringname, PCRE_UCHAR16 *buffer, int buffersize); int pcre16_copy_substring(PCRE_SPTR16 subject, int *ovector, int stringcount, int stringnumber, PCRE_UCHAR16 *buffer, int buffersize); int pcre16_get_named_substring(const pcre16 *code, PCRE_SPTR16 subject, int *ovector, int stringcount, PCRE_SPTR16 stringname, PCRE_SPTR16 *stringptr); int pcre16_get_stringnumber(const pcre16 *code, PCRE_SPTR16 name); int pcre16_get_stringtable_entries(const pcre16 *code, PCRE_SPTR16 name, PCRE_UCHAR16 **first, PCRE_UCHAR16 **last); int pcre16_get_substring(PCRE_SPTR16 subject, int *ovector, int stringcount, int stringnumber, PCRE_SPTR16 *stringptr); int pcre16_get_substring_list(PCRE_SPTR16 subject, int *ovector, int stringcount, PCRE_SPTR16 **listptr); void pcre16_free_substring(PCRE_SPTR16 stringptr); void pcre16_free_substring_list(PCRE_SPTR16 *stringptr); PCRE 16-BIT API AUXILIARY FUNCTIONS pcre16_jit_stack *pcre16_jit_stack_alloc(int startsize, int maxsize); void pcre16_jit_stack_free(pcre16_jit_stack *stack); void pcre16_assign_jit_stack(pcre16_extra *extra, pcre16_jit_callback callback, void *data); const unsigned char *pcre16_maketables(void); int pcre16_fullinfo(const pcre16 *code, const pcre16_extra *extra, int what, void *where); int pcre16_refcount(pcre16 *code, int adjust); int pcre16_config(int what, void *where); const char *pcre16_version(void); int pcre16_pattern_to_host_byte_order(pcre16 *code, pcre16_extra *extra, const unsigned char *tables); PCRE 16-BIT API INDIRECTED FUNCTIONS void *(*pcre16_malloc)(size_t); void (*pcre16_free)(void *); void *(*pcre16_stack_malloc)(size_t); void (*pcre16_stack_free)(void *); int (*pcre16_callout)(pcre16_callout_block *); PCRE 16-BIT API 16-BIT-ONLY FUNCTION int pcre16_utf16_to_host_byte_order(PCRE_UCHAR16 *output, PCRE_SPTR16 input, int length, int *byte_order, int keep_boms); THE PCRE 16-BIT LIBRARY Starting with release 8.30, it is possible to compile a PCRE library that supports 16-bit character strings, including UTF-16 strings, as well as or instead of the original 8-bit library. The majority of the work to make this possible was done by Zoltan Herczeg. The two libraries contain identical sets of functions, used in exactly the same way. Only the names of the functions and the data types of their argu- ments and results are different. To avoid over-complication and reduce the documentation maintenance load, most of the PCRE documentation describes the 8-bit library, with only occasional references to the 16-bit library. This page describes what is different when you use the 16-bit library. WARNING: A single application can be linked with both libraries, but you must take care when processing any particular pattern to use func- tions from just one library. For example, if you want to study a pat- tern that was compiled with pcre16_compile(), you must do so with pcre16_study(), not pcre_study(), and you must free the study data with pcre16_free_study(). THE HEADER FILE There is only one header file, pcre.h. It contains prototypes for all the functions in all libraries, as well as definitions of flags, struc- tures, error codes, etc. THE LIBRARY NAME In Unix-like systems, the 16-bit library is called libpcre16, and can normally be accesss by adding -lpcre16 to the command for linking an application that uses PCRE. STRING TYPES In the 8-bit library, strings are passed to PCRE library functions as vectors of bytes with the C type "char *". In the 16-bit library, strings are passed as vectors of unsigned 16-bit quantities. The macro PCRE_UCHAR16 specifies an appropriate data type, and PCRE_SPTR16 is defined as "const PCRE_UCHAR16 *". In very many environments, "short int" is a 16-bit data type. When PCRE is built, it defines PCRE_UCHAR16 as "unsigned short int", but checks that it really is a 16-bit data type. If it is not, the build fails with an error message telling the maintainer to modify the definition appropriately. STRUCTURE TYPES The types of the opaque structures that are used for compiled 16-bit patterns and JIT stacks are pcre16 and pcre16_jit_stack respectively. The type of the user-accessible structure that is returned by pcre16_study() is pcre16_extra, and the type of the structure that is used for passing data to a callout function is pcre16_callout_block. These structures contain the same fields, with the same names, as their 8-bit counterparts. The only difference is that pointers to character strings are 16-bit instead of 8-bit types. 16-BIT FUNCTIONS For every function in the 8-bit library there is a corresponding func- tion in the 16-bit library with a name that starts with pcre16_ instead of pcre_. The prototypes are listed above. In addition, there is one extra function, pcre16_utf16_to_host_byte_order(). This is a utility function that converts a UTF-16 character string to host byte order if necessary. The other 16-bit functions expect the strings they are passed to be in host byte order. The input and output arguments of pcre16_utf16_to_host_byte_order() may point to the same address, that is, conversion in place is supported. The output buffer must be at least as long as the input. The length argument specifies the number of 16-bit data units in the input string; a negative value specifies a zero-terminated string. If byte_order is NULL, it is assumed that the string starts off in host byte order. This may be changed by byte-order marks (BOMs) anywhere in the string (commonly as the first character). If byte_order is not NULL, a non-zero value of the integer to which it points means that the input starts off in host byte order, otherwise the opposite order is assumed. Again, BOMs in the string can change this. The final byte order is passed back at the end of processing. If keep_boms is not zero, byte-order mark characters (0xfeff) are copied into the output string. Otherwise they are discarded. The result of the function is the number of 16-bit units placed into the output buffer, including the zero terminator if the string was zero-terminated. SUBJECT STRING OFFSETS The lengths and starting offsets of subject strings must be specified in 16-bit data units, and the offsets within subject strings that are returned by the matching functions are in also 16-bit units rather than bytes. NAMED SUBPATTERNS The name-to-number translation table that is maintained for named sub- patterns uses 16-bit characters. The pcre16_get_stringtable_entries() function returns the length of each entry in the table as the number of 16-bit data units. OPTION NAMES There are two new general option names, PCRE_UTF16 and PCRE_NO_UTF16_CHECK, which correspond to PCRE_UTF8 and PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options define the same bits in the options word. There is a discussion about the validity of UTF-16 strings in the pcreunicode page. For the pcre16_config() function there is an option PCRE_CONFIG_UTF16 that returns 1 if UTF-16 support is configured, otherwise 0. If this option is given to pcre_config() or pcre32_config(), or if the PCRE_CONFIG_UTF8 or PCRE_CONFIG_UTF32 option is given to pcre16_con- fig(), the result is the PCRE_ERROR_BADOPTION error. CHARACTER CODES In 16-bit mode, when PCRE_UTF16 is not set, character values are treated in the same way as in 8-bit, non UTF-8 mode, except, of course, that they can range from 0 to 0xffff instead of 0 to 0xff. Character types for characters less than 0xff can therefore be influenced by the locale in the same way as before. Characters greater than 0xff have only one case, and no "type" (such as letter or digit). In UTF-16 mode, the character code is Unicode, in the range 0 to 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff because those are "surrogate" values that are used in pairs to encode values greater than 0xffff. A UTF-16 string can indicate its endianness by special code knows as a byte-order mark (BOM). The PCRE functions do not handle this, expecting strings to be in host byte order. A utility function called pcre16_utf16_to_host_byte_order() is provided to help with this (see above). ERROR NAMES The errors PCRE_ERROR_BADUTF16_OFFSET and PCRE_ERROR_SHORTUTF16 corre- spond to their 8-bit counterparts. The error PCRE_ERROR_BADMODE is given when a compiled pattern is passed to a function that processes patterns in the other mode, for example, if a pattern compiled with pcre_compile() is passed to pcre16_exec(). There are new error codes whose names begin with PCRE_UTF16_ERR for invalid UTF-16 strings, corresponding to the PCRE_UTF8_ERR codes for UTF-8 strings that are described in the section entitled "Reason codes for invalid UTF-8 strings" in the main pcreapi page. The UTF-16 errors are: PCRE_UTF16_ERR1 Missing low surrogate at end of string PCRE_UTF16_ERR2 Invalid low surrogate follows high surrogate PCRE_UTF16_ERR3 Isolated low surrogate PCRE_UTF16_ERR4 Non-character ERROR TEXTS If there is an error while compiling a pattern, the error text that is passed back by pcre16_compile() or pcre16_compile2() is still an 8-bit character string, zero-terminated. CALLOUTS The subject and mark fields in the callout block that is passed to a callout function point to 16-bit vectors. TESTING The pcretest program continues to operate with 8-bit input and output files, but it can be used for testing the 16-bit library. If it is run with the command line option -16, patterns and subject strings are con- verted from 8-bit to 16-bit before being passed to PCRE, and the 16-bit library functions are used instead of the 8-bit ones. Returned 16-bit strings are converted to 8-bit for output. If both the 8-bit and the 32-bit libraries were not compiled, pcretest defaults to 16-bit and the -16 option is ignored. When PCRE is being built, the RunTest script that is called by "make check" uses the pcretest -C option to discover which of the 8-bit, 16-bit and 32-bit libraries has been built, and runs the tests appro- priately. NOT SUPPORTED IN 16-BIT MODE Not all the features of the 8-bit library are available with the 16-bit library. The C++ and POSIX wrapper functions support only the 8-bit library, and the pcregrep program is at present 8-bit only. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 12 May 2013 Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------ PCRE(3) Library Functions Manual PCRE(3) NAME PCRE - Perl-compatible regular expressions #include <pcre.h> PCRE 32-BIT API BASIC FUNCTIONS pcre32 *pcre32_compile(PCRE_SPTR32 pattern, int options, const char **errptr, int *erroffset, const unsigned char *tableptr); pcre32 *pcre32_compile2(PCRE_SPTR32 pattern, int options, int *errorcodeptr, const unsigned char *tableptr); pcre32_extra *pcre32_study(const pcre32 *code, int options, const char **errptr); void pcre32_free_study(pcre32_extra *extra); int pcre32_exec(const pcre32 *code, const pcre32_extra *extra, PCRE_SPTR32 subject, int length, int startoffset, int options, int *ovector, int ovecsize); int pcre32_dfa_exec(const pcre32 *code, const pcre32_extra *extra, PCRE_SPTR32 subject, int length, int startoffset, int options, int *ovector, int ovecsize, int *workspace, int wscount); PCRE 32-BIT API STRING EXTRACTION FUNCTIONS int pcre32_copy_named_substring(const pcre32 *code, PCRE_SPTR32 subject, int *ovector, int stringcount, PCRE_SPTR32 stringname, PCRE_UCHAR32 *buffer, int buffersize); int pcre32_copy_substring(PCRE_SPTR32 subject, int *ovector, int stringcount, int stringnumber, PCRE_UCHAR32 *buffer, int buffersize); int pcre32_get_named_substring(const pcre32 *code, PCRE_SPTR32 subject, int *ovector, int stringcount, PCRE_SPTR32 stringname, PCRE_SPTR32 *stringptr); int pcre32_get_stringnumber(const pcre32 *code, PCRE_SPTR32 name); int pcre32_get_stringtable_entries(const pcre32 *code, PCRE_SPTR32 name, PCRE_UCHAR32 **first, PCRE_UCHAR32 **last); int pcre32_get_substring(PCRE_SPTR32 subject, int *ovector, int stringcount, int stringnumber, PCRE_SPTR32 *stringptr); int pcre32_get_substring_list(PCRE_SPTR32 subject, int *ovector, int stringcount, PCRE_SPTR32 **listptr); void pcre32_free_substring(PCRE_SPTR32 stringptr); void pcre32_free_substring_list(PCRE_SPTR32 *stringptr); PCRE 32-BIT API AUXILIARY FUNCTIONS pcre32_jit_stack *pcre32_jit_stack_alloc(int startsize, int maxsize); void pcre32_jit_stack_free(pcre32_jit_stack *stack); void pcre32_assign_jit_stack(pcre32_extra *extra, pcre32_jit_callback callback, void *data); const unsigned char *pcre32_maketables(void); int pcre32_fullinfo(const pcre32 *code, const pcre32_extra *extra, int what, void *where); int pcre32_refcount(pcre32 *code, int adjust); int pcre32_config(int what, void *where); const char *pcre32_version(void); int pcre32_pattern_to_host_byte_order(pcre32 *code, pcre32_extra *extra, const unsigned char *tables); PCRE 32-BIT API INDIRECTED FUNCTIONS void *(*pcre32_malloc)(size_t); void (*pcre32_free)(void *); void *(*pcre32_stack_malloc)(size_t); void (*pcre32_stack_free)(void *); int (*pcre32_callout)(pcre32_callout_block *); PCRE 32-BIT API 32-BIT-ONLY FUNCTION int pcre32_utf32_to_host_byte_order(PCRE_UCHAR32 *output, PCRE_SPTR32 input, int length, int *byte_order, int keep_boms); THE PCRE 32-BIT LIBRARY Starting with release 8.32, it is possible to compile a PCRE library that supports 32-bit character strings, including UTF-32 strings, as well as or instead of the original 8-bit library. This work was done by Christian Persch, based on the work done by Zoltan Herczeg for the 16-bit library. All three libraries contain identical sets of func- tions, used in exactly the same way. Only the names of the functions and the data types of their arguments and results are different. To avoid over-complication and reduce the documentation maintenance load, most of the PCRE documentation describes the 8-bit library, with only occasional references to the 16-bit and 32-bit libraries. This page describes what is different when you use the 32-bit library. WARNING: A single application can be linked with all or any of the three libraries, but you must take care when processing any particular pattern to use functions from just one library. For example, if you want to study a pattern that was compiled with pcre32_compile(), you must do so with pcre32_study(), not pcre_study(), and you must free the study data with pcre32_free_study(). THE HEADER FILE There is only one header file, pcre.h. It contains prototypes for all the functions in all libraries, as well as definitions of flags, struc- tures, error codes, etc. THE LIBRARY NAME In Unix-like systems, the 32-bit library is called libpcre32, and can normally be accesss by adding -lpcre32 to the command for linking an application that uses PCRE. STRING TYPES In the 8-bit library, strings are passed to PCRE library functions as vectors of bytes with the C type "char *". In the 32-bit library, strings are passed as vectors of unsigned 32-bit quantities. The macro PCRE_UCHAR32 specifies an appropriate data type, and PCRE_SPTR32 is defined as "const PCRE_UCHAR32 *". In very many environments, "unsigned int" is a 32-bit data type. When PCRE is built, it defines PCRE_UCHAR32 as "unsigned int", but checks that it really is a 32-bit data type. If it is not, the build fails with an error message telling the maintainer to modify the definition appropriately. STRUCTURE TYPES The types of the opaque structures that are used for compiled 32-bit patterns and JIT stacks are pcre32 and pcre32_jit_stack respectively. The type of the user-accessible structure that is returned by pcre32_study() is pcre32_extra, and the type of the structure that is used for passing data to a callout function is pcre32_callout_block. These structures contain the same fields, with the same names, as their 8-bit counterparts. The only difference is that pointers to character strings are 32-bit instead of 8-bit types. 32-BIT FUNCTIONS For every function in the 8-bit library there is a corresponding func- tion in the 32-bit library with a name that starts with pcre32_ instead of pcre_. The prototypes are listed above. In addition, there is one extra function, pcre32_utf32_to_host_byte_order(). This is a utility function that converts a UTF-32 character string to host byte order if necessary. The other 32-bit functions expect the strings they are passed to be in host byte order. The input and output arguments of pcre32_utf32_to_host_byte_order() may point to the same address, that is, conversion in place is supported. The output buffer must be at least as long as the input. The length argument specifies the number of 32-bit data units in the input string; a negative value specifies a zero-terminated string. If byte_order is NULL, it is assumed that the string starts off in host byte order. This may be changed by byte-order marks (BOMs) anywhere in the string (commonly as the first character). If byte_order is not NULL, a non-zero value of the integer to which it points means that the input starts off in host byte order, otherwise the opposite order is assumed. Again, BOMs in the string can change this. The final byte order is passed back at the end of processing. If keep_boms is not zero, byte-order mark characters (0xfeff) are copied into the output string. Otherwise they are discarded. The result of the function is the number of 32-bit units placed into the output buffer, including the zero terminator if the string was zero-terminated. SUBJECT STRING OFFSETS The lengths and starting offsets of subject strings must be specified in 32-bit data units, and the offsets within subject strings that are returned by the matching functions are in also 32-bit units rather than bytes. NAMED SUBPATTERNS The name-to-number translation table that is maintained for named sub- patterns uses 32-bit characters. The pcre32_get_stringtable_entries() function returns the length of each entry in the table as the number of 32-bit data units. OPTION NAMES There are two new general option names, PCRE_UTF32 and PCRE_NO_UTF32_CHECK, which correspond to PCRE_UTF8 and PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options define the same bits in the options word. There is a discussion about the validity of UTF-32 strings in the pcreunicode page. For the pcre32_config() function there is an option PCRE_CONFIG_UTF32 that returns 1 if UTF-32 support is configured, otherwise 0. If this option is given to pcre_config() or pcre16_config(), or if the PCRE_CONFIG_UTF8 or PCRE_CONFIG_UTF16 option is given to pcre32_con- fig(), the result is the PCRE_ERROR_BADOPTION error. CHARACTER CODES In 32-bit mode, when PCRE_UTF32 is not set, character values are treated in the same way as in 8-bit, non UTF-8 mode, except, of course, that they can range from 0 to 0x7fffffff instead of 0 to 0xff. Charac- ter types for characters less than 0xff can therefore be influenced by the locale in the same way as before. Characters greater than 0xff have only one case, and no "type" (such as letter or digit). In UTF-32 mode, the character code is Unicode, in the range 0 to 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff because those are "surrogate" values that are ill-formed in UTF-32. A UTF-32 string can indicate its endianness by special code knows as a byte-order mark (BOM). The PCRE functions do not handle this, expecting strings to be in host byte order. A utility function called pcre32_utf32_to_host_byte_order() is provided to help with this (see above). ERROR NAMES The error PCRE_ERROR_BADUTF32 corresponds to its 8-bit counterpart. The error PCRE_ERROR_BADMODE is given when a compiled pattern is passed to a function that processes patterns in the other mode, for example, if a pattern compiled with pcre_compile() is passed to pcre32_exec(). There are new error codes whose names begin with PCRE_UTF32_ERR for invalid UTF-32 strings, corresponding to the PCRE_UTF8_ERR codes for UTF-8 strings that are described in the section entitled "Reason codes for invalid UTF-8 strings" in the main pcreapi page. The UTF-32 errors are: PCRE_UTF32_ERR1 Surrogate character (range from 0xd800 to 0xdfff) PCRE_UTF32_ERR2 Non-character PCRE_UTF32_ERR3 Character > 0x10ffff ERROR TEXTS If there is an error while compiling a pattern, the error text that is passed back by pcre32_compile() or pcre32_compile2() is still an 8-bit character string, zero-terminated. CALLOUTS The subject and mark fields in the callout block that is passed to a callout function point to 32-bit vectors. TESTING The pcretest program continues to operate with 8-bit input and output files, but it can be used for testing the 32-bit library. If it is run with the command line option -32, patterns and subject strings are con- verted from 8-bit to 32-bit before being passed to PCRE, and the 32-bit library functions are used instead of the 8-bit ones. Returned 32-bit strings are converted to 8-bit for output. If both the 8-bit and the 16-bit libraries were not compiled, pcretest defaults to 32-bit and the -32 option is ignored. When PCRE is being built, the RunTest script that is called by "make check" uses the pcretest -C option to discover which of the 8-bit, 16-bit and 32-bit libraries has been built, and runs the tests appro- priately. NOT SUPPORTED IN 32-BIT MODE Not all the features of the 8-bit library are available with the 32-bit library. The C++ and POSIX wrapper functions support only the 8-bit library, and the pcregrep program is at present 8-bit only. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 12 May 2013 Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------ PCREBUILD(3) Library Functions Manual PCREBUILD(3) NAME PCRE - Perl-compatible regular expressions BUILDING PCRE PCRE is distributed with a configure script that can be used to build the library in Unix-like environments using the applications known as Autotools. Also in the distribution are files to support building using CMake instead of configure. The text file README contains general information about building with Autotools (some of which is repeated below), and also has some comments about building on various operating systems. There is a lot more information about building PCRE without using Autotools (including information about using CMake and building "by hand") in the text file called NON-AUTOTOOLS-BUILD. You should consult this file as well as the README file if you are building in a non-Unix-like environment. PCRE BUILD-TIME OPTIONS The rest of this document describes the optional features of PCRE that can be selected when the library is compiled. It assumes use of the configure script, where the optional features are selected or dese- lected by providing options to configure before running the make com- mand. However, the same options can be selected in both Unix-like and non-Unix-like environments using the GUI facility of cmake-gui if you are using CMake instead of configure to build PCRE. If you are not using Autotools or CMake, option selection can be done by editing the config.h file, or by passing parameter settings to the compiler, as described in NON-AUTOTOOLS-BUILD. The complete list of options for configure (which includes the standard ones such as the selection of the installation directory) can be obtained by running ./configure --help The following sections include descriptions of options whose names begin with --enable or --disable. These settings specify changes to the defaults for the configure command. Because of the way that configure works, --enable and --disable always come in pairs, so the complemen- tary option always exists as well, but as it specifies the default, it is not described. BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES By default, a library called libpcre is built, containing functions that take string arguments contained in vectors of bytes, either as single-byte characters, or interpreted as UTF-8 strings. You can also build a separate library, called libpcre16, in which strings are con- tained in vectors of 16-bit data units and interpreted either as sin- gle-unit characters or UTF-16 strings, by adding --enable-pcre16 to the configure command. You can also build yet another separate library, called libpcre32, in which strings are contained in vectors of 32-bit data units and interpreted either as single-unit characters or UTF-32 strings, by adding --enable-pcre32 to the configure command. If you do not want the 8-bit library, add --disable-pcre8 as well. At least one of the three libraries must be built. Note that the C++ and POSIX wrappers are for the 8-bit library only, and that pcregrep is an 8-bit program. None of these are built if you select only the 16-bit or 32-bit libraries. BUILDING SHARED AND STATIC LIBRARIES The Autotools PCRE building process uses libtool to build both shared and static libraries by default. You can suppress one of these by adding one of --disable-shared --disable-static to the configure command, as required. C++ SUPPORT By default, if the 8-bit library is being built, the configure script will search for a C++ compiler and C++ header files. If it finds them, it automatically builds the C++ wrapper library (which supports only 8-bit strings). You can disable this by adding --disable-cpp to the configure command. UTF-8, UTF-16 AND UTF-32 SUPPORT To build PCRE with support for UTF Unicode character strings, add --enable-utf to the configure command. This setting applies to all three libraries, adding support for UTF-8 to the 8-bit library, support for UTF-16 to the 16-bit library, and support for UTF-32 to the to the 32-bit library. There are no separate options for enabling UTF-8, UTF-16 and UTF-32 independently because that would allow ridiculous settings such as requesting UTF-16 support while building only the 8-bit library. It is not possible to build one library with UTF support and another with- out in the same configuration. (For backwards compatibility, --enable- utf8 is a synonym of --enable-utf.) Of itself, this setting does not make PCRE treat strings as UTF-8, UTF-16 or UTF-32. As well as compiling PCRE with this option, you also have have to set the PCRE_UTF8, PCRE_UTF16 or PCRE_UTF32 option (as appropriate) when you call one of the pattern compiling functions. If you set --enable-utf when compiling in an EBCDIC environment, PCRE expects its input to be either ASCII or UTF-8 (depending on the run- time option). It is not possible to support both EBCDIC and UTF-8 codes in the same version of the library. Consequently, --enable-utf and --enable-ebcdic are mutually exclusive. UNICODE CHARACTER PROPERTY SUPPORT UTF support allows the libraries to process character codepoints up to 0x10ffff in the strings that they handle. On its own, however, it does not provide any facilities for accessing the properties of such charac- ters. If you want to be able to use the pattern escapes \P, \p, and \X, which refer to Unicode character properties, you must add --enable-unicode-properties to the configure command. This implies UTF support, even if you have not explicitly requested it. Including Unicode property support adds around 30K of tables to the PCRE library. Only the general category properties such as Lu and Nd are supported. Details are given in the pcrepattern documentation. JUST-IN-TIME COMPILER SUPPORT Just-in-time compiler support is included in the build by specifying --enable-jit This support is available only for certain hardware architectures. If this option is set for an unsupported architecture, a compile time error occurs. See the pcrejit documentation for a discussion of JIT usage. When JIT support is enabled, pcregrep automatically makes use of it, unless you add --disable-pcregrep-jit to the "configure" command. CODE VALUE OF NEWLINE By default, PCRE interprets the linefeed (LF) character as indicating the end of a line. This is the normal newline character on Unix-like systems. You can compile PCRE to use carriage return (CR) instead, by adding --enable-newline-is-cr to the configure command. There is also a --enable-newline-is-lf option, which explicitly specifies linefeed as the newline character. Alternatively, you can specify that line endings are to be indicated by the two character sequence CRLF. If you want this, add --enable-newline-is-crlf to the configure command. There is a fourth option, specified by --enable-newline-is-anycrlf which causes PCRE to recognize any of the three sequences CR, LF, or CRLF as indicating a line ending. Finally, a fifth option, specified by --enable-newline-is-any causes PCRE to recognize any Unicode newline sequence. Whatever line ending convention is selected when PCRE is built can be overridden when the library functions are called. At build time it is conventional to use the standard for your operating system. WHAT \R MATCHES By default, the sequence \R in a pattern matches any Unicode newline sequence, whatever has been selected as the line ending sequence. If you specify --enable-bsr-anycrlf the default is changed so that \R matches only CR, LF, or CRLF. What- ever is selected when PCRE is built can be overridden when the library functions are called. POSIX MALLOC USAGE When the 8-bit library is called through the POSIX interface (see the pcreposix documentation), additional working storage is required for holding the pointers to capturing substrings, because PCRE requires three integers per substring, whereas the POSIX interface provides only two. If the number of expected substrings is small, the wrapper func- tion uses space on the stack, because this is faster than using mal- loc() for each call. The default threshold above which the stack is no longer used is 10; it can be changed by adding a setting such as --with-posix-malloc-threshold=20 to the configure command. HANDLING VERY LARGE PATTERNS Within a compiled pattern, offset values are used to point from one part to another (for example, from an opening parenthesis to an alter- nation metacharacter). By default, in the 8-bit and 16-bit libraries, two-byte values are used for these offsets, leading to a maximum size for a compiled pattern of around 64K. This is sufficient to handle all but the most gigantic patterns. Nevertheless, some people do want to process truly enormous patterns, so it is possible to compile PCRE to use three-byte or four-byte offsets by adding a setting such as --with-link-size=3 to the configure command. The value given must be 2, 3, or 4. For the 16-bit library, a value of 3 is rounded up to 4. In these libraries, using longer offsets slows down the operation of PCRE because it has to load additional data when handling them. For the 32-bit library the value is always 4 and cannot be overridden; the value of --with-link- size is ignored. AVOIDING EXCESSIVE STACK USAGE When matching with the pcre_exec() function, PCRE implements backtrack- ing by making recursive calls to an internal function called match(). In environments where the size of the stack is limited, this can se- verely limit PCRE's operation. (The Unix environment does not usually suffer from this problem, but it may sometimes be necessary to increase the maximum stack size. There is a discussion in the pcrestack docu- mentation.) An alternative approach to recursion that uses memory from the heap to remember data, instead of using recursive function calls, has been implemented to work round the problem of limited stack size. If you want to build a version of PCRE that works this way, add --disable-stack-for-recursion to the configure command. With this configuration, PCRE will use the pcre_stack_malloc and pcre_stack_free variables to call memory manage- ment functions. By default these point to malloc() and free(), but you can replace the pointers so that your own functions are used instead. Separate functions are provided rather than using pcre_malloc and pcre_free because the usage is very predictable: the block sizes requested are always the same, and the blocks are always freed in reverse order. A calling program might be able to implement optimized functions that perform better than malloc() and free(). PCRE runs noticeably more slowly when built in this way. This option affects only the pcre_exec() function; it is not relevant for pcre_dfa_exec(). LIMITING PCRE RESOURCE USAGE Internally, PCRE has a function called match(), which it calls repeat- edly (sometimes recursively) when matching a pattern with the pcre_exec() function. By controlling the maximum number of times this function may be called during a single matching operation, a limit can be placed on the resources used by a single call to pcre_exec(). The limit can be changed at run time, as described in the pcreapi documen- tation. The default is 10 million, but this can be changed by adding a setting such as --with-match-limit=500000 to the configure command. This setting has no effect on the pcre_dfa_exec() matching function. In some environments it is desirable to limit the depth of recursive calls of match() more strictly than the total number of calls, in order to restrict the maximum amount of stack (or heap, if --disable-stack- for-recursion is specified) that is used. A second limit controls this; it defaults to the value that is set for --with-match-limit, which imposes no additional constraints. However, you can set a lower limit by adding, for example, --with-match-limit-recursion=10000 to the configure command. This value can also be overridden at run time. CREATING CHARACTER TABLES AT BUILD TIME PCRE uses fixed tables for processing characters whose code values are less than 256. By default, PCRE is built with a set of tables that are distributed in the file pcre_chartables.c.dist. These tables are for ASCII codes only. If you add --enable-rebuild-chartables to the configure command, the distributed tables are no longer used. Instead, a program called dftables is compiled and run. This outputs the source for new set of tables, created in the default locale of your C run-time system. (This method of replacing the tables does not work if you are cross compiling, because dftables is run on the local host. If you need to create alternative tables when cross compiling, you will have to do so "by hand".) USING EBCDIC CODE PCRE assumes by default that it will run in an environment where the character code is ASCII (or Unicode, which is a superset of ASCII). This is the case for most computer operating systems. PCRE can, how- ever, be compiled to run in an EBCDIC environment by adding --enable-ebcdic to the configure command. This setting implies --enable-rebuild-charta- bles. You should only use it if you know that you are in an EBCDIC environment (for example, an IBM mainframe operating system). The --enable-ebcdic option is incompatible with --enable-utf. The EBCDIC character that corresponds to an ASCII LF is assumed to have the value 0x15 by default. However, in some EBCDIC environments, 0x25 is used. In such an environment you should use --enable-ebcdic-nl25 as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR has the same value as in ASCII, namely, 0x0d. Whichever of 0x15 and 0x25 is not chosen as LF is made to correspond to the Unicode NEL char- acter (which, in Unicode, is 0x85). The options that select newline behaviour, such as --enable-newline-is- cr, and equivalent run-time options, refer to these character values in an EBCDIC environment. PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT By default, pcregrep reads all files as plain text. You can build it so that it recognizes files whose names end in .gz or .bz2, and reads them with libz or libbz2, respectively, by adding one or both of --enable-pcregrep-libz --enable-pcregrep-libbz2 to the configure command. These options naturally require that the rel- evant libraries are installed on your system. Configuration will fail if they are not. PCREGREP BUFFER SIZE pcregrep uses an internal buffer to hold a "window" on the file it is scanning, in order to be able to output "before" and "after" lines when it finds a match. The size of the buffer is controlled by a parameter whose default value is 20K. The buffer itself is three times this size, but because of the way it is used for holding "before" lines, the long- est line that is guaranteed to be processable is the parameter size. You can change the default parameter value by adding, for example, --with-pcregrep-bufsize=50K to the configure command. The caller of pcregrep can, however, override this value by specifying a run-time option. PCRETEST OPTION FOR LIBREADLINE SUPPORT If you add --enable-pcretest-libreadline to the configure command, pcretest is linked with the libreadline library, and when its input is from a terminal, it reads it using the readline() function. This provides line-editing and history facilities. Note that libreadline is GPL-licensed, so if you distribute a binary of pcretest linked in this way, there may be licensing issues. Setting this option causes the -lreadline option to be added to the pcretest build. In many operating environments with a sytem-installed libreadline this is sufficient. However, in some environments (e.g. if an unmodified distribution version of readline is in use), some extra configuration may be necessary. The INSTALL file for libreadline says this: "Readline uses the termcap functions, but does not link with the termcap or curses library itself, allowing applications which link with readline the to choose an appropriate library." If your environment has not been set up so that an appropriate library is automatically included, you may need to add something like LIBS="-ncurses" immediately before the configure command. DEBUGGING WITH VALGRIND SUPPORT By adding the --enable-valgrind option to to the configure command, PCRE will use valgrind annotations to mark certain memory regions as unaddressable. This allows it to detect invalid memory accesses, and is mostly useful for debugging PCRE itself. CODE COVERAGE REPORTING If your C compiler is gcc, you can build a version of PCRE that can generate a code coverage report for its test suite. To enable this, you must install lcov version 1.6 or above. Then specify --enable-coverage to the configure command and build PCRE in the usual way. Note that using ccache (a caching C compiler) is incompatible with code coverage reporting. If you have configured ccache to run automatically on your system, you must set the environment variable CCACHE_DISABLE=1 before running make to build PCRE, so that ccache is not used. When --enable-coverage is used, the following addition targets are added to the Makefile: make coverage This creates a fresh coverage report for the PCRE test suite. It is equivalent to running "make coverage-reset", "make coverage-baseline", "make check", and then "make coverage-report". make coverage-reset This zeroes the coverage counters, but does nothing else. make coverage-baseline This captures baseline coverage information. make coverage-report This creates the coverage report. make coverage-clean-report This removes the generated coverage report without cleaning the cover- age data itself. make coverage-clean-data This removes the captured coverage data without removing the coverage files created at compile time (*.gcno). make coverage-clean This cleans all coverage data including the generated coverage report. For more information about code coverage, see the gcov and lcov docu- mentation. SEE ALSO pcreapi(3), pcre16, pcre32, pcre_config(3). AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 12 May 2013 Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------ PCREMATCHING(3) Library Functions Manual PCREMATCHING(3) NAME PCRE - Perl-compatible regular expressions PCRE MATCHING ALGORITHMS This document describes the two different algorithms that are available in PCRE for matching a compiled regular expression against a given sub- ject string. The "standard" algorithm is the one provided by the pcre_exec(), pcre16_exec() and pcre32_exec() functions. These work in the same as as Perl's matching function, and provide a Perl-compatible matching operation. The just-in-time (JIT) optimization that is described in the pcrejit documentation is compatible with these func- tions. An alternative algorithm is provided by the pcre_dfa_exec(), pcre16_dfa_exec() and pcre32_dfa_exec() functions; they operate in a different way, and are not Perl-compatible. This alternative has advan- tages and disadvantages compared with the standard algorithm, and these are described below. When there is only one possible way in which a given subject string can match a pattern, the two algorithms give the same answer. A difference arises, however, when there are multiple possibilities. For example, if the pattern ^<.*> is matched against the string <something> <something else> <something further> there are three possible answers. The standard algorithm finds only one of them, whereas the alternative algorithm finds all three. REGULAR EXPRESSIONS AS TREES The set of strings that are matched by a regular expression can be rep- resented as a tree structure. An unlimited repetition in the pattern makes the tree of infinite size, but it is still a tree. Matching the pattern to a given subject string (from a given starting point) can be thought of as a search of the tree. There are two ways to search a tree: depth-first and breadth-first, and these correspond to the two matching algorithms provided by PCRE. THE STANDARD MATCHING ALGORITHM In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- sions", the standard algorithm is an "NFA algorithm". It conducts a depth-first search of the pattern tree. That is, it proceeds along a single path through the tree, checking that the subject matches what is required. When there is a mismatch, the algorithm tries any alterna- tives at the current point, and if they all fail, it backs up to the previous branch point in the tree, and tries the next alternative branch at that level. This often involves backing up (moving to the left) in the subject string as well. The order in which repetition branches are tried is controlled by the greedy or ungreedy nature of the quantifier. If a leaf node is reached, a matching string has been found, and at that point the algorithm stops. Thus, if there is more than one possi- ble match, this algorithm returns the first one that it finds. Whether this is the shortest, the longest, or some intermediate length depends on the way the greedy and ungreedy repetition quantifiers are specified in the pattern. Because it ends up with a single path through the tree, it is rela- tively straightforward for this algorithm to keep track of the sub- strings that are matched by portions of the pattern in parentheses. This provides support for capturing parentheses and back references. THE ALTERNATIVE MATCHING ALGORITHM This algorithm conducts a breadth-first search of the tree. Starting from the first matching point in the subject, it scans the subject string from left to right, once, character by character, and as it does this, it remembers all the paths through the tree that represent valid matches. In Friedl's terminology, this is a kind of "DFA algorithm", though it is not implemented as a traditional finite state machine (it keeps multiple states active simultaneously). Although the general principle of this matching algorithm is that it scans the subject string only once, without backtracking, there is one exception: when a lookaround assertion is encountered, the characters following or preceding the current point have to be independently inspected. The scan continues until either the end of the subject is reached, or there are no more unterminated paths. At this point, terminated paths represent the different matching possibilities (if there are none, the match has failed). Thus, if there is more than one possible match, this algorithm finds all of them, and in particular, it finds the long- est. The matches are returned in decreasing order of length. There is an option to stop the algorithm after the first match (which is neces- sarily the shortest) is found. Note that all the matches that are found start at the same point in the subject. If the pattern cat(er(pillar)?)? is matched against the string "the caterpillar catchment", the result will be the three strings "caterpillar", "cater", and "cat" that start at the fifth character of the subject. The algorithm does not automati- cally move on to find matches that start at later positions. PCRE's "auto-possessification" optimization usually applies to charac- ter repeats at the end of a pattern (as well as internally). For exam- ple, the pattern "a\d+" is compiled as if it were "a\d++" because there is no point even considering the possibility of backtracking into the repeated digits. For DFA matching, this means that only one possible match is found. If you really do want multiple matches in such cases, either use an ungreedy repeat ("a\d+?") or set the PCRE_NO_AUTO_POSSESS option when compiling. There are a number of features of PCRE regular expressions that are not supported by the alternative matching algorithm. They are as follows: 1. Because the algorithm finds all possible matches, the greedy or ungreedy nature of repetition quantifiers is not relevant. Greedy and ungreedy quantifiers are treated in exactly the same way. However, pos- sessive quantifiers can make a difference when what follows could also match what is quantified, for example in a pattern like this: ^a++\w! This pattern matches "aaab!" but not "aaa!", which would be matched by a non-possessive quantifier. Similarly, if an atomic group is present, it is matched as if it were a standalone pattern at the current point, and the longest match is then "locked in" for the rest of the overall pattern. 2. When dealing with multiple paths through the tree simultaneously, it is not straightforward to keep track of captured substrings for the different matching possibilities, and PCRE's implementation of this algorithm does not attempt to do this. This means that no captured sub- strings are available. 3. Because no substrings are captured, back references within the pat- tern are not supported, and cause errors if encountered. 4. For the same reason, conditional expressions that use a backrefer- ence as the condition or test for a specific group recursion are not supported. 5. Because many paths through the tree may be active, the \K escape sequence, which resets the start of the match when encountered (but may be on some paths and not on others), is not supported. It causes an error if encountered. 6. Callouts are supported, but the value of the capture_top field is always 1, and the value of the capture_last field is always -1. 7. The \C escape sequence, which (in the standard algorithm) always matches a single data unit, even in UTF-8, UTF-16 or UTF-32 modes, is not supported in these modes, because the alternative algorithm moves through the subject string one character (not data unit) at a time, for all active paths through the tree. 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not supported. (*FAIL) is supported, and behaves like a failing negative assertion. ADVANTAGES OF THE ALTERNATIVE ALGORITHM Using the alternative matching algorithm provides the following advan- tages: 1. All possible matches (at a single point in the subject) are automat- ically found, and in particular, the longest match is found. To find more than one match using the standard algorithm, you have to do kludgy things with callouts. 2. Because the alternative algorithm scans the subject string just once, and never needs to backtrack (except for lookbehinds), it is pos- sible to pass very long subject strings to the matching function in several pieces, checking for partial matching each time. Although it is possible to do multi-segment matching using the standard algorithm by retaining partially matched substrings, it is more complicated. The pcrepartial documentation gives details of partial matching and dis- cusses multi-segment matching. DISADVANTAGES OF THE ALTERNATIVE ALGORITHM The alternative algorithm suffers from a number of disadvantages: 1. It is substantially slower than the standard algorithm. This is partly because it has to search for all possible matches, but is also because it is less susceptible to optimization. 2. Capturing parentheses and back references are not supported. 3. Although atomic groups are supported, their use does not provide the performance advantage that it does for the standard algorithm. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 12 November 2013 Copyright (c) 1997-2012 University of Cambridge. ------------------------------------------------------------------------------ PCREAPI(3) Library Functions Manual PCREAPI(3) NAME PCRE - Perl-compatible regular expressions #include <pcre.h> PCRE NATIVE API BASIC FUNCTIONS pcre *pcre_compile(const char *pattern, int options, const char **errptr, int *erroffset, const unsigned char *tableptr); pcre *pcre_compile2(const char *pattern, int options, int *errorcodeptr, const char **errptr, int *erroffset, const unsigned char *tableptr); pcre_extra *pcre_study(const pcre *code, int options, const char **errptr); void pcre_free_study(pcre_extra *extra); int pcre_exec(const pcre *code, const pcre_extra *extra, const char *subject, int length, int startoffset, int options, int *ovector, int ovecsize); int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, const char *subject, int length, int startoffset, int options, int *ovector, int ovecsize, int *workspace, int wscount); PCRE NATIVE API STRING EXTRACTION FUNCTIONS int pcre_copy_named_substring(const pcre *code, const char *subject, int *ovector, int stringcount, const char *stringname, char *buffer, int buffersize); int pcre_copy_substring(const char *subject, int *ovector, int stringcount, int stringnumber, char *buffer, int buffersize); int pcre_get_named_substring(const pcre *code, const char *subject, int *ovector, int stringcount, const char *stringname, const char **stringptr); int pcre_get_stringnumber(const pcre *code, const char *name); int pcre_get_stringtable_entries(const pcre *code, const char *name, char **first, char **last); int pcre_get_substring(const char *subject, int *ovector, int stringcount, int stringnumber, const char **stringptr); int pcre_get_substring_list(const char *subject, int *ovector, int stringcount, const char ***listptr); void pcre_free_substring(const char *stringptr); void pcre_free_substring_list(const char **stringptr); PCRE NATIVE API AUXILIARY FUNCTIONS int pcre_jit_exec(const pcre *code, const pcre_extra *extra, const char *subject, int length, int startoffset, int options, int *ovector, int ovecsize, pcre_jit_stack *jstack); pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize); void pcre_jit_stack_free(pcre_jit_stack *stack); void pcre_assign_jit_stack(pcre_extra *extra, pcre_jit_callback callback, void *data); const unsigned char *pcre_maketables(void); int pcre_fullinfo(const pcre *code, const pcre_extra *extra, int what, void *where); int pcre_refcount(pcre *code, int adjust); int pcre_config(int what, void *where); const char *pcre_version(void); int pcre_pattern_to_host_byte_order(pcre *code, pcre_extra *extra, const unsigned char *tables); PCRE NATIVE API INDIRECTED FUNCTIONS void *(*pcre_malloc)(size_t); void (*pcre_free)(void *); void *(*pcre_stack_malloc)(size_t); void (*pcre_stack_free)(void *); int (*pcre_callout)(pcre_callout_block *); int (*pcre_stack_guard)(void); PCRE 8-BIT, 16-BIT, AND 32-BIT LIBRARIES As well as support for 8-bit character strings, PCRE also supports 16-bit strings (from release 8.30) and 32-bit strings (from release 8.32), by means of two additional libraries. They can be built as well as, or instead of, the 8-bit library. To avoid too much complication, this document describes the 8-bit versions of the functions, with only occasional references to the 16-bit and 32-bit libraries. The 16-bit and 32-bit functions operate in the same way as their 8-bit counterparts; they just use different data types for their arguments and results, and their names start with pcre16_ or pcre32_ instead of pcre_. For every option that has UTF8 in its name (for example, PCRE_UTF8), there are corresponding 16-bit and 32-bit names with UTF8 replaced by UTF16 or UTF32, respectively. This facility is in fact just cosmetic; the 16-bit and 32-bit option names define the same bit val- ues. References to bytes and UTF-8 in this document should be read as refer- ences to 16-bit data units and UTF-16 when using the 16-bit library, or 32-bit data units and UTF-32 when using the 32-bit library, unless specified otherwise. More details of the specific differences for the 16-bit and 32-bit libraries are given in the pcre16 and pcre32 pages. PCRE API OVERVIEW PCRE has its own native API, which is described in this document. There are also some wrapper functions (for the 8-bit library only) that cor- respond to the POSIX regular expression API, but they do not give access to all the functionality. They are described in the pcreposix documentation. Both of these APIs define a set of C function calls. A C++ wrapper (again for the 8-bit library only) is also distributed with PCRE. It is documented in the pcrecpp page. The native API C function prototypes are defined in the header file pcre.h, and on Unix-like systems the (8-bit) library itself is called libpcre. It can normally be accessed by adding -lpcre to the command for linking an application that uses PCRE. The header file defines the macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release numbers for the library. Applications can use these to include support for different releases of PCRE. In a Windows environment, if you want to statically link an application program against a non-dll pcre.a file, you must define PCRE_STATIC before including pcre.h or pcrecpp.h, because otherwise the pcre_mal- loc() and pcre_free() exported functions will be declared __declspec(dllimport), with unwanted results. The functions pcre_compile(), pcre_compile2(), pcre_study(), and pcre_exec() are used for compiling and matching regular expressions in a Perl-compatible manner. A sample program that demonstrates the sim- plest way of using them is provided in the file called pcredemo.c in the PCRE source distribution. A listing of this program is given in the pcredemo documentation, and the pcresample documentation describes how to compile and run it. Just-in-time compiler support is an optional feature of PCRE that can be built in appropriate hardware environments. It greatly speeds up the matching performance of many patterns. Simple programs can easily request that it be used if available, by setting an option that is ignored when it is not relevant. More complicated programs might need to make use of the functions pcre_jit_stack_alloc(), pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control the JIT code's memory usage. From release 8.32 there is also a direct interface for JIT execution, which gives improved performance. The JIT-specific functions are dis- cussed in the pcrejit documentation. A second matching function, pcre_dfa_exec(), which is not Perl-compati- ble, is also provided. This uses a different algorithm for the match- ing. The alternative algorithm finds all possible matches (at a given point in the subject), and scans the subject just once (unless there are lookbehind assertions). However, this algorithm does not return captured substrings. A description of the two matching algorithms and their advantages and disadvantages is given in the pcrematching docu- mentation. In addition to the main compiling and matching functions, there are convenience functions for extracting captured substrings from a subject string that is matched by pcre_exec(). They are: pcre_copy_substring() pcre_copy_named_substring() pcre_get_substring() pcre_get_named_substring() pcre_get_substring_list() pcre_get_stringnumber() pcre_get_stringtable_entries() pcre_free_substring() and pcre_free_substring_list() are also provided, to free the memory used for extracted strings. The function pcre_maketables() is used to build a set of character tables in the current locale for passing to pcre_compile(), pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is provided for specialist use. Most commonly, no special tables are passed, in which case internal tables that are generated when PCRE is built are used. The function pcre_fullinfo() is used to find out information about a compiled pattern. The function pcre_version() returns a pointer to a string containing the version of PCRE and its date of release. The function pcre_refcount() maintains a reference count in a data block containing a compiled pattern. This is provided for the benefit of object-oriented applications. The global variables pcre_malloc and pcre_free initially contain the entry points of the standard malloc() and free() functions, respec- tively. PCRE calls the memory management functions via these variables, so a calling program can replace them if it wishes to intercept the calls. This should be done before calling any PCRE functions. The global variables pcre_stack_malloc and pcre_stack_free are also indirections to memory management functions. These special functions are used only when PCRE is compiled to use the heap for remembering data, instead of recursive function calls, when running the pcre_exec() function. See the pcrebuild documentation for details of how to do this. It is a non-standard way of building PCRE, for use in environ- ments that have limited stacks. Because of the greater use of memory management, it runs more slowly. Separate functions are provided so that special-purpose external code can be used for this case. When used, these functions always allocate memory blocks of the same size. There is a discussion about PCRE's stack usage in the pcrestack docu- mentation. The global variable pcre_callout initially contains NULL. It can be set by the caller to a "callout" function, which PCRE will then call at specified points during a matching operation. Details are given in the pcrecallout documentation. The global variable pcre_stack_guard initially contains NULL. It can be set by the caller to a function that is called by PCRE whenever it starts to compile a parenthesized part of a pattern. When parentheses are nested, PCRE uses recursive function calls, which use up the system stack. This function is provided so that applications with restricted stacks can force a compilation error if the stack runs out. The func- tion should return zero if all is well, or non-zero to force an error. NEWLINES PCRE supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (line- feed) character, the two-character sequence CRLF, any of the three pre- ceding, or any Unicode newline sequence. The Unicode newline sequences are the three just mentioned, plus the single characters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029). Each of the first three conventions is used by at least one operating system as its standard newline sequence. When PCRE is built, a default can be specified. The default default is LF, which is the Unix stan- dard. When PCRE is run, the default can be overridden, either when a pattern is compiled, or when it is matched. At compile time, the newline convention can be specified by the options argument of pcre_compile(), or it can be specified by special text at the start of the pattern itself; this overrides any other settings. See the pcrepattern page for details of the special character sequences. In the PCRE documentation the word "newline" is used to mean "the char- acter or pair of characters that indicate a line break". The choice of newline convention affects the handling of the dot, circumflex, and dollar metacharacters, the handling of #-comments in /x mode, and, when CRLF is a recognized line ending sequence, the match position advance- ment for a non-anchored pattern. There is more detail about this in the section on pcre_exec() options below. The choice of newline convention does not affect the interpretation of the \n or \r escape sequences, nor does it affect what \R matches, which is controlled in a similar way, but by separate options. MULTITHREADING The PCRE functions can be used in multi-threading applications, with the proviso that the memory management functions pointed to by pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the callout and stack-checking functions pointed to by pcre_callout and pcre_stack_guard, are shared by all threads. The compiled form of a regular expression is not altered during match- ing, so the same compiled pattern can safely be used by several threads at once. If the just-in-time optimization feature is being used, it needs sepa- rate memory stack areas for each thread. See the pcrejit documentation for more details. SAVING PRECOMPILED PATTERNS FOR LATER USE The compiled form of a regular expression can be saved and re-used at a later time, possibly by a different program, and even on a host other than the one on which it was compiled. Details are given in the pcreprecompile documentation, which includes a description of the pcre_pattern_to_host_byte_order() function. However, compiling a regu- lar expression with one version of PCRE for use with a different ver- sion is not guaranteed to work and may cause crashes. CHECKING BUILD-TIME OPTIONS int pcre_config(int what, void *where); The function pcre_config() makes it possible for a PCRE client to dis- cover which optional features have been compiled into the PCRE library. The pcrebuild documentation has more details about these optional fea- tures. The first argument for pcre_config() is an integer, specifying which information is required; the second argument is a pointer to a variable into which the information is placed. The returned value is zero on success, or the negative error code PCRE_ERROR_BADOPTION if the value in the first argument is not recognized. The following information is available: PCRE_CONFIG_UTF8 The output is an integer that is set to one if UTF-8 support is avail- able; otherwise it is set to zero. This value should normally be given to the 8-bit version of this function, pcre_config(). If it is given to the 16-bit or 32-bit version of this function, the result is PCRE_ERROR_BADOPTION. PCRE_CONFIG_UTF16 The output is an integer that is set to one if UTF-16 support is avail- able; otherwise it is set to zero. This value should normally be given to the 16-bit version of this function, pcre16_config(). If it is given to the 8-bit or 32-bit version of this function, the result is PCRE_ERROR_BADOPTION. PCRE_CONFIG_UTF32 The output is an integer that is set to one if UTF-32 support is avail- able; otherwise it is set to zero. This value should normally be given to the 32-bit version of this function, pcre32_config(). If it is given to the 8-bit or 16-bit version of this function, the result is PCRE_ERROR_BADOPTION. PCRE_CONFIG_UNICODE_PROPERTIES The output is an integer that is set to one if support for Unicode character properties is available; otherwise it is set to zero. PCRE_CONFIG_JIT The output is an integer that is set to one if support for just-in-time compiling is available; otherwise it is set to zero. PCRE_CONFIG_JITTARGET The output is a pointer to a zero-terminated "const char *" string. If JIT support is available, the string contains the name of the architec- ture for which the JIT compiler is configured, for example "x86 32bit (little endian + unaligned)". If JIT support is not available, the result is NULL. PCRE_CONFIG_NEWLINE The output is an integer whose value specifies the default character sequence that is recognized as meaning "newline". The values that are supported in ASCII/Unicode environments are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF, and -1 for ANY. In EBCDIC environments, CR, ANYCRLF, and ANY yield the same values. However, the value for LF is normally 21, though some EBCDIC environments use 37. The corresponding values for CRLF are 3349 and 3365. The default should normally corre- spond to the standard sequence for your operating system. PCRE_CONFIG_BSR The output is an integer whose value indicates what character sequences the \R escape sequence matches by default. A value of 0 means that \R matches any Unicode line ending sequence; a value of 1 means that \R matches only CR, LF, or CRLF. The default can be overridden when a pat- tern is compiled or matched. PCRE_CONFIG_LINK_SIZE The output is an integer that contains the number of bytes used for internal linkage in compiled regular expressions. For the 8-bit library, the value can be 2, 3, or 4. For the 16-bit library, the value is either 2 or 4 and is still a number of bytes. For the 32-bit library, the value is either 2 or 4 and is still a number of bytes. The default value of 2 is sufficient for all but the most massive patterns, since it allows the compiled pattern to be up to 64K in size. Larger values allow larger regular expressions to be compiled, at the expense of slower matching. PCRE_CONFIG_POSIX_MALLOC_THRESHOLD The output is an integer that contains the threshold above which the POSIX interface uses malloc() for output vectors. Further details are given in the pcreposix documentation. PCRE_CONFIG_PARENS_LIMIT The output is a long integer that gives the maximum depth of nesting of parentheses (of any kind) in a pattern. This limit is imposed to cap the amount of system stack used when a pattern is compiled. It is spec- ified when PCRE is built; the default is 250. This limit does not take into account the stack that may already be used by the calling applica- tion. For finer control over compilation stack usage, you can set a pointer to an external checking function in pcre_stack_guard. PCRE_CONFIG_MATCH_LIMIT The output is a long integer that gives the default limit for the num- ber of internal matching function calls in a pcre_exec() execution. Further details are given with pcre_exec() below. PCRE_CONFIG_MATCH_LIMIT_RECURSION The output is a long integer that gives the default limit for the depth of recursion when calling the internal matching function in a pcre_exec() execution. Further details are given with pcre_exec() below. PCRE_CONFIG_STACKRECURSE The output is an integer that is set to one if internal recursion when running pcre_exec() is implemented by recursive function calls that use the stack to remember their state. This is the usual way that PCRE is compiled. The output is zero if PCRE was compiled to use blocks of data on the heap instead of recursive function calls. In this case, pcre_stack_malloc and pcre_stack_free are called to manage memory blocks on the heap, thus avoiding the use of the stack. COMPILING A PATTERN pcre *pcre_compile(const char *pattern, int options, const char **errptr, int *erroffset, const unsigned char *tableptr); pcre *pcre_compile2(const char *pattern, int options, int *errorcodeptr, const char **errptr, int *erroffset, const unsigned char *tableptr); Either of the functions pcre_compile() or pcre_compile2() can be called to compile a pattern into an internal form. The only difference between the two interfaces is that pcre_compile2() has an additional argument, errorcodeptr, via which a numerical error code can be returned. To avoid too much repetition, we refer just to pcre_compile() below, but the information applies equally to pcre_compile2(). The pattern is a C string terminated by a binary zero, and is passed in the pattern argument. A pointer to a single block of memory that is obtained via pcre_malloc is returned. This contains the compiled code and related data. The pcre type is defined for the returned block; this is a typedef for a structure whose contents are not externally defined. It is up to the caller to free the memory (via pcre_free) when it is no longer required. Although the compiled code of a PCRE regex is relocatable, that is, it does not depend on memory location, the complete pcre data block is not fully relocatable, because it may contain a copy of the tableptr argu- ment, which is an address (see below). The options argument contains various bit settings that affect the com- pilation. It should be zero if no options are required. The available options are described below. Some of them (in particular, those that are compatible with Perl, but some others as well) can also be set and unset from within the pattern (see the detailed description in the pcrepattern documentation). For those options that can be different in different parts of the pattern, the contents of the options argument specifies their settings at the start of compilation and execution. The PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and PCRE_NO_START_OPTIMIZE options can be set at the time of matching as well as at compile time. If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, if compilation of a pattern fails, pcre_compile() returns NULL, and sets the variable pointed to by errptr to point to a textual error mes- sage. This is a static string that is part of the library. You must not try to free it. Normally, the offset from the start of the pattern to the data unit that was being processed when the error was discovered is placed in the variable pointed to by erroffset, which must not be NULL (if it is, an immediate error is given). However, for an invalid UTF-8 or UTF-16 string, the offset is that of the first data unit of the failing character. Some errors are not detected until the whole pattern has been scanned; in these cases, the offset passed back is the length of the pattern. Note that the offset is in data units, not characters, even in a UTF mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char- acter. If pcre_compile2() is used instead of pcre_compile(), and the error- codeptr argument is not NULL, a non-zero error code number is returned via this argument in the event of an error. This is in addition to the textual error message. Error codes and messages are listed below. If the final argument, tableptr, is NULL, PCRE uses a default set of character tables that are built when PCRE is compiled, using the default C locale. Otherwise, tableptr must be an address that is the result of a call to pcre_maketables(). This value is stored with the compiled pattern, and used again by pcre_exec() and pcre_dfa_exec() when the pattern is matched. For more discussion, see the section on locale support below. This code fragment shows a typical straightforward call to pcre_com- pile(): pcre *re; const char *error; int erroffset; re = pcre_compile( "^A.*Z", /* the pattern */ 0, /* default options */ &error, /* for error message */ &erroffset, /* for error offset */ NULL); /* use default character tables */ The following names for option bits are defined in the pcre.h header file: PCRE_ANCHORED If this bit is set, the pattern is forced to be "anchored", that is, it is constrained to match only at the first matching point in the string that is being searched (the "subject string"). This effect can also be achieved by appropriate constructs in the pattern itself, which is the only way to do it in Perl. PCRE_AUTO_CALLOUT If this bit is set, pcre_compile() automatically inserts callout items, all with number 255, before each pattern item. For discussion of the callout facility, see the pcrecallout documentation. PCRE_BSR_ANYCRLF PCRE_BSR_UNICODE These options (which are mutually exclusive) control what the \R escape sequence matches. The choice is either to match only CR, LF, or CRLF, or to match any Unicode newline sequence. The default is specified when PCRE is built. It can be overridden from within the pattern, or by set- ting an option when a compiled pattern is matched. PCRE_CASELESS If this bit is set, letters in the pattern match both upper and lower case letters. It is equivalent to Perl's /i option, and it can be changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE always understands the concept of case for characters whose values are less than 128, so caseless matching is always possible. For characters with higher values, the concept of case is supported if PCRE is com- piled with Unicode property support, but not otherwise. If you want to use caseless matching for characters 128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with UTF-8 support. PCRE_DOLLAR_ENDONLY If this bit is set, a dollar metacharacter in the pattern matches only at the end of the subject string. Without this option, a dollar also matches immediately before a newline at the end of the string (but not before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. There is no equivalent to this option in Perl, and no way to set it within a pattern. PCRE_DOTALL If this bit is set, a dot metacharacter in the pattern matches a char- acter of any value, including one that indicates a newline. However, it only ever matches one character, even if newlines are coded as CRLF. Without this option, a dot does not match when the current position is at a newline. This option is equivalent to Perl's /s option, and it can be changed within a pattern by a (?s) option setting. A negative class such as [^a] always matches newline characters, independent of the set- ting of this option. PCRE_DUPNAMES If this bit is set, names used to identify capturing subpatterns need not be unique. This can be helpful for certain types of pattern when it is known that only one instance of the named subpattern can ever be matched. There are more details of named subpatterns below; see also the pcrepattern documentation. PCRE_EXTENDED If this bit is set, most white space characters in the pattern are totally ignored except when escaped or inside a character class. How- ever, white space is not allowed within sequences such as (?> that introduce various parenthesized subpatterns, nor within a numerical quantifier such as {1,3}. However, ignorable white space is permitted between an item and a following quantifier and between a quantifier and a following + that indicates possessiveness. White space did not used to include the VT character (code 11), because Perl did not treat this character as white space. However, Perl changed at release 5.18, so PCRE followed at release 8.34, and VT is now treated as white space. PCRE_EXTENDED also causes characters between an unescaped # outside a character class and the next newline, inclusive, to be ignored. PCRE_EXTENDED is equivalent to Perl's /x option, and it can be changed within a pattern by a (?x) option setting. Which characters are interpreted as newlines is controlled by the options passed to pcre_compile() or by a special sequence at the start of the pattern, as described in the section entitled "Newline conven- tions" in the pcrepattern documentation. Note that the end of this type of comment is a literal newline sequence in the pattern; escape sequences that happen to represent a newline do not count. This option makes it possible to include comments inside complicated patterns. Note, however, that this applies only to data characters. White space characters may never appear within special character sequences in a pattern, for example within the sequence (?( that intro- duces a conditional subpattern. PCRE_EXTRA This option was invented in order to turn on additional functionality of PCRE that is incompatible with Perl, but it is currently of very little use. When set, any backslash in a pattern that is followed by a letter that has no special meaning causes an error, thus reserving these combinations for future expansion. By default, as in Perl, a backslash followed by a letter with no special meaning is treated as a literal. (Perl can, however, be persuaded to give an error for this, by running it with the -w option.) There are at present no other features controlled by this option. It can also be set by a (?X) option setting within a pattern. PCRE_FIRSTLINE If this option is set, an unanchored pattern is required to match before or at the first newline in the subject string, though the matched text may continue over the newline. PCRE_JAVASCRIPT_COMPAT If this option is set, PCRE's behaviour is changed in some ways so that it is compatible with JavaScript rather than Perl. The changes are as follows: (1) A lone closing square bracket in a pattern causes a compile-time error, because this is illegal in JavaScript (by default it is treated as a data character). Thus, the pattern AB]CD becomes illegal when this option is set. (2) At run time, a back reference to an unset subpattern group matches an empty string (by default this causes the current matching alterna- tive to fail). A pattern such as (\1)(a) succeeds when this option is set (assuming it can find an "a" in the subject), whereas it fails by default, for Perl compatibility. (3) \U matches an upper case "U" character; by default \U causes a com- pile time error (Perl uses \U to upper case subsequent characters). (4) \u matches a lower case "u" character unless it is followed by four hexadecimal digits, in which case the hexadecimal number defines the code point to match. By default, \u causes a compile time error (Perl uses it to upper case the following character). (5) \x matches a lower case "x" character unless it is followed by two hexadecimal digits, in which case the hexadecimal number defines the code point to match. By default, as in Perl, a hexadecimal number is always expected after \x, but it may have zero, one, or two digits (so, for example, \xz matches a binary zero character followed by z). PCRE_MULTILINE By default, for the purposes of matching "start of line" and "end of line", PCRE treats the subject string as consisting of a single line of characters, even if it actually contains newlines. The "start of line" metacharacter (^) matches only at the start of the string, and the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (except when PCRE_DOLLAR_ENDONLY is set). Note, however, that unless PCRE_DOTALL is set, the "any character" metacharacter (.) does not match at a newline. This behaviour (for ^, $, and dot) is the same as Perl. When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs match immediately following or immediately before internal newlines in the subject string, respectively, as well as at the very start and end. This is equivalent to Perl's /m option, and it can be changed within a pattern by a (?m) option setting. If there are no new- lines in a subject string, or no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no effect. PCRE_NEVER_UTF This option locks out interpretation of the pattern as UTF-8 (or UTF-16 or UTF-32 in the 16-bit and 32-bit libraries). In particular, it pre- vents the creator of the pattern from switching to UTF interpretation by starting the pattern with (*UTF). This may be useful in applications that process patterns from external sources. The combination of PCRE_UTF8 and PCRE_NEVER_UTF also causes an error. PCRE_NEWLINE_CR PCRE_NEWLINE_LF PCRE_NEWLINE_CRLF PCRE_NEWLINE_ANYCRLF PCRE_NEWLINE_ANY These options override the default newline definition that was chosen when PCRE was built. Setting the first or the second specifies that a newline is indicated by a single character (CR or LF, respectively). Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies that any of the three preceding sequences should be recognized. Setting PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be recognized. In an ASCII/Unicode environment, the Unicode newline sequences are the three just mentioned, plus the single characters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line sep- arator, U+2028), and PS (paragraph separator, U+2029). For the 8-bit library, the last two are recognized only in UTF-8 mode. When PCRE is compiled to run in an EBCDIC (mainframe) environment, the code for CR is 0x0d, the same as ASCII. However, the character code for LF is normally 0x15, though in some EBCDIC environments 0x25 is used. Whichever of these is not LF is made to correspond to Unicode's NEL character. EBCDIC codes are all less than 256. For more details, see the pcrebuild documentation. The newline setting in the options word uses three bits that are treated as a number, giving eight possibilities. Currently only six are used (default plus the five values above). This means that if you set more than one newline option, the combination may or may not be sensi- ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and cause an error. The only time that a line break in a pattern is specially recognized when compiling is when PCRE_EXTENDED is set. CR and LF are white space characters, and so are ignored in this mode. Also, an unescaped # out- side a character class indicates a comment that lasts until after the next line break sequence. In other circumstances, line break sequences in patterns are treated as literal data. The newline option that is set at compile time becomes the default that is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden. PCRE_NO_AUTO_CAPTURE If this option is set, it disables the use of numbered capturing paren- theses in the pattern. Any opening parenthesis that is not followed by ? behaves as if it were followed by ?: but named parentheses can still be used for capturing (and they acquire numbers in the usual way). There is no equivalent of this option in Perl. PCRE_NO_AUTO_POSSESS If this option is set, it disables "auto-possessification". This is an optimization that, for example, turns a+b into a++b in order to avoid backtracks into a+ that can never be successful. However, if callouts are in use, auto-possessification means that some of them are never taken. You can set this option if you want the matching functions to do a full unoptimized search and run all the callouts, but it is mainly provided for testing purposes. PCRE_NO_START_OPTIMIZE This is an option that acts at matching time; that is, it is really an option for pcre_exec() or pcre_dfa_exec(). If it is set at compile time, it is remembered with the compiled pattern and assumed at match- ing time. This is necessary if you want to use JIT execution, because the JIT compiler needs to know whether or not this option is set. For details see the discussion of PCRE_NO_START_OPTIMIZE below. PCRE_UCP This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes. By default, only ASCII characters are recognized, but if PCRE_UCP is set, Unicode properties are used instead to classify characters. More details are given in the section on generic character types in the pcrepattern page. If you set PCRE_UCP, matching one of the items it affects takes much longer. The option is available only if PCRE has been compiled with Unicode prop- erty support. PCRE_UNGREEDY This option inverts the "greediness" of the quantifiers so that they are not greedy by default, but become greedy if followed by "?". It is not compatible with Perl. It can also be set by a (?U) option setting within the pattern. PCRE_UTF8 This option causes PCRE to regard both the pattern and the subject as strings of UTF-8 characters instead of single-byte strings. However, it is available only when PCRE is built to include UTF support. If not, the use of this option provokes an error. Details of how this option changes the behaviour of PCRE are given in the pcreunicode page. PCRE_NO_UTF8_CHECK When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is automatically checked. There is a discussion about the validity of UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is found, pcre_compile() returns an error. If you already know that your pattern is valid, and you want to skip this check for performance rea- sons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the effect of passing an invalid UTF-8 string as a pattern is undefined. It may cause your program to crash or loop. Note that this option can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the validity checking of subject strings only. If the same string is being matched many times, the option can be safely set for the second and subsequent matchings to improve performance. COMPILATION ERROR CODES The following table lists the error codes than may be returned by pcre_compile2(), along with the error messages that may be returned by both compiling functions. Note that error messages are always 8-bit ASCII strings, even in 16-bit or 32-bit mode. As PCRE has developed, some error codes have fallen out of use. To avoid confusion, they have not been re-used. 0 no error 1 \ at end of pattern 2 \c at end of pattern 3 unrecognized character follows \ 4 numbers out of order in {} quantifier 5 number too big in {} quantifier 6 missing terminating ] for character class 7 invalid escape sequence in character class 8 range out of order in character class 9 nothing to repeat 10 [this code is not in use] 11 internal error: unexpected repeat 12 unrecognized character after (? or (?- 13 POSIX named classes are supported only within a class 14 missing ) 15 reference to non-existent subpattern 16 erroffset passed as NULL 17 unknown option bit(s) set 18 missing ) after comment 19 [this code is not in use] 20 regular expression is too large 21 failed to get memory 22 unmatched parentheses 23 internal error: code overflow 24 unrecognized character after (?< 25 lookbehind assertion is not fixed length 26 malformed number or name after (?( 27 conditional group contains more than two branches 28 assertion expected after (?( 29 (?R or (?[+-]digits must be followed by ) 30 unknown POSIX class name 31 POSIX collating elements are not supported 32 this version of PCRE is compiled without UTF support 33 [this code is not in use] 34 character value in \x{} or \o{} is too large 35 invalid condition (?(0) 36 \C not allowed in lookbehind assertion 37 PCRE does not support \L, \l, \N{name}, \U, or \u 38 number after (?C is > 255 39 closing ) for (?C expected 40 recursive call could loop indefinitely 41 unrecognized character after (?P 42 syntax error in subpattern name (missing terminator) 43 two named subpatterns have the same name 44 invalid UTF-8 string (specifically UTF-8) 45 support for \P, \p, and \X has not been compiled 46 malformed \P or \p sequence 47 unknown property name after \P or \p 48 subpattern name is too long (maximum 32 characters) 49 too many named subpatterns (maximum 10000) 50 [this code is not in use] 51 octal value is greater than \377 in 8-bit non-UTF-8 mode 52 internal error: overran compiling workspace 53 internal error: previously-checked referenced subpattern not found 54 DEFINE group contains more than one branch 55 repeating a DEFINE group is not allowed 56 inconsistent NEWLINE options 57 \g is not followed by a braced, angle-bracketed, or quoted name/number or by a plain number 58 a numbered reference must not be zero 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT) 60 (*VERB) not recognized or malformed 61 number is too big 62 subpattern name expected 63 digit expected after (?+ 64 ] is an invalid data character in JavaScript compatibility mode 65 different names for subpatterns of the same number are not allowed 66 (*MARK) must have an argument 67 this version of PCRE is not compiled with Unicode property support 68 \c must be followed by an ASCII character 69 \k is not followed by a braced, angle-bracketed, or quoted name 70 internal error: unknown opcode in find_fixedlength() 71 \N is not supported in a class 72 too many forward references 73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff) 74 invalid UTF-16 string (specifically UTF-16) 75 name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN) 76 character value in \u.... sequence is too large 77 invalid UTF-32 string (specifically UTF-32) 78 setting UTF is disabled by the application 79 non-hex character in \x{} (closing brace missing?) 80 non-octal character in \o{} (closing brace missing?) 81 missing opening brace after \o 82 parentheses are too deeply nested 83 invalid range in character class 84 group name must start with a non-digit 85 parentheses are too deeply nested (stack check) The numbers 32 and 10000 in errors 48 and 49 are defaults; different values may be used if the limits were changed when PCRE was built. STUDYING A PATTERN pcre_extra *pcre_study(const pcre *code, int options, const char **errptr); If a compiled pattern is going to be used several times, it is worth spending more time analyzing it in order to speed up the time taken for matching. The function pcre_study() takes a pointer to a compiled pat- tern as its first argument. If studying the pattern produces additional information that will help speed up matching, pcre_study() returns a pointer to a pcre_extra block, in which the study_data field points to the results of the study. The returned value from pcre_study() can be passed directly to pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con- tains other fields that can be set by the caller before the block is passed; these are described below in the section on matching a pattern. If studying the pattern does not produce any useful information, pcre_study() returns NULL by default. In that circumstance, if the calling program wants to pass any of the other fields to pcre_exec() or pcre_dfa_exec(), it must set up its own pcre_extra block. However, if pcre_study() is called with the PCRE_STUDY_EXTRA_NEEDED option, it returns a pcre_extra block even if studying did not find any additional information. It may still return NULL, however, if an error occurs in pcre_study(). The second argument of pcre_study() contains option bits. There are three further options in addition to PCRE_STUDY_EXTRA_NEEDED: PCRE_STUDY_JIT_COMPILE PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE If any of these are set, and the just-in-time compiler is available, the pattern is further compiled into machine code that executes much faster than the pcre_exec() interpretive matching function. If the just-in-time compiler is not available, these options are ignored. All undefined bits in the options argument must be zero. JIT compilation is a heavyweight optimization. It can take some time for patterns to be analyzed, and for one-off matches and simple pat- terns the benefit of faster execution might be offset by a much slower study time. Not all patterns can be optimized by the JIT compiler. For those that cannot be handled, matching automatically falls back to the pcre_exec() interpreter. For more details, see the pcrejit documenta- tion. The third argument for pcre_study() is a pointer for an error message. If studying succeeds (even if no data is returned), the variable it points to is set to NULL. Otherwise it is set to point to a textual error message. This is a static string that is part of the library. You must not try to free it. You should test the error pointer for NULL after calling pcre_study(), to be sure that it has run successfully. When you are finished with a pattern, you can free the memory used for the study data by calling pcre_free_study(). This function was added to the API for release 8.20. For earlier versions, the memory could be freed with pcre_free(), just like the pattern itself. This will still work in cases where JIT optimization is not used, but it is advisable to change to the new function when convenient. This is a typical way in which pcre_study() is used (except that in a real application there should be tests for errors): int rc; pcre *re; pcre_extra *sd; re = pcre_compile("pattern", 0, &error, &erroroffset, NULL); sd = pcre_study( re, /* result of pcre_compile() */ 0, /* no options */ &error); /* set to NULL or points to a message */ rc = pcre_exec( /* see below for details of pcre_exec() options */ re, sd, "subject", 7, 0, 0, ovector, 30); ... pcre_free_study(sd); pcre_free(re); Studying a pattern does two things: first, a lower bound for the length of subject string that is needed to match the pattern is computed. This does not mean that there are any strings of that length that match, but it does guarantee that no shorter strings match. The value is used to avoid wasting time by trying to match strings that are shorter than the lower bound. You can find out the value in a calling program via the pcre_fullinfo() function. Studying a pattern is also useful for non-anchored patterns that do not have a single fixed starting character. A bitmap of possible starting bytes is created. This speeds up finding a position in the subject at which to start matching. (In 16-bit mode, the bitmap is used for 16-bit values less than 256. In 32-bit mode, the bitmap is used for 32-bit values less than 256.) These two optimizations apply to both pcre_exec() and pcre_dfa_exec(), and the information is also used by the JIT compiler. The optimiza- tions can be disabled by setting the PCRE_NO_START_OPTIMIZE option. You might want to do this if your pattern contains callouts or (*MARK) and you want to make use of these facilities in cases where matching fails. PCRE_NO_START_OPTIMIZE can be specified at either compile time or exe- cution time. However, if PCRE_NO_START_OPTIMIZE is passed to pcre_exec(), (that is, after any JIT compilation has happened) JIT exe- cution is disabled. For JIT execution to work with PCRE_NO_START_OPTI- MIZE, the option must be set at compile time. There is a longer discussion of PCRE_NO_START_OPTIMIZE below. LOCALE SUPPORT PCRE handles caseless matching, and determines whether characters are letters, digits, or whatever, by reference to a set of tables, indexed by character code point. When running in UTF-8 mode, or in the 16- or 32-bit libraries, this applies only to characters with code points less than 256. By default, higher-valued code points never match escapes such as \w or \d. However, if PCRE is built with Unicode property sup- port, all characters can be tested with \p and \P, or, alternatively, the PCRE_UCP option can be set when a pattern is compiled; this causes \w and friends to use Unicode property support instead of the built-in tables. The use of locales with Unicode is discouraged. If you are handling characters with code points greater than 128, you should either use Unicode support, or use locales, but not try to mix the two. PCRE contains an internal set of tables that are used when the final argument of pcre_compile() is NULL. These are sufficient for many applications. Normally, the internal tables recognize only ASCII char- acters. However, when PCRE is built, it is possible to cause the inter- nal tables to be rebuilt in the default "C" locale of the local system, which may cause them to be different. The internal tables can always be overridden by tables supplied by the application that calls PCRE. These may be created in a different locale from the default. As more and more applications change to using Uni- code, the need for this locale support is expected to die away. External tables are built by calling the pcre_maketables() function, which has no arguments, in the relevant locale. The result can then be passed to pcre_compile() as often as necessary. For example, to build and use tables that are appropriate for the French locale (where accented characters with values greater than 128 are treated as let- ters), the following code could be used: setlocale(LC_CTYPE, "fr_FR"); tables = pcre_maketables(); re = pcre_compile(..., tables); The locale name "fr_FR" is used on Linux and other Unix-like systems; if you are using Windows, the name for the French locale is "french". When pcre_maketables() runs, the tables are built in memory that is obtained via pcre_malloc. It is the caller's responsibility to ensure that the memory containing the tables remains available for as long as it is needed. The pointer that is passed to pcre_compile() is saved with the compiled pattern, and the same tables are used via this pointer by pcre_study() and also by pcre_exec() and pcre_dfa_exec(). Thus, for any single pat- tern, compilation, studying and matching all happen in the same locale, but different patterns can be processed in different locales. It is possible to pass a table pointer or NULL (indicating the use of the internal tables) to pcre_exec() or pcre_dfa_exec() (see the discus- sion below in the section on matching a pattern). This facility is pro- vided for use with pre-compiled patterns that have been saved and reloaded. Character tables are not saved with patterns, so if a non- standard table was used at compile time, it must be provided again when the reloaded pattern is matched. Attempting to use this facility to match a pattern in a different locale from the one in which it was com- piled is likely to lead to anomalous (usually incorrect) results. INFORMATION ABOUT A PATTERN int pcre_fullinfo(const pcre *code, const pcre_extra *extra, int what, void *where); The pcre_fullinfo() function returns information about a compiled pat- tern. It replaces the pcre_info() function, which was removed from the library at version 8.30, after more than 10 years of obsolescence. The first argument for pcre_fullinfo() is a pointer to the compiled pattern. The second argument is the result of pcre_study(), or NULL if the pattern was not studied. The third argument specifies which piece of information is required, and the fourth argument is a pointer to a variable to receive the data. The yield of the function is zero for success, or one of the following negative numbers: PCRE_ERROR_NULL the argument code was NULL the argument where was NULL PCRE_ERROR_BADMAGIC the "magic number" was not found PCRE_ERROR_BADENDIANNESS the pattern was compiled with different endianness PCRE_ERROR_BADOPTION the value of what was invalid PCRE_ERROR_UNSET the requested field is not set The "magic number" is placed at the start of each compiled pattern as an simple check against passing an arbitrary memory pointer. The endi- anness error can occur if a compiled pattern is saved and reloaded on a different host. Here is a typical call of pcre_fullinfo(), to obtain the length of the compiled pattern: int rc; size_t length; rc = pcre_fullinfo( re, /* result of pcre_compile() */ sd, /* result of pcre_study(), or NULL */ PCRE_INFO_SIZE, /* what is required */ &length); /* where to put the data */ The possible values for the third argument are defined in pcre.h, and are as follows: PCRE_INFO_BACKREFMAX Return the number of the highest back reference in the pattern. The fourth argument should point to an int variable. Zero is returned if there are no back references. PCRE_INFO_CAPTURECOUNT Return the number of capturing subpatterns in the pattern. The fourth argument should point to an int variable. PCRE_INFO_DEFAULT_TABLES Return a pointer to the internal default character tables within PCRE. The fourth argument should point to an unsigned char * variable. This information call is provided for internal use by the pcre_study() func- tion. External callers can cause PCRE to use its internal tables by passing a NULL table pointer. PCRE_INFO_FIRSTBYTE (deprecated) Return information about the first data unit of any matched string, for a non-anchored pattern. The name of this option refers to the 8-bit library, where data units are bytes. The fourth argument should point to an int variable. Negative values are used for special cases. How- ever, this means that when the 32-bit library is in non-UTF-32 mode, the full 32-bit range of characters cannot be returned. For this rea- son, this value is deprecated; use PCRE_INFO_FIRSTCHARACTERFLAGS and PCRE_INFO_FIRSTCHARACTER instead. If there is a fixed first value, for example, the letter "c" from a pattern such as (cat|cow|coyote), its value is returned. In the 8-bit library, the value is always less than 256. In the 16-bit library the value can be up to 0xffff. In the 32-bit library the value can be up to 0x10ffff. If there is no fixed first value, and if either (a) the pattern was compiled with the PCRE_MULTILINE option, and every branch starts with "^", or (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set (if it were set, the pattern would be anchored), -1 is returned, indicating that the pattern matches only at the start of a subject string or after any newline within the string. Otherwise -2 is returned. For anchored patterns, -2 is returned. PCRE_INFO_FIRSTCHARACTER Return the value of the first data unit (non-UTF character) of any matched string in the situation where PCRE_INFO_FIRSTCHARACTERFLAGS returns 1; otherwise return 0. The fourth argument should point to an uint_t variable. In the 8-bit library, the value is always less than 256. In the 16-bit library the value can be up to 0xffff. In the 32-bit library in UTF-32 mode the value can be up to 0x10ffff, and up to 0xffffffff when not using UTF-32 mode. PCRE_INFO_FIRSTCHARACTERFLAGS Return information about the first data unit of any matched string, for a non-anchored pattern. The fourth argument should point to an int variable. If there is a fixed first value, for example, the letter "c" from a pattern such as (cat|cow|coyote), 1 is returned, and the character value can be retrieved using PCRE_INFO_FIRSTCHARACTER. If there is no fixed first value, and if either (a) the pattern was compiled with the PCRE_MULTILINE option, and every branch starts with "^", or (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set (if it were set, the pattern would be anchored), 2 is returned, indicating that the pattern matches only at the start of a subject string or after any newline within the string. Otherwise 0 is returned. For anchored patterns, 0 is returned. PCRE_INFO_FIRSTTABLE If the pattern was studied, and this resulted in the construction of a 256-bit table indicating a fixed set of values for the first data unit in any matching string, a pointer to the table is returned. Otherwise NULL is returned. The fourth argument should point to an unsigned char * variable. PCRE_INFO_HASCRORLF Return 1 if the pattern contains any explicit matches for CR or LF characters, otherwise 0. The fourth argument should point to an int variable. An explicit match is either a literal CR or LF character, or \r or \n. PCRE_INFO_JCHANGED Return 1 if the (?J) or (?-J) option setting is used in the pattern, otherwise 0. The fourth argument should point to an int variable. (?J) and (?-J) set and unset the local PCRE_DUPNAMES option, respectively. PCRE_INFO_JIT Return 1 if the pattern was studied with one of the JIT options, and just-in-time compiling was successful. The fourth argument should point to an int variable. A return value of 0 means that JIT support is not available in this version of PCRE, or that the pattern was not studied with a JIT option, or that the JIT compiler could not handle this par- ticular pattern. See the pcrejit documentation for details of what can and cannot be handled. PCRE_INFO_JITSIZE If the pattern was successfully studied with a JIT option, return the size of the JIT compiled code, otherwise return zero. The fourth argu- ment should point to a size_t variable. PCRE_INFO_LASTLITERAL Return the value of the rightmost literal data unit that must exist in any matched string, other than at its start, if such a value has been recorded. The fourth argument should point to an int variable. If there is no such value, -1 is returned. For anchored patterns, a last literal value is recorded only if it follows something of variable length. For example, for the pattern /^a\d+z\d+/ the returned value is "z", but for /^a\dz\d/ the returned value is -1. Since for the 32-bit library using the non-UTF-32 mode, this function is unable to return the full 32-bit range of characters, this value is deprecated; instead the PCRE_INFO_REQUIREDCHARFLAGS and PCRE_INFO_REQUIREDCHAR values should be used. PCRE_INFO_MATCH_EMPTY Return 1 if the pattern can match an empty string, otherwise 0. The fourth argument should point to an int variable. PCRE_INFO_MATCHLIMIT If the pattern set a match limit by including an item of the form (*LIMIT_MATCH=nnnn) at the start, the value is returned. The fourth argument should point to an unsigned 32-bit integer. If no such value has been set, the call to pcre_fullinfo() returns the error PCRE_ERROR_UNSET. PCRE_INFO_MAXLOOKBEHIND Return the number of characters (NB not data units) in the longest lookbehind assertion in the pattern. This information is useful when doing multi-segment matching using the partial matching facilities. Note that the simple assertions \b and \B require a one-character look- behind. \A also registers a one-character lookbehind, though it does not actually inspect the previous character. This is to ensure that at least one character from the old segment is retained when a new segment is processed. Otherwise, if there are no lookbehinds in the pattern, \A might match incorrectly at the start of a new segment. PCRE_INFO_MINLENGTH If the pattern was studied and a minimum length for matching subject strings was computed, its value is returned. Otherwise the returned value is -1. The value is a number of characters, which in UTF mode may be different from the number of data units. The fourth argument should point to an int variable. A non-negative value is a lower bound to the length of any matching string. There may not be any strings of that length that do actually match, but every string that does match is at least that long. PCRE_INFO_NAMECOUNT PCRE_INFO_NAMEENTRYSIZE PCRE_INFO_NAMETABLE PCRE supports the use of named as well as numbered capturing parenthe- ses. The names are just an additional way of identifying the parenthe- ses, which still acquire numbers. Several convenience functions such as pcre_get_named_substring() are provided for extracting captured sub- strings by name. It is also possible to extract the data directly, by first converting the name to a number in order to access the correct pointers in the output vector (described with pcre_exec() below). To do the conversion, you need to use the name-to-number map, which is described by these three values. The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size of each entry; both of these return an int value. The entry size depends on the length of the longest name. PCRE_INFO_NAMETABLE returns a pointer to the first entry of the table. This is a pointer to char in the 8-bit library, where the first two bytes of each entry are the num- ber of the capturing parenthesis, most significant byte first. In the 16-bit library, the pointer points to 16-bit data units, the first of which contains the parenthesis number. In the 32-bit library, the pointer points to 32-bit data units, the first of which contains the parenthesis number. The rest of the entry is the corresponding name, zero terminated. The names are in alphabetical order. If (?| is used to create multiple groups with the same number, as described in the section on duplicate subpattern numbers in the pcrepattern page, the groups may be given the same name, but there is only one entry in the table. Different names for groups of the same number are not permitted. Duplicate names for subpatterns with different numbers are permitted, but only if PCRE_DUP- NAMES is set. They appear in the table in the order in which they were found in the pattern. In the absence of (?| this is the order of increasing number; when (?| is used this is not necessarily the case because later subpatterns may have lower numbers. As a simple example of the name/number table, consider the following pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is set, so white space - including newlines - is ignored): (?<date> (?<year>(\d\d)?\d\d) - (?<month>\d\d) - (?<day>\d\d) ) There are four named subpatterns, so the table has four entries, and each entry in the table is eight bytes long. The table is as follows, with non-printing bytes shows in hexadecimal, and undefined bytes shown as ??: 00 01 d a t e 00 ?? 00 05 d a y 00 ?? ?? 00 04 m o n t h 00 00 02 y e a r 00 ?? When writing code to extract data from named subpatterns using the name-to-number map, remember that the length of the entries is likely to be different for each compiled pattern. PCRE_INFO_OKPARTIAL Return 1 if the pattern can be used for partial matching with pcre_exec(), otherwise 0. The fourth argument should point to an int variable. From release 8.00, this always returns 1, because the restrictions that previously applied to partial matching have been lifted. The pcrepartial documentation gives details of partial match- ing. PCRE_INFO_OPTIONS Return a copy of the options with which the pattern was compiled. The fourth argument should point to an unsigned long int variable. These option bits are those specified in the call to pcre_compile(), modified by any top-level option settings at the start of the pattern itself. In other words, they are the options that will be in force when matching starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE, and PCRE_EXTENDED. A pattern is automatically anchored by PCRE if all of its top-level alternatives begin with one of the following: ^ unless PCRE_MULTILINE is set \A always \G always .* if PCRE_DOTALL is set and there are no back references to the subpattern in which .* appears For such patterns, the PCRE_ANCHORED bit is set in the options returned by pcre_fullinfo(). PCRE_INFO_RECURSIONLIMIT If the pattern set a recursion limit by including an item of the form (*LIMIT_RECURSION=nnnn) at the start, the value is returned. The fourth argument should point to an unsigned 32-bit integer. If no such value has been set, the call to pcre_fullinfo() returns the error PCRE_ERROR_UNSET. PCRE_INFO_SIZE Return the size of the compiled pattern in bytes (for all three libraries). The fourth argument should point to a size_t variable. This value does not include the size of the pcre structure that is returned by pcre_compile(). The value that is passed as the argument to pcre_malloc() when pcre_compile() is getting memory in which to place the compiled data is the value returned by this option plus the size of the pcre structure. Studying a compiled pattern, with or without JIT, does not alter the value returned by this option. PCRE_INFO_STUDYSIZE Return the size in bytes (for all three libraries) of the data block pointed to by the study_data field in a pcre_extra block. If pcre_extra is NULL, or there is no study data, zero is returned. The fourth argu- ment should point to a size_t variable. The study_data field is set by pcre_study() to record information that will speed up matching (see the section entitled "Studying a pattern" above). The format of the study_data block is private, but its length is made available via this option so that it can be saved and restored (see the pcreprecompile documentation for details). PCRE_INFO_REQUIREDCHARFLAGS Returns 1 if there is a rightmost literal data unit that must exist in any matched string, other than at its start. The fourth argument should point to an int variable. If there is no such value, 0 is returned. If returning 1, the character value itself can be retrieved using PCRE_INFO_REQUIREDCHAR. For anchored patterns, a last literal value is recorded only if it fol- lows something of variable length. For example, for the pattern /^a\d+z\d+/ the returned value 1 (with "z" returned from PCRE_INFO_REQUIREDCHAR), but for /^a\dz\d/ the returned value is 0. PCRE_INFO_REQUIREDCHAR Return the value of the rightmost literal data unit that must exist in any matched string, other than at its start, if such a value has been recorded. The fourth argument should point to an uint32_t variable. If there is no such value, 0 is returned. REFERENCE COUNTS int pcre_refcount(pcre *code, int adjust); The pcre_refcount() function is used to maintain a reference count in the data block that contains a compiled pattern. It is provided for the benefit of applications that operate in an object-oriented manner, where different parts of the application may be using the same compiled pattern, but you want to free the block when they are all done. When a pattern is compiled, the reference count field is initialized to zero. It is changed only by calling this function, whose action is to add the adjust value (which may be positive or negative) to it. The yield of the function is the new value. However, the value of the count is constrained to lie between 0 and 65535, inclusive. If the new value is outside these limits, it is forced to the appropriate limit value. Except when it is zero, the reference count is not correctly preserved if a pattern is compiled on one host and then transferred to a host whose byte-order is different. (This seems a highly unlikely scenario.) MATCHING A PATTERN: THE TRADITIONAL FUNCTION int pcre_exec(const pcre *code, const pcre_extra *extra, const char *subject, int length, int startoffset, int options, int *ovector, int ovecsize); The function pcre_exec() is called to match a subject string against a compiled pattern, which is passed in the code argument. If the pattern was studied, the result of the study should be passed in the extra argument. You can call pcre_exec() with the same code and extra argu- ments as many times as you like, in order to match different subject strings with the same pattern. This function is the main matching facility of the library, and it operates in a Perl-like manner. For specialist use there is also an alternative matching function, which is described below in the section about the pcre_dfa_exec() function. In most applications, the pattern will have been compiled (and option- ally studied) in the same process that calls pcre_exec(). However, it is possible to save compiled patterns and study data, and then use them later in different processes, possibly even on different hosts. For a discussion about this, see the pcreprecompile documentation. Here is an example of a simple call to pcre_exec(): int rc; int ovector[30]; rc = pcre_exec( re, /* result of pcre_compile() */ NULL, /* we didn't study the pattern */ "some string", /* the subject string */ 11, /* the length of the subject string */ 0, /* start at offset 0 in the subject */ 0, /* default options */ ovector, /* vector of integers for substring information */ 30); /* number of elements (NOT size in bytes) */ Extra data for pcre_exec() If the extra argument is not NULL, it must point to a pcre_extra data block. The pcre_study() function returns such a block (when it doesn't return NULL), but you can also create one for yourself, and pass addi- tional information in it. The pcre_extra block contains the following fields (not necessarily in this order): unsigned long int flags; void *study_data; void *executable_jit; unsigned long int match_limit; unsigned long int match_limit_recursion; void *callout_data; const unsigned char *tables; unsigned char **mark; In the 16-bit version of this structure, the mark field has type "PCRE_UCHAR16 **". In the 32-bit version of this structure, the mark field has type "PCRE_UCHAR32 **". The flags field is used to specify which of the other fields are set. The flag bits are: PCRE_EXTRA_CALLOUT_DATA PCRE_EXTRA_EXECUTABLE_JIT PCRE_EXTRA_MARK PCRE_EXTRA_MATCH_LIMIT PCRE_EXTRA_MATCH_LIMIT_RECURSION PCRE_EXTRA_STUDY_DATA PCRE_EXTRA_TABLES Other flag bits should be set to zero. The study_data field and some- times the executable_jit field are set in the pcre_extra block that is returned by pcre_study(), together with the appropriate flag bits. You should not set these yourself, but you may add to the block by setting other fields and their corresponding flag bits. The match_limit field provides a means of preventing PCRE from using up a vast amount of resources when running patterns that are not going to match, but which have a very large number of possibilities in their search trees. The classic example is a pattern that uses nested unlim- ited repeats. Internally, pcre_exec() uses a function called match(), which it calls repeatedly (sometimes recursively). The limit set by match_limit is imposed on the number of times this function is called during a match, which has the effect of limiting the amount of backtracking that can take place. For patterns that are not anchored, the count restarts from zero for each position in the subject string. When pcre_exec() is called with a pattern that was successfully studied with a JIT option, the way that the matching is executed is entirely different. However, there is still the possibility of runaway matching that goes on for a very long time, and so the match_limit value is also used in this case (but in a different way) to limit how long the match- ing can continue. The default value for the limit can be set when PCRE is built; the default default is 10 million, which handles all but the most extreme cases. You can override the default by suppling pcre_exec() with a pcre_extra block in which match_limit is set, and PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT. A value for the match limit may also be supplied by an item at the start of a pattern of the form (*LIMIT_MATCH=d) where d is a decimal number. However, such a setting is ignored unless d is less than the limit set by the caller of pcre_exec() or, if no such limit is set, less than the default. The match_limit_recursion field is similar to match_limit, but instead of limiting the total number of times that match() is called, it limits the depth of recursion. The recursion depth is a smaller number than the total number of calls, because not all calls to match() are recur- sive. This limit is of use only if it is set smaller than match_limit. Limiting the recursion depth limits the amount of machine stack that can be used, or, when PCRE has been compiled to use memory on the heap instead of the stack, the amount of heap memory that can be used. This limit is not relevant, and is ignored, when matching is done using JIT compiled code. The default value for match_limit_recursion can be set when PCRE is built; the default default is the same value as the default for match_limit. You can override the default by suppling pcre_exec() with a pcre_extra block in which match_limit_recursion is set, and PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT. A value for the recursion limit may also be supplied by an item at the start of a pattern of the form (*LIMIT_RECURSION=d) where d is a decimal number. However, such a setting is ignored unless d is less than the limit set by the caller of pcre_exec() or, if no such limit is set, less than the default. The callout_data field is used in conjunction with the "callout" fea- ture, and is described in the pcrecallout documentation. The tables field is provided for use with patterns that have been pre- compiled using custom character tables, saved to disc or elsewhere, and then reloaded, because the tables that were used to compile a pattern are not saved with it. See the pcreprecompile documentation for a dis- cussion of saving compiled patterns for later use. If NULL is passed using this mechanism, it forces PCRE's internal tables to be used. Warning: The tables that pcre_exec() uses must be the same as those that were used when the pattern was compiled. If this is not the case, the behaviour of pcre_exec() is undefined. Therefore, when a pattern is compiled and matched in the same process, this field should never be set. In this (the most common) case, the correct table pointer is auto- matically passed with the compiled pattern from pcre_compile() to pcre_exec(). If PCRE_EXTRA_MARK is set in the flags field, the mark field must be set to point to a suitable variable. If the pattern contains any back- tracking control verbs such as (*MARK:NAME), and the execution ends up with a name to pass back, a pointer to the name string (zero termi- nated) is placed in the variable pointed to by the mark field. The names are within the compiled pattern; if you wish to retain such a name you must copy it before freeing the memory of a compiled pattern. If there is no name to pass back, the variable pointed to by the mark field is set to NULL. For details of the backtracking control verbs, see the section entitled "Backtracking control" in the pcrepattern doc- umentation. Option bits for pcre_exec() The unused bits of the options argument for pcre_exec() must be zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If the pattern was successfully studied with one of the just-in-time (JIT) compile options, the only supported options for JIT execution are PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If an unsupported option is used, JIT execution is disabled and the normal interpretive code in pcre_exec() is run. PCRE_ANCHORED The PCRE_ANCHORED option limits pcre_exec() to matching at the first matching position. If a pattern was compiled with PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it cannot be made unachored at matching time. PCRE_BSR_ANYCRLF PCRE_BSR_UNICODE These options (which are mutually exclusive) control what the \R escape sequence matches. The choice is either to match only CR, LF, or CRLF, or to match any Unicode newline sequence. These options override the choice that was made or defaulted when the pattern was compiled. PCRE_NEWLINE_CR PCRE_NEWLINE_LF PCRE_NEWLINE_CRLF PCRE_NEWLINE_ANYCRLF PCRE_NEWLINE_ANY These options override the newline definition that was chosen or defaulted when the pattern was compiled. For details, see the descrip- tion of pcre_compile() above. During matching, the newline choice affects the behaviour of the dot, circumflex, and dollar metacharac- ters. It may also alter the way the match position is advanced after a match failure for an unanchored pattern. When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is set, and a match attempt for an unanchored pattern fails when the cur- rent position is at a CRLF sequence, and the pattern contains no explicit matches for CR or LF characters, the match position is advanced by two characters instead of one, in other words, to after the CRLF. The above rule is a compromise that makes the most common cases work as expected. For example, if the pattern is .+A (and the PCRE_DOTALL option is not set), it does not match the string "\r\nA" because, after failing at the start, it skips both the CR and the LF before retrying. However, the pattern [\r\n]A does match that string, because it con- tains an explicit CR or LF reference, and so advances only by one char- acter after the first failure. An explicit match for CR of LF is either a literal appearance of one of those characters, or one of the \r or \n escape sequences. Implicit matches such as [^X] do not count, nor does \s (which includes CR and LF in the characters that it matches). Notwithstanding the above, anomalous effects may still occur when CRLF is a valid newline sequence and explicit \r or \n escapes appear in the pattern. PCRE_NOTBOL This option specifies that first character of the subject string is not the beginning of a line, so the circumflex metacharacter should not match before it. Setting this without PCRE_MULTILINE (at compile time) causes circumflex never to match. This option affects only the behav- iour of the circumflex metacharacter. It does not affect \A. PCRE_NOTEOL This option specifies that the end of the subject string is not the end of a line, so the dollar metacharacter should not match it nor (except in multiline mode) a newline immediately before it. Setting this with- out PCRE_MULTILINE (at compile time) causes dollar never to match. This option affects only the behaviour of the dollar metacharacter. It does not affect \Z or \z. PCRE_NOTEMPTY An empty string is not considered to be a valid match if this option is set. If there are alternatives in the pattern, they are tried. If all the alternatives match the empty string, the entire match fails. For example, if the pattern a?b? is applied to a string not beginning with "a" or "b", it matches an empty string at the start of the subject. With PCRE_NOTEMPTY set, this match is not valid, so PCRE searches further into the string for occur- rences of "a" or "b". PCRE_NOTEMPTY_ATSTART This is like PCRE_NOTEMPTY, except that an empty string match that is not at the start of the subject is permitted. If the pattern is anchored, such a match can occur only if the pattern contains \K. Perl has no direct equivalent of PCRE_NOTEMPTY or PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern match of the empty string within its split() function, and when using the /g modifier. It is possible to emulate Perl's behaviour after matching a null string by first trying the match again at the same off- set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that fails, by advancing the starting offset (see below) and trying an ordi- nary match again. There is some code that demonstrates how to do this in the pcredemo sample program. In the most general case, you have to check to see if the newline convention recognizes CRLF as a newline, and if so, and the current character is CR followed by LF, advance the starting offset by two characters instead of one. PCRE_NO_START_OPTIMIZE There are a number of optimizations that pcre_exec() uses at the start of a match, in order to speed up the process. For example, if it is known that an unanchored match must start with a specific character, it searches the subject for that character, and fails immediately if it cannot find it, without actually running the main matching function. This means that a special item such as (*COMMIT) at the start of a pat- tern is not considered until after a suitable starting point for the match has been found. Also, when callouts or (*MARK) items are in use, these "start-up" optimizations can cause them to be skipped if the pat- tern is never actually used. The start-up optimizations are in effect a pre-scan of the subject that takes place before the pattern is run. The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations, possibly causing performance to suffer, but ensuring that in cases where the result is "no match", the callouts do occur, and that items such as (*COMMIT) and (*MARK) are considered at every possible starting position in the subject string. If PCRE_NO_START_OPTIMIZE is set at compile time, it cannot be unset at matching time. The use of PCRE_NO_START_OPTIMIZE at matching time (that is, passing it to pcre_exec()) disables JIT execution; in this situation, matching is always done using interpretively. Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching operation. Consider the pattern (*COMMIT)ABC When this is compiled, PCRE records the fact that a match must start with the character "A". Suppose the subject string is "DEFABC". The start-up optimization scans along the subject, finds "A" and runs the first match attempt from there. The (*COMMIT) item means that the pat- tern must match the current starting position, which in this case, it does. However, if the same match is run with PCRE_NO_START_OPTIMIZE set, the initial scan along the subject string does not happen. The first match attempt is run starting from "D" and when this fails, (*COMMIT) prevents any further matches being tried, so the overall result is "no match". If the pattern is studied, more start-up opti- mizations may be used. For example, a minimum length for the subject may be recorded. Consider the pattern (*MARK:A)(X|Y) The minimum length for a match is one character. If the subject is "ABC", there will be attempts to match "ABC", "BC", "C", and then finally an empty string. If the pattern is studied, the final attempt does not take place, because PCRE knows that the subject is too short, and so the (*MARK) is never encountered. In this case, studying the pattern does not affect the overall match result, which is still "no match", but it does affect the auxiliary information that is returned. PCRE_NO_UTF8_CHECK When PCRE_UTF8 is set at compile time, the validity of the subject as a UTF-8 string is automatically checked when pcre_exec() is subsequently called. The entire string is checked before any other processing takes place. The value of startoffset is also checked to ensure that it points to the start of a UTF-8 character. There is a discussion about the validity of UTF-8 strings in the pcreunicode page. If an invalid sequence of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In both cases, information about the precise nature of the error may also be returned (see the descriptions of these errors in the section enti- tled Error return values from pcre_exec() below). If startoffset con- tains a value that does not point to the start of a UTF-8 character (or to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned. If you already know that your subject is valid, and you want to skip these checks for performance reasons, you can set the PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to do this for the second and subsequent calls to pcre_exec() if you are making repeated calls to find all the matches in a single subject string. However, you should be sure that the value of startoffset points to the start of a character (or the end of the subject). When PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as a subject or an invalid value of startoffset is undefined. Your program may crash or loop. PCRE_PARTIAL_HARD PCRE_PARTIAL_SOFT These options turn on the partial matching feature. For backwards com- patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial match occurs if the end of the subject string is reached successfully, but there are not enough subject characters to complete the match. If this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set, matching continues by testing any remaining alternatives. Only if no complete match can be found is PCRE_ERROR_PARTIAL returned instead of PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the caller is prepared to handle a partial match, but only if no complete match can be found. If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this case, if a partial match is found, pcre_exec() immediately returns PCRE_ERROR_PARTIAL, without considering any other alternatives. In other words, when PCRE_PARTIAL_HARD is set, a partial match is consid- ered to be more important that an alternative complete match. In both cases, the portion of the string that was inspected when the partial match was found is set as the first matching string. There is a more detailed discussion of partial and multi-segment matching, with examples, in the pcrepartial documentation. The string to be matched by pcre_exec() The subject string is passed to pcre_exec() as a pointer in subject, a length in length, and a starting offset in startoffset. The units for length and startoffset are bytes for the 8-bit library, 16-bit data items for the 16-bit library, and 32-bit data items for the 32-bit library. If startoffset is negative or greater than the length of the subject, pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is zero, the search for a match starts at the beginning of the subject, and this is by far the most common case. In UTF-8 or UTF-16 mode, the offset must point to the start of a character, or the end of the sub- ject (in UTF-32 mode, one data unit equals one character, so all off- sets are valid). Unlike the pattern string, the subject may contain binary zeroes. A non-zero starting offset is useful when searching for another match in the same subject by calling pcre_exec() again after a previous suc- cess. Setting startoffset differs from just passing over a shortened string and setting PCRE_NOTBOL in the case of a pattern that begins with any kind of lookbehind. For example, consider the pattern \Biss\B which finds occurrences of "iss" in the middle of words. (\B matches only if the current position in the subject is not a word boundary.) When applied to the string "Mississipi" the first call to pcre_exec() finds the first occurrence. If pcre_exec() is called again with just the remainder of the subject, namely "issipi", it does not match, because \B is always false at the start of the subject, which is deemed to be a word boundary. However, if pcre_exec() is passed the entire string again, but with startoffset set to 4, it finds the second occur- rence of "iss" because it is able to look behind the starting point to discover that it is preceded by a letter. Finding all the matches in a subject is tricky when the pattern can match an empty string. It is possible to emulate Perl's /g behaviour by first trying the match again at the same offset, with the PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that fails, advancing the starting offset and trying an ordinary match again. There is some code that demonstrates how to do this in the pcre- demo sample program. In the most general case, you have to check to see if the newline convention recognizes CRLF as a newline, and if so, and the current character is CR followed by LF, advance the starting offset by two characters instead of one. If a non-zero starting offset is passed when the pattern is anchored, one attempt to match at the given offset is made. This can only succeed if the pattern does not require the match to be at the start of the subject. How pcre_exec() returns captured substrings In general, a pattern matches a certain portion of the subject, and in addition, further substrings from the subject may be picked out by parts of the pattern. Following the usage in Jeffrey Friedl's book, this is called "capturing" in what follows, and the phrase "capturing subpattern" is used for a fragment of a pattern that picks out a sub- string. PCRE supports several other kinds of parenthesized subpattern that do not cause substrings to be captured. Captured substrings are returned to the caller via a vector of integers whose address is passed in ovector. The number of elements in the vec- tor is passed in ovecsize, which must be a non-negative number. Note: this argument is NOT the size of ovector in bytes. The first two-thirds of the vector is used to pass back captured sub- strings, each substring using a pair of integers. The remaining third of the vector is used as workspace by pcre_exec() while matching cap- turing subpatterns, and is not available for passing back information. The number passed in ovecsize should always be a multiple of three. If it is not, it is rounded down. When a match is successful, information about captured substrings is returned in pairs of integers, starting at the beginning of ovector, and continuing up to two-thirds of its length at the most. The first element of each pair is set to the offset of the first character in a substring, and the second is set to the offset of the first character after the end of a substring. These values are always data unit off- sets, even in UTF mode. They are byte offsets in the 8-bit library, 16-bit data item offsets in the 16-bit library, and 32-bit data item offsets in the 32-bit library. Note: they are not character counts. The first pair of integers, ovector[0] and ovector[1], identify the portion of the subject string matched by the entire pattern. The next pair is used for the first capturing subpattern, and so on. The value returned by pcre_exec() is one more than the highest numbered pair that has been set. For example, if two substrings have been captured, the returned value is 3. If there are no capturing subpatterns, the return value from a successful match is 1, indicating that just the first pair of offsets has been set. If a capturing subpattern is matched repeatedly, it is the last portion of the string that it matched that is returned. If the vector is too small to hold all the captured substring offsets, it is used as far as possible (up to two-thirds of its length), and the function returns a value of zero. If neither the actual string matched nor any captured substrings are of interest, pcre_exec() may be called with ovector passed as NULL and ovecsize as zero. However, if the pat- tern contains back references and the ovector is not big enough to remember the related substrings, PCRE has to get additional memory for use during matching. Thus it is usually advisable to supply an ovector of reasonable size. There are some cases where zero is returned (indicating vector over- flow) when in fact the vector is exactly the right size for the final match. For example, consider the pattern (a)(?:(b)c|bd) If a vector of 6 elements (allowing for only 1 captured substring) is given with subject string "abd", pcre_exec() will try to set the second captured string, thereby recording a vector overflow, before failing to match "c" and backing up to try the second alternative. The zero return, however, does correctly indicate that the maximum number of slots (namely 2) have been filled. In similar cases where there is tem- porary overflow, but the final number of used slots is actually less than the maximum, a non-zero value is returned. The pcre_fullinfo() function can be used to find out how many capturing subpatterns there are in a compiled pattern. The smallest size for ovector that will allow for n captured substrings, in addition to the offsets of the substring matched by the whole pattern, is (n+1)*3. It is possible for capturing subpattern number n+1 to match some part of the subject when subpattern n has not been used at all. For example, if the string "abc" is matched against the pattern (a|(z))(bc) the return from the function is 4, and subpatterns 1 and 3 are matched, but 2 is not. When this happens, both values in the offset pairs corre- sponding to unused subpatterns are set to -1. Offset values that correspond to unused subpatterns at the end of the expression are also set to -1. For example, if the string "abc" is matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not matched. The return from the function is 2, because the highest used capturing subpattern number is 1, and the offsets for for the second and third capturing subpatterns (assuming the vector is large enough, of course) are set to -1. Note: Elements in the first two-thirds of ovector that do not corre- spond to capturing parentheses in the pattern are never changed. That is, if a pattern contains n capturing parentheses, no more than ovec- tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in the first two-thirds) retain whatever values they previously had. Some convenience functions are provided for extracting the captured substrings as separate strings. These are described below. Error return values from pcre_exec() If pcre_exec() fails, it returns a negative number. The following are defined in the header file: PCRE_ERROR_NOMATCH (-1) The subject string did not match the pattern. PCRE_ERROR_NULL (-2) Either code or subject was passed as NULL, or ovector was NULL and ovecsize was not zero. PCRE_ERROR_BADOPTION (-3) An unrecognized bit was set in the options argument. PCRE_ERROR_BADMAGIC (-4) PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch the case when it is passed a junk pointer and to detect when a pattern that was compiled in an environment of one endianness is run in an environment with the other endianness. This is the error that PCRE gives when the magic number is not present. PCRE_ERROR_UNKNOWN_OPCODE (-5) While running the pattern match, an unknown item was encountered in the compiled pattern. This error could be caused by a bug in PCRE or by overwriting of the compiled pattern. PCRE_ERROR_NOMEMORY (-6) If a pattern contains back references, but the ovector that is passed to pcre_exec() is not big enough to remember the referenced substrings, PCRE gets a block of memory at the start of matching to use for this purpose. If the call via pcre_malloc() fails, this error is given. The memory is automatically freed at the end of matching. This error is also given if pcre_stack_malloc() fails in pcre_exec(). This can happen only when PCRE has been compiled with --disable-stack- for-recursion. PCRE_ERROR_NOSUBSTRING (-7) This error is used by the pcre_copy_substring(), pcre_get_substring(), and pcre_get_substring_list() functions (see below). It is never returned by pcre_exec(). PCRE_ERROR_MATCHLIMIT (-8) The backtracking limit, as specified by the match_limit field in a pcre_extra structure (or defaulted) was reached. See the description above. PCRE_ERROR_CALLOUT (-9) This error is never generated by pcre_exec() itself. It is provided for use by callout functions that want to yield a distinctive error code. See the pcrecallout documentation for details. PCRE_ERROR_BADUTF8 (-10) A string that contains an invalid UTF-8 byte sequence was passed as a subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of the output vector (ovecsize) is at least 2, the byte offset to the start of the the invalid UTF-8 character is placed in the first ele- ment, and a reason code is placed in the second element. The reason codes are listed in the following section. For backward compatibility, if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char- acter at the end of the subject (reason codes 1 to 5), PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8. PCRE_ERROR_BADUTF8_OFFSET (-11) The UTF-8 byte sequence that was passed as a subject was checked and found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the value of startoffset did not point to the beginning of a UTF-8 charac- ter or the end of the subject. PCRE_ERROR_PARTIAL (-12) The subject string did not match, but it did match partially. See the pcrepartial documentation for details of partial matching. PCRE_ERROR_BADPARTIAL (-13) This code is no longer in use. It was formerly returned when the PCRE_PARTIAL option was used with a compiled pattern containing items that were not supported for partial matching. From release 8.00 onwards, there are no restrictions on partial matching. PCRE_ERROR_INTERNAL (-14) An unexpected internal error has occurred. This error could be caused by a bug in PCRE or by overwriting of the compiled pattern. PCRE_ERROR_BADCOUNT (-15) This error is given if the value of the ovecsize argument is negative. PCRE_ERROR_RECURSIONLIMIT (-21) The internal recursion limit, as specified by the match_limit_recursion field in a pcre_extra structure (or defaulted) was reached. See the description above. PCRE_ERROR_BADNEWLINE (-23) An invalid combination of PCRE_NEWLINE_xxx options was given. PCRE_ERROR_BADOFFSET (-24) The value of startoffset was negative or greater than the length of the subject, that is, the value in length. PCRE_ERROR_SHORTUTF8 (-25) This error is returned instead of PCRE_ERROR_BADUTF8 when the subject string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD option is set. Information about the failure is returned as for PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but this special error code for PCRE_PARTIAL_HARD precedes the implementa- tion of returned information; it is retained for backwards compatibil- ity. PCRE_ERROR_RECURSELOOP (-26) This error is returned when pcre_exec() detects a recursion loop within the pattern. Specifically, it means that either the whole pattern or a subpattern has been called recursively for the second time at the same position in the subject string. Some simple patterns that might do this are detected and faulted at compile time, but more complicated cases, in particular mutual recursions between two different subpatterns, can- not be detected until run time. PCRE_ERROR_JIT_STACKLIMIT (-27) This error is returned when a pattern that was successfully studied using a JIT compile option is being matched, but the memory available for the just-in-time processing stack is not large enough. See the pcrejit documentation for more details. PCRE_ERROR_BADMODE (-28) This error is given if a pattern that was compiled by the 8-bit library is passed to a 16-bit or 32-bit library function, or vice versa. PCRE_ERROR_BADENDIANNESS (-29) This error is given if a pattern that was compiled and saved is reloaded on a host with different endianness. The utility function pcre_pattern_to_host_byte_order() can be used to convert such a pattern so that it runs on the new host. PCRE_ERROR_JIT_BADOPTION This error is returned when a pattern that was successfully studied using a JIT compile option is being matched, but the matching mode (partial or complete match) does not correspond to any JIT compilation mode. When the JIT fast path function is used, this error may be also given for invalid options. See the pcrejit documentation for more details. PCRE_ERROR_BADLENGTH (-32) This error is given if pcre_exec() is called with a negative value for the length argument. Error numbers -16 to -20, -22, and 30 are not used by pcre_exec(). Reason codes for invalid UTF-8 strings This section applies only to the 8-bit library. The corresponding information for the 16-bit and 32-bit libraries is given in the pcre16 and pcre32 pages. When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT- UTF8, and the size of the output vector (ovecsize) is at least 2, the offset of the start of the invalid UTF-8 character is placed in the first output vector element (ovector[0]) and a reason code is placed in the second element (ovector[1]). The reason codes are given names in the pcre.h header file: PCRE_UTF8_ERR1 PCRE_UTF8_ERR2 PCRE_UTF8_ERR3 PCRE_UTF8_ERR4 PCRE_UTF8_ERR5 The string ends with a truncated UTF-8 character; the code specifies how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 characters to be no longer than 4 bytes, the encoding scheme (origi- nally defined by RFC 2279) allows for up to 6 bytes, and this is checked first; hence the possibility of 4 or 5 missing bytes. PCRE_UTF8_ERR6 PCRE_UTF8_ERR7 PCRE_UTF8_ERR8 PCRE_UTF8_ERR9 PCRE_UTF8_ERR10 The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of the character do not have the binary value 0b10 (that is, either the most significant bit is 0, or the next bit is 1). PCRE_UTF8_ERR11 PCRE_UTF8_ERR12 A character that is valid by the RFC 2279 rules is either 5 or 6 bytes long; these code points are excluded by RFC 3629. PCRE_UTF8_ERR13 A 4-byte character has a value greater than 0x10fff; these code points are excluded by RFC 3629. PCRE_UTF8_ERR14 A 3-byte character has a value in the range 0xd800 to 0xdfff; this range of code points are reserved by RFC 3629 for use with UTF-16, and so are excluded from UTF-8. PCRE_UTF8_ERR15 PCRE_UTF8_ERR16 PCRE_UTF8_ERR17 PCRE_UTF8_ERR18 PCRE_UTF8_ERR19 A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes for a value that can be represented by fewer bytes, which is invalid. For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor- rect coding uses just one byte. PCRE_UTF8_ERR20 The two most significant bits of the first byte of a character have the binary value 0b10 (that is, the most significant bit is 1 and the sec- ond is 0). Such a byte can only validly occur as the second or subse- quent byte of a multi-byte character. PCRE_UTF8_ERR21 The first byte of a character has the value 0xfe or 0xff. These values can never occur in a valid UTF-8 string. PCRE_UTF8_ERR22 This error code was formerly used when the presence of a so-called "non-character" caused an error. Unicode corrigendum #9 makes it clear that such characters should not cause a string to be rejected, and so this code is no longer in use and is never returned. EXTRACTING CAPTURED SUBSTRINGS BY NUMBER int pcre_copy_substring(const char *subject, int *ovector, int stringcount, int stringnumber, char *buffer, int buffersize); int pcre_get_substring(const char *subject, int *ovector, int stringcount, int stringnumber, const char **stringptr); int pcre_get_substring_list(const char *subject, int *ovector, int stringcount, const char ***listptr); Captured substrings can be accessed directly by using the offsets returned by pcre_exec() in ovector. For convenience, the functions pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub- string_list() are provided for extracting captured substrings as new, separate, zero-terminated strings. These functions identify substrings by number. The next section describes functions for extracting named substrings. A substring that contains a binary zero is correctly extracted and has a further zero added on the end, but the result is not, of course, a C string. However, you can process such a string by referring to the length that is returned by pcre_copy_substring() and pcre_get_sub- string(). Unfortunately, the interface to pcre_get_substring_list() is not adequate for handling strings containing binary zeros, because the end of the final string is not independently indicated. The first three arguments are the same for all three of these func- tions: subject is the subject string that has just been successfully matched, ovector is a pointer to the vector of integer offsets that was passed to pcre_exec(), and stringcount is the number of substrings that were captured by the match, including the substring that matched the entire regular expression. This is the value returned by pcre_exec() if it is greater than zero. If pcre_exec() returned zero, indicating that it ran out of space in ovector, the value passed as stringcount should be the number of elements in the vector divided by three. The functions pcre_copy_substring() and pcre_get_substring() extract a single substring, whose number is given as stringnumber. A value of zero extracts the substring that matched the entire pattern, whereas higher values extract the captured substrings. For pcre_copy_sub- string(), the string is placed in buffer, whose length is given by buffersize, while for pcre_get_substring() a new block of memory is obtained via pcre_malloc, and its address is returned via stringptr. The yield of the function is the length of the string, not including the terminating zero, or one of these error codes: PCRE_ERROR_NOMEMORY (-6) The buffer was too small for pcre_copy_substring(), or the attempt to get memory failed for pcre_get_substring(). PCRE_ERROR_NOSUBSTRING (-7) There is no substring whose number is stringnumber. The pcre_get_substring_list() function extracts all available sub- strings and builds a list of pointers to them. All this is done in a single block of memory that is obtained via pcre_malloc. The address of the memory block is returned via listptr, which is also the start of the list of string pointers. The end of the list is marked by a NULL pointer. The yield of the function is zero if all went well, or the error code PCRE_ERROR_NOMEMORY (-6) if the attempt to get the memory block failed. When any of these functions encounter a substring that is unset, which can happen when capturing subpattern number n+1 matches some part of the subject, but subpattern n has not been used at all, they return an empty string. This can be distinguished from a genuine zero-length sub- string by inspecting the appropriate offset in ovector, which is nega- tive for unset substrings. The two convenience functions pcre_free_substring() and pcre_free_sub- string_list() can be used to free the memory returned by a previous call of pcre_get_substring() or pcre_get_substring_list(), respec- tively. They do nothing more than call the function pointed to by pcre_free, which of course could be called directly from a C program. However, PCRE is used in some situations where it is linked via a spe- cial interface to another programming language that cannot use pcre_free directly; it is for these cases that the functions are pro- vided. EXTRACTING CAPTURED SUBSTRINGS BY NAME int pcre_get_stringnumber(const pcre *code, const char *name); int pcre_copy_named_substring(const pcre *code, const char *subject, int *ovector, int stringcount, const char *stringname, char *buffer, int buffersize); int pcre_get_named_substring(const pcre *code, const char *subject, int *ovector, int stringcount, const char *stringname, const char **stringptr); To extract a substring by name, you first have to find associated num- ber. For example, for this pattern (a+)b(?<xxx>\d+)... the number of the subpattern called "xxx" is 2. If the name is known to be unique (PCRE_DUPNAMES was not set), you can find the number from the name by calling pcre_get_stringnumber(). The first argument is the com- piled pattern, and the second is the name. The yield of the function is the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no subpattern of that name. Given the number, you can extract the substring directly, or use one of the functions described in the previous section. For convenience, there are also two functions that do the whole job. Most of the arguments of pcre_copy_named_substring() and pcre_get_named_substring() are the same as those for the similarly named functions that extract by number. As these are described in the previous section, they are not re-described here. There are just two differences: First, instead of a substring number, a substring name is given. Sec- ond, there is an extra argument, given at the start, which is a pointer to the compiled pattern. This is needed in order to gain access to the name-to-number translation table. These functions call pcre_get_stringnumber(), and if it succeeds, they then call pcre_copy_substring() or pcre_get_substring(), as appropri- ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the behaviour may not be what you want (see the next section). Warning: If the pattern uses the (?| feature to set up multiple subpat- terns with the same number, as described in the section on duplicate subpattern numbers in the pcrepattern page, you cannot use names to distinguish the different subpatterns, because names are not included in the compiled code. The matching process uses only numbers. For this reason, the use of different names for subpatterns of the same number causes an error at compile time. DUPLICATE SUBPATTERN NAMES int pcre_get_stringtable_entries(const pcre *code, const char *name, char **first, char **last); When a pattern is compiled with the PCRE_DUPNAMES option, names for subpatterns are not required to be unique. (Duplicate names are always allowed for subpatterns with the same number, created by using the (?| feature. Indeed, if such subpatterns are named, they are required to use the same names.) Normally, patterns with duplicate names are such that in any one match, only one of the named subpatterns participates. An example is shown in the pcrepattern documentation. When duplicates are present, pcre_copy_named_substring() and pcre_get_named_substring() return the first substring corresponding to the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING (-7) is returned; no data is returned. The pcre_get_stringnumber() function returns one of the numbers that are associated with the name, but it is not defined which it is. If you want to get full details of all captured substrings for a given name, you must use the pcre_get_stringtable_entries() function. The first argument is the compiled pattern, and the second is the name. The third and fourth are pointers to variables which are updated by the function. After it has run, they point to the first and last entries in the name-to-number table for the given name. The function itself returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if there are none. The format of the table is described above in the sec- tion entitled Information about a pattern above. Given all the rele- vant entries for the name, you can extract each of their numbers, and hence the captured data, if any. FINDING ALL POSSIBLE MATCHES The traditional matching function uses a similar algorithm to Perl, which stops when it finds the first match, starting at a given point in the subject. If you want to find all possible matches, or the longest possible match, consider using the alternative matching function (see below) instead. If you cannot use the alternative function, but still need to find all possible matches, you can kludge it up by making use of the callout facility, which is described in the pcrecallout documen- tation. What you have to do is to insert a callout right at the end of the pat- tern. When your callout function is called, extract and save the cur- rent matched substring. Then return 1, which forces pcre_exec() to backtrack and try other alternatives. Ultimately, when it runs out of matches, pcre_exec() will yield PCRE_ERROR_NOMATCH. OBTAINING AN ESTIMATE OF STACK USAGE Matching certain patterns using pcre_exec() can use a lot of process stack, which in certain environments can be rather limited in size. Some users find it helpful to have an estimate of the amount of stack that is used by pcre_exec(), to help them set recursion limits, as described in the pcrestack documentation. The estimate that is output by pcretest when called with the -m and -C options is obtained by call- ing pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its first five arguments. Normally, if its first argument is NULL, pcre_exec() immediately returns the negative error code PCRE_ERROR_NULL, but with this special combination of arguments, it returns instead a negative number whose absolute value is the approximate stack frame size in bytes. (A nega- tive number is used so that it is clear that no match has happened.) The value is approximate because in some cases, recursive calls to pcre_exec() occur when there are one or two additional variables on the stack. If PCRE has been compiled to use the heap instead of the stack for recursion, the value returned is the size of each block that is obtained from the heap. MATCHING A PATTERN: THE ALTERNATIVE FUNCTION int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, const char *subject, int length, int startoffset, int options, int *ovector, int ovecsize, int *workspace, int wscount); The function pcre_dfa_exec() is called to match a subject string against a compiled pattern, using a matching algorithm that scans the subject string just once, and does not backtrack. This has different characteristics to the normal algorithm, and is not compatible with Perl. Some of the features of PCRE patterns are not supported. Never- theless, there are times when this kind of matching can be useful. For a discussion of the two matching algorithms, and a list of features that pcre_dfa_exec() does not support, see the pcrematching documenta- tion. The arguments for the pcre_dfa_exec() function are the same as for pcre_exec(), plus two extras. The ovector argument is used in a differ- ent way, and this is described below. The other common arguments are used in the same way as for pcre_exec(), so their description is not repeated here. The two additional arguments provide workspace for the function. The workspace vector should contain at least 20 elements. It is used for keeping track of multiple paths through the pattern tree. More workspace will be needed for patterns and subjects where there are a lot of potential matches. Here is an example of a simple call to pcre_dfa_exec(): int rc; int ovector[10]; int wspace[20]; rc = pcre_dfa_exec( re, /* result of pcre_compile() */ NULL, /* we didn't study the pattern */ "some string", /* the subject string */ 11, /* the length of the subject string */ 0, /* start at offset 0 in the subject */ 0, /* default options */ ovector, /* vector of integers for substring information */ 10, /* number of elements (NOT size in bytes) */ wspace, /* working space vector */ 20); /* number of elements (NOT size in bytes) */ Option bits for pcre_dfa_exec() The unused bits of the options argument for pcre_dfa_exec() must be zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW- LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF, PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR- TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last four of these are exactly the same as for pcre_exec(), so their description is not repeated here. PCRE_PARTIAL_HARD PCRE_PARTIAL_SOFT These have the same general effect as they do for pcre_exec(), but the details are slightly different. When PCRE_PARTIAL_HARD is set for pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub- ject is reached and there is still at least one matching possibility that requires additional characters. This happens even if some complete matches have also been found. When PCRE_PARTIAL_SOFT is set, the return code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the subject is reached, there have been no complete matches, but there is still at least one matching possibility. The portion of the string that was inspected when the longest partial match was found is set as the first matching string in both cases. There is a more detailed discussion of partial and multi-segment matching, with exam- ples, in the pcrepartial documentation. PCRE_DFA_SHORTEST Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to stop as soon as it has found one match. Because of the way the alterna- tive algorithm works, this is necessarily the shortest possible match at the first possible matching point in the subject string. PCRE_DFA_RESTART When pcre_dfa_exec() returns a partial match, it is possible to call it again, with additional subject characters, and have it continue with the same match. The PCRE_DFA_RESTART option requests this action; when it is set, the workspace and wscount options must reference the same vector as before because data about the match so far is left in them after a partial match. There is more discussion of this facility in the pcrepartial documentation. Successful returns from pcre_dfa_exec() When pcre_dfa_exec() succeeds, it may have matched more than one sub- string in the subject. Note, however, that all the matches from one run of the function start at the same point in the subject. The shorter matches are all initial substrings of the longer matches. For example, if the pattern <.*> is matched against the string This is <something> <something else> <something further> no more the three matched strings are <something> <something> <something else> <something> <something else> <something further> On success, the yield of the function is a number greater than zero, which is the number of matched substrings. The substrings themselves are returned in ovector. Each string uses two elements; the first is the offset to the start, and the second is the offset to the end. In fact, all the strings have the same start offset. (Space could have been saved by giving this only once, but it was decided to retain some compatibility with the way pcre_exec() returns data, even though the meaning of the strings is different.) The strings are returned in reverse order of length; that is, the long- est matching string is given first. If there were too many matches to fit into ovector, the yield of the function is zero, and the vector is filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec() can use the entire ovector for returning matched strings. NOTE: PCRE's "auto-possessification" optimization usually applies to character repeats at the end of a pattern (as well as internally). For example, the pattern "a\d+" is compiled as if it were "a\d++" because there is no point even considering the possibility of backtracking into the repeated digits. For DFA matching, this means that only one possi- ble match is found. If you really do want multiple matches in such cases, either use an ungreedy repeat ("a\d+?") or set the PCRE_NO_AUTO_POSSESS option when compiling. Error returns from pcre_dfa_exec() The pcre_dfa_exec() function returns a negative number when it fails. Many of the errors are the same as for pcre_exec(), and these are described above. There are in addition the following errors that are specific to pcre_dfa_exec(): PCRE_ERROR_DFA_UITEM (-16) This return is given if pcre_dfa_exec() encounters an item in the pat- tern that it does not support, for instance, the use of \C or a back reference. PCRE_ERROR_DFA_UCOND (-17) This return is given if pcre_dfa_exec() encounters a condition item that uses a back reference for the condition, or a test for recursion in a specific group. These are not supported. PCRE_ERROR_DFA_UMLIMIT (-18) This return is given if pcre_dfa_exec() is called with an extra block that contains a setting of the match_limit or match_limit_recursion fields. This is not supported (these fields are meaningless for DFA matching). PCRE_ERROR_DFA_WSSIZE (-19) This return is given if pcre_dfa_exec() runs out of space in the workspace vector. PCRE_ERROR_DFA_RECURSE (-20) When a recursive subpattern is processed, the matching function calls itself recursively, using private vectors for ovector and workspace. This error is given if the output vector is not large enough. This should be extremely rare, as a vector of size 1000 is used. PCRE_ERROR_DFA_BADRESTART (-30) When pcre_dfa_exec() is called with the PCRE_DFA_RESTART option, some plausibility checks are made on the contents of the workspace, which should contain data about the previous partial match. If any of these checks fail, this error is given. SEE ALSO pcre16(3), pcre32(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcre- sample(3), pcrestack(3). AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 18 December 2015 Copyright (c) 1997-2015 University of Cambridge. ------------------------------------------------------------------------------ PCRECALLOUT(3) Library Functions Manual PCRECALLOUT(3) NAME PCRE - Perl-compatible regular expressions SYNOPSIS #include <pcre.h> int (*pcre_callout)(pcre_callout_block *); int (*pcre16_callout)(pcre16_callout_block *); int (*pcre32_callout)(pcre32_callout_block *); DESCRIPTION PCRE provides a feature called "callout", which is a means of temporar- ily passing control to the caller of PCRE in the middle of pattern matching. The caller of PCRE provides an external function by putting its entry point in the global variable pcre_callout (pcre16_callout for the 16-bit library, pcre32_callout for the 32-bit library). By default, this variable contains NULL, which disables all calling out. Within a regular expression, (?C) indicates the points at which the external function is to be called. Different callout points can be identified by putting a number less than 256 after the letter C. The default value is zero. For example, this pattern has two callout points: (?C1)abc(?C2)def If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE automatically inserts callouts, all with number 255, before each item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern A(\d{2}|--) it is processed as if it were (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) Notice that there is a callout before and after each parenthesis and alternation bar. If the pattern contains a conditional group whose con- dition is an assertion, an automatic callout is inserted immediately before the condition. Such a callout may also be inserted explicitly, for example: (?(?C9)(?=a)ab|de) This applies only to assertion conditions (because they are themselves independent groups). Automatic callouts can be used for tracking the progress of pattern matching. The pcretest program has a pattern qualifier (/C) that sets automatic callouts; when it is used, the output indicates how the pat- tern is being matched. This is useful information when you are trying to optimize the performance of a particular pattern. MISSING CALLOUTS You should be aware that, because of optimizations in the way PCRE com- piles and matches patterns, callouts sometimes do not happen exactly as you might expect. At compile time, PCRE "auto-possessifies" repeated items when it knows that what follows cannot be part of the repeat. For example, a+[bc] is compiled as if it were a++[bc]. The pcretest output when this pattern is anchored and then applied with automatic callouts to the string "aaaa" is: --->aaaa +0 ^ ^ +1 ^ a+ +3 ^ ^ [bc] No match This indicates that when matching [bc] fails, there is no backtracking into a+ and therefore the callouts that would be taken for the back- tracks do not occur. You can disable the auto-possessify feature by passing PCRE_NO_AUTO_POSSESS to pcre_compile(), or starting the pattern with (*NO_AUTO_POSSESS). If this is done in pcretest (using the /O qualifier), the output changes to this: --->aaaa +0 ^ ^ +1 ^ a+ +3 ^ ^ [bc] +3 ^ ^ [bc] +3 ^ ^ [bc] +3 ^^ [bc] No match This time, when matching [bc] fails, the matcher backtracks into a+ and tries again, repeatedly, until a+ itself fails. Other optimizations that provide fast "no match" results also affect callouts. For example, if the pattern is ab(?C4)cd PCRE knows that any matching string must contain the letter "d". If the subject string is "abyz", the lack of "d" means that matching doesn't ever start, and the callout is never reached. However, with "abyd", though the result is still no match, the callout is obeyed. If the pattern is studied, PCRE knows the minimum length of a matching string, and will immediately give a "no match" return without actually running a match if the subject is not long enough, or, for unanchored patterns, if it has been scanned far enough. You can disable these optimizations by passing the PCRE_NO_START_OPTI- MIZE option to the matching function, or by starting the pattern with (*NO_START_OPT). This slows down the matching process, but does ensure that callouts such as the example above are obeyed. THE CALLOUT INTERFACE During matching, when PCRE reaches a callout point, the external func- tion defined by pcre_callout or pcre[16|32]_callout is called (if it is set). This applies to both normal and DFA matching. The only argument to the callout function is a pointer to a pcre_callout or pcre[16|32]_callout block. These structures contains the following fields: int version; int callout_number; int *offset_vector; const char *subject; (8-bit version) PCRE_SPTR16 subject; (16-bit version) PCRE_SPTR32 subject; (32-bit version) int subject_length; int start_match; int current_position; int capture_top; int capture_last; void *callout_data; int pattern_position; int next_item_length; const unsigned char *mark; (8-bit version) const PCRE_UCHAR16 *mark; (16-bit version) const PCRE_UCHAR32 *mark; (32-bit version) The version field is an integer containing the version number of the block format. The initial version was 0; the current version is 2. The version number will change again in future if additional fields are added, but the intention is never to remove any of the existing fields. The callout_number field contains the number of the callout, as com- piled into the pattern (that is, the number after ?C for manual call- outs, and 255 for automatically generated callouts). The offset_vector field is a pointer to the vector of offsets that was passed by the caller to the matching function. When pcre_exec() or pcre[16|32]_exec() is used, the contents can be inspected, in order to extract substrings that have been matched so far, in the same way as for extracting substrings after a match has completed. For the DFA matching functions, this field is not useful. The subject and subject_length fields contain copies of the values that were passed to the matching function. The start_match field normally contains the offset within the subject at which the current match attempt started. However, if the escape sequence \K has been encountered, this value is changed to reflect the modified starting point. If the pattern is not anchored, the callout function may be called several times from the same point in the pattern for different starting points in the subject. The current_position field contains the offset within the subject of the current match pointer. When the pcre_exec() or pcre[16|32]_exec() is used, the capture_top field contains one more than the number of the highest numbered cap- tured substring so far. If no substrings have been captured, the value of capture_top is one. This is always the case when the DFA functions are used, because they do not support captured substrings. The capture_last field contains the number of the most recently cap- tured substring. However, when a recursion exits, the value reverts to what it was outside the recursion, as do the values of all captured substrings. If no substrings have been captured, the value of cap- ture_last is -1. This is always the case for the DFA matching func- tions. The callout_data field contains a value that is passed to a matching function specifically so that it can be passed back in callouts. It is passed in the callout_data field of a pcre_extra or pcre[16|32]_extra data structure. If no such data was passed, the value of callout_data in a callout block is NULL. There is a description of the pcre_extra structure in the pcreapi documentation. The pattern_position field is present from version 1 of the callout structure. It contains the offset to the next item to be matched in the pattern string. The next_item_length field is present from version 1 of the callout structure. It contains the length of the next item to be matched in the pattern string. When the callout immediately precedes an alternation bar, a closing parenthesis, or the end of the pattern, the length is zero. When the callout precedes an opening parenthesis, the length is that of the entire subpattern. The pattern_position and next_item_length fields are intended to help in distinguishing between different automatic callouts, which all have the same callout number. However, they are set for all callouts. The mark field is present from version 2 of the callout structure. In callouts from pcre_exec() or pcre[16|32]_exec() it contains a pointer to the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or (*THEN) item in the match, or NULL if no such items have been passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In callouts from the DFA matching func- tions this field always contains NULL. RETURN VALUES The external callout function returns an integer to PCRE. If the value is zero, matching proceeds as normal. If the value is greater than zero, matching fails at the current point, but the testing of other matching possibilities goes ahead, just as if a lookahead assertion had failed. If the value is less than zero, the match is abandoned, the matching function returns the negative value. Negative values should normally be chosen from the set of PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan- dard "no match" failure. The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions; it will never be used by PCRE itself. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 12 November 2013 Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------ PCRECOMPAT(3) Library Functions Manual PCRECOMPAT(3) NAME PCRE - Perl-compatible regular expressions DIFFERENCES BETWEEN PCRE AND PERL This document describes the differences in the ways that PCRE and Perl handle regular expressions. The differences described here are with respect to Perl versions 5.10 and above. 1. PCRE has only a subset of Perl's Unicode support. Details of what it does have are given in the pcreunicode page. 2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do not mean what you might think. For example, (?!a){3} does not assert that the next three characters are not "a". It just asserts that the next character is not "a" three times (in principle: PCRE optimizes this to run the assertion just once). Perl allows repeat quantifiers on other assertions such as \b, but these do not seem to have any use. 3. Capturing subpatterns that occur inside negative lookahead asser- tions are counted, but their entries in the offsets vector are never set. Perl sometimes (but not always) sets its numerical variables from inside negative assertions. 4. Though binary zero characters are supported in the subject string, they are not allowed in a pattern string because it is passed as a nor- mal C string, terminated by zero. The escape sequence \0 can be used in the pattern to represent a binary zero. 5. The following Perl escape sequences are not supported: \l, \u, \L, \U, and \N when followed by a character name or Unicode value. (\N on its own, matching a non-newline character, is supported.) In fact these are implemented by Perl's general string-handling and are not part of its pattern matching engine. If any of these are encountered by PCRE, an error is generated by default. However, if the PCRE_JAVASCRIPT_COM- PAT option is set, \U and \u are interpreted as JavaScript interprets them. 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is built with Unicode character property support. The properties that can be tested with \p and \P are limited to the general category prop- erties such as Lu and Nd, script names such as Greek or Han, and the derived properties Any and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the Perl documentation says "Because Perl hides the need for the user to understand the internal representa- tion of Unicode characters, there is no need to implement the somewhat messy concept of surrogates." 7. PCRE does support the \Q...\E escape for quoting substrings. Charac- ters in between are treated as literals. This is slightly different from Perl in that $ and @ are also handled as literals inside the quotes. In Perl, they cause variable interpolation (but of course PCRE does not have variables). Note the following examples: Pattern PCRE matches Perl matches \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz \Qabc\$xyz\E abc\$xyz abc\$xyz \Qabc\E\$\Qxyz\E abc$xyz abc$xyz The \Q...\E sequence is recognized both inside and outside character classes. 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) constructions. However, there is support for recursive patterns. This is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout" feature allows an external function to be called during pat- tern matching. See the pcrecallout documentation for details. 9. Subpatterns that are called as subroutines (whether or not recur- sively) are always treated as atomic groups in PCRE. This is like Python, but unlike Perl. Captured values that are set outside a sub- routine call can be reference from inside in PCRE, but not in Perl. There is a discussion that explains these differences in more detail in the section on recursion differences from Perl in the pcrepattern page. 10. If any of the backtracking control verbs are used in a subpattern that is called as a subroutine (whether or not recursively), their effect is confined to that subpattern; it does not extend to the sur- rounding pattern. This is not always the case in Perl. In particular, if (*THEN) is present in a group that is called as a subroutine, its action is limited to that group, even if the group does not contain any | characters. Note that such subpatterns are processed as anchored at the point where they are tested. 11. If a pattern contains more than one backtracking control verb, the first one that is backtracked onto acts. For example, in the pattern A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the same as PCRE, but there are examples where it differs. 12. Most backtracking verbs in assertions have their normal actions. They are not confined to the assertion. 13. There are some differences that are concerned with the settings of captured strings when part of a pattern is repeated. For example, matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b". 14. PCRE's handling of duplicate subpattern numbers and duplicate sub- pattern names is not as general as Perl's. This is a consequence of the fact the PCRE works internally just with numbers, using an external ta- ble to translate between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b>B), where the two capturing parentheses have the same number but different names, is not supported, and causes an error at compile time. If it were allowed, it would not be possible to distinguish which parentheses matched, because both names map to cap- turing subpattern number 1. To avoid this confusing situation, an error is given at compile time. 15. Perl recognizes comments in some places that PCRE does not, for example, between the ( and ? at the start of a subpattern. If the /x modifier is set, Perl allows white space between ( and ? (though cur- rent Perls warn that this is deprecated) but PCRE never does, even if the PCRE_EXTENDED option is set. 16. Perl, when in warning mode, gives warnings for character classes such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter- als. PCRE has no warning features, so it gives an error in these cases because they are almost certainly user mistakes. 17. In PCRE, the upper/lower case character properties Lu and Ll are not affected when case-independent matching is specified. For example, \p{Lu} always matches an upper case letter. I think Perl has changed in this respect; in the release at the time of writing (5.16), \p{Lu} and \p{Ll} match all letters, regardless of case, when case independence is specified. 18. PCRE provides some extensions to the Perl regular expression facil- ities. Perl 5.10 includes new features that are not in earlier ver- sions of Perl, some of which (such as named parentheses) have been in PCRE for some time. This list is with respect to Perl 5.10: (a) Although lookbehind assertions in PCRE must match fixed length strings, each alternative branch of a lookbehind assertion can match a different length of string. Perl requires them all to have the same length. (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-character matches only at the very end of the string. (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe- cial meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored. (Perl can be made to issue a warning.) (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti- fiers is inverted, that is, by default they are not greedy, but if fol- lowed by a question mark they are. (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried only at the first matching position in the subject string. (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva- lents. (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF by the PCRE_BSR_ANYCRLF option. (h) The callout facility is PCRE-specific. (i) The partial matching facility is PCRE-specific. (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on different hosts that have the other endianness. However, this does not apply to optimized data created by the just-in-time compiler. (k) The alternative matching functions (pcre_dfa_exec(), pcre16_dfa_exec() and pcre32_dfa_exec(),) match in a different way and are not Perl-compatible. (l) PCRE recognizes some special sequences such as (*CR) at the start of a pattern that set overall options that cannot be changed within the pattern. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 10 November 2013 Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------ PCREPATTERN(3) Library Functions Manual PCREPATTERN(3) NAME PCRE - Perl-compatible regular expressions PCRE REGULAR EXPRESSION DETAILS The syntax and semantics of the regular expressions that are supported by PCRE are described in detail below. There is a quick-reference syn- tax summary in the pcresyntax page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE also supports some alternative regular expression syntax (which does not conflict with the Perl syn- tax) in order to provide some compatibility with regular expressions in Python, .NET, and Oniguruma. Perl's regular expressions are described in its own documentation, and regular expressions in general are covered in a number of books, some of which have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers regular expressions in great detail. This description of PCRE's regular expressions is intended as reference material. This document discusses the patterns that are supported by PCRE when one its main matching functions, pcre_exec() (8-bit) or pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has alternative matching functions, pcre_dfa_exec() and pcre[16|32_dfa_exec(), which match using a different algorithm that is not Perl-compatible. Some of the features discussed below are not available when DFA matching is used. The advantages and disadvantages of the alternative functions, and how they differ from the normal functions, are discussed in the pcrematching page. SPECIAL START-OF-PATTERN ITEMS A number of options that can be passed to pcre_compile() can also be set by special items at the start of a pattern. These are not Perl-com- patible, but are provided to make these options accessible to pattern writers who are not able to change the program that processes the pat- tern. Any number of these items may appear, but they must all be together right at the start of the pattern string, and the letters must be in upper case. UTF support The original operation of PCRE was on strings of one-byte characters. However, there is now also support for UTF-8 strings in the original library, an extra library that supports 16-bit and UTF-16 character strings, and a third library that supports 32-bit and UTF-32 character strings. To use these features, PCRE must be built to include appropri- ate support. When using UTF strings you must either call the compiling function with the PCRE_UTF8, PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of these special sequences: (*UTF8) (*UTF16) (*UTF32) (*UTF) (*UTF) is a generic sequence that can be used with any of the libraries. Starting a pattern with such a sequence is equivalent to setting the relevant option. How setting a UTF mode affects pattern matching is mentioned in several places below. There is also a summary of features in the pcreunicode page. Some applications that allow their users to supply patterns may wish to restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF option is set at compile time, (*UTF) etc. are not allowed, and their appearance causes an error. Unicode property support Another special sequence that may appear at the start of a pattern is (*UCP). This has the same effect as setting the PCRE_UCP option: it causes sequences such as \d and \w to use Unicode properties to deter- mine character types, instead of recognizing only characters with codes less than 128 via a lookup table. Disabling auto-possessification If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making quantifiers possessive when what follows cannot match the repeated item. For example, by default a+b is treated as a++b. For more details, see the pcreapi documentation. Disabling start-up optimizations If a pattern starts with (*NO_START_OPT), it has the same effect as setting the PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables several optimizations for quickly reaching "no match" results. For more details, see the pcreapi documentation. Newline conventions PCRE supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (line- feed) character, the two-character sequence CRLF, any of the three pre- ceding, or any Unicode newline sequence. The pcreapi page has further discussion about newlines, and shows how to set the newline convention in the options arguments for the compiling and matching functions. It is also possible to specify a newline convention by starting a pat- tern string with one of the following five sequences: (*CR) carriage return (*LF) linefeed (*CRLF) carriage return, followed by linefeed (*ANYCRLF) any of the three above (*ANY) all Unicode newline sequences These override the default and the options given to the compiling func- tion. For example, on a Unix system where LF is the default newline sequence, the pattern (*CR)a.b changes the convention to CR. That pattern matches "a\nb" because LF is no longer a newline. If more than one of these settings is present, the last one is used. The newline convention affects where the circumflex and dollar asser- tions are true. It also affects the interpretation of the dot metachar- acter when PCRE_DOTALL is not set, and the behaviour of \N. However, it does not affect what the \R escape sequence matches. By default, this is any Unicode newline sequence, for Perl compatibility. However, this can be changed; see the description of \R in the section entitled "New- line sequences" below. A change of \R setting can be combined with a change of newline convention. Setting match and recursion limits The caller of pcre_exec() can set a limit on the number of times the internal match() function is called and on the maximum depth of recur- sive calls. These facilities are provided to catch runaway matches that are provoked by patterns with huge matching trees (a typical example is a pattern with nested unlimited repeats) and to avoid running out of system stack by too much recursion. When one of these limits is reached, pcre_exec() gives an error return. The limits can also be set by items at the start of the pattern of the form (*LIMIT_MATCH=d) (*LIMIT_RECURSION=d) where d is any number of decimal digits. However, the value of the set- ting must be less than the value set (or defaulted) by the caller of pcre_exec() for it to have any effect. In other words, the pattern writer can lower the limits set by the programmer, but not raise them. If there is more than one setting of one of these limits, the lower value is used. EBCDIC CHARACTER CODES PCRE can be compiled to run in an environment that uses EBCDIC as its character code rather than ASCII or Unicode (typically a mainframe sys- tem). In the sections below, character code values are ASCII or Uni- code; in an EBCDIC environment these characters may have different code values, and there are no code points greater than 255. CHARACTERS AND METACHARACTERS A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for themselves in a pattern, and match the corresponding characters in the subject. As a trivial example, the pattern The quick brown fox matches a portion of a subject string that is identical to itself. When caseless matching is specified (the PCRE_CASELESS option), letters are matched independently of case. In a UTF mode, PCRE always understands the concept of case for characters whose values are less than 128, so caseless matching is always possible. For characters with higher val- ues, the concept of case is supported if PCRE is compiled with Unicode property support, but not otherwise. If you want to use caseless matching for characters 128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with UTF support. The power of regular expressions comes from the ability to include alternatives and repetitions in the pattern. These are encoded in the pattern by the use of metacharacters, which do not stand for themselves but instead are interpreted in some special way. There are two different sets of metacharacters: those that are recog- nized anywhere in the pattern except within square brackets, and those that are recognized within square brackets. Outside square brackets, the metacharacters are as follows: \ general escape character with several uses ^ assert start of string (or line, in multiline mode) $ assert end of string (or line, in multiline mode) . match any character except newline (by default) [ start character class definition | start of alternative branch ( start subpattern ) end subpattern ? extends the meaning of ( also 0 or 1 quantifier also quantifier minimizer * 0 or more quantifier + 1 or more quantifier also "possessive quantifier" { start min/max quantifier Part of a pattern that is in square brackets is called a "character class". In a character class the only metacharacters are: \ general escape character ^ negate the class, but only if the first character - indicates character range [ POSIX character class (only if followed by POSIX syntax) ] terminates the character class The following sections describe the use of each of the metacharacters. BACKSLASH The backslash character has several uses. Firstly, if it is followed by a character that is not a number or a letter, it takes away any special meaning that character may have. This use of backslash as an escape character applies both inside and outside character classes. For example, if you want to match a * character, you write \* in the pattern. This escaping action applies whether or not the following character would otherwise be interpreted as a metacharacter, so it is always safe to precede a non-alphanumeric with backslash to specify that it stands for itself. In particular, if you want to match a back- slash, you write \\. In a UTF mode, only ASCII numbers and letters have any special meaning after a backslash. All other characters (in particular, those whose codepoints are greater than 127) are treated as literals. If a pattern is compiled with the PCRE_EXTENDED option, most white space in the pattern (other than in a character class), and characters between a # outside a character class and the next newline, inclusive, are ignored. An escaping backslash can be used to include a white space or # character as part of the pattern. If you want to remove the special meaning from a sequence of charac- ters, you can do so by putting them between \Q and \E. This is differ- ent from Perl in that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- tion. Note the following examples: Pattern PCRE matches Perl matches \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz \Qabc\$xyz\E abc\$xyz abc\$xyz \Qabc\E\$\Qxyz\E abc$xyz abc$xyz The \Q...\E sequence is recognized both inside and outside character classes. An isolated \E that is not preceded by \Q is ignored. If \Q is not followed by \E later in the pattern, the literal interpretation continues to the end of the pattern (that is, \E is assumed at the end). If the isolated \Q is inside a character class, this causes an error, because the character class is not terminated. Non-printing characters A second use of backslash provides a way of encoding non-printing char- acters in patterns in a visible manner. There is no restriction on the appearance of non-printing characters, apart from the binary zero that terminates a pattern, but when a pattern is being prepared by text editing, it is often easier to use one of the following escape sequences than the binary character it represents. In an ASCII or Uni- code environment, these escapes are as follows: \a alarm, that is, the BEL character (hex 07) \cx "control-x", where x is any ASCII character \e escape (hex 1B) \f form feed (hex 0C) \n linefeed (hex 0A) \r carriage return (hex 0D) \t tab (hex 09) \0dd character with octal code 0dd \ddd character with octal code ddd, or back reference \o{ddd..} character with octal code ddd.. \xhh character with hex code hh \x{hhh..} character with hex code hhh.. (non-JavaScript mode) \uhhhh character with hex code hhhh (JavaScript mode only) The precise effect of \cx on ASCII characters is as follows: if x is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the data item (byte or 16-bit value) following \c has a value greater than 127, a compile-time error occurs. This locks out non-ASCII characters in all modes. When PCRE is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t gener- ate the appropriate EBCDIC code values. The \c escape is processed as specified for Perl in the perlebcdic document. The only characters that are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?. Any other character provokes a compile-time error. The sequence \c@ encodes character code 0; after \c the letters (in either case) encode characters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31 (hex 1B to hex 1F), and \c? becomes either 255 (hex FF) or 95 (hex 5F). Thus, apart from \c?, these escapes generate the same character code values as they do in an ASCII environment, though the meanings of the values mostly differ. For example, \cG always generates code value 7, which is BEL in ASCII but DEL in EBCDIC. The sequence \c? generates DEL (127, hex 7F) in an ASCII environment, but because 127 is not a control character in EBCDIC, Perl makes it generate the APC character. Unfortunately, there are several variants of EBCDIC. In most of them the APC character has the value 255 (hex FF), but in the one Perl calls POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC values, PCRE makes \c? generate 95; otherwise it generates 255. After \0 up to two further octal digits are read. If there are fewer than two digits, just those that are present are used. Thus the sequence \0\x\015 specifies two binary zeros followed by a CR character (code value 13). Make sure you supply two digits after the initial zero if the pattern character that follows is itself an octal digit. The escape \o must be followed by a sequence of octal digits, enclosed in braces. An error occurs if this is not the case. This escape is a recent addition to Perl; it provides way of specifying character code points as octal numbers greater than 0777, and it also allows octal numbers and back references to be unambiguously specified. For greater clarity and unambiguity, it is best to avoid following \ by a digit greater than zero. Instead, use \o{} or \x{} to specify charac- ter numbers, and \g{} to specify back references. The following para- graphs describe the old, ambiguous syntax. The handling of a backslash followed by a digit other than 0 is compli- cated, and Perl has changed in recent releases, causing PCRE also to change. Outside a character class, PCRE reads the digit and any follow- ing digits as a decimal number. If the number is less than 8, or if there have been at least that many previous capturing left parentheses in the expression, the entire sequence is taken as a back reference. A description of how this works is given later, following the discussion of parenthesized subpatterns. Inside a character class, or if the decimal number following \ is greater than 7 and there have not been that many capturing subpatterns, PCRE handles \8 and \9 as the literal characters "8" and "9", and oth- erwise re-reads up to three octal digits following the backslash, using them to generate a data character. Any subsequent digits stand for themselves. For example: \040 is another way of writing an ASCII space \40 is the same, provided there are fewer than 40 previous capturing subpatterns \7 is always a back reference \11 might be a back reference, or another way of writing a tab \011 is always a tab \0113 is a tab followed by the character "3" \113 might be a back reference, otherwise the character with octal code 113 \377 might be a back reference, otherwise the value 255 (decimal) \81 is either a back reference, or the two characters "8" and "1" Note that octal values of 100 or greater that are specified using this syntax must not be introduced by a leading zero, because no more than three octal digits are ever read. By default, after \x that is not followed by {, from zero to two hexa- decimal digits are read (letters can be in upper or lower case). Any number of hexadecimal digits may appear between \x{ and }. If a charac- ter other than a hexadecimal digit appears between \x{ and }, or if there is no terminating }, an error occurs. If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x is as just described only when it is followed by two hexadecimal dig- its. Otherwise, it matches a literal "x" character. In JavaScript mode, support for code points greater than 256 is provided by \u, which must be followed by four hexadecimal digits; otherwise it matches a literal "u" character. Characters whose value is less than 256 can be defined by either of the two syntaxes for \x (or by \u in JavaScript mode). There is no differ- ence in the way they are handled. For example, \xdc is exactly the same as \x{dc} (or \u00dc in JavaScript mode). Constraints on character values Characters that are specified using octal or hexadecimal numbers are limited to certain values, as follows: 8-bit non-UTF mode less than 0x100 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint 16-bit non-UTF mode less than 0x10000 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint 32-bit non-UTF mode less than 0x100000000 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so- called "surrogate" codepoints), and 0xffef. Escape sequences in character classes All the sequences that define a single character value can be used both inside and outside character classes. In addition, inside a character class, \b is interpreted as the backspace character (hex 08). \N is not allowed in a character class. \B, \R, and \X are not special inside a character class. Like other unrecognized escape sequences, they are treated as the literal characters "B", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is set. Outside a character class, these sequences have different meanings. Unsupported escape sequences In Perl, the sequences \l, \L, \u, and \U are recognized by its string handler and used to modify the case of following characters. By default, PCRE does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and \u can be used to define a character by code point, as described in the previous section. Absolute and relative back references The sequence \g followed by an unsigned or a negative number, option- ally enclosed in braces, is an absolute or relative back reference. A named back reference can be coded as \g{name}. Back references are dis- cussed later, following the discussion of parenthesized subpatterns. Absolute and relative subroutine calls For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative syntax for referencing a subpattern as a "subroutine". Details are discussed later. Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a back reference; the latter is a subroutine call. Generic character types Another use of backslash is for specifying generic character types: \d any decimal digit \D any character that is not a decimal digit \h any horizontal white space character \H any character that is not a horizontal white space character \s any white space character \S any character that is not a white space character \v any vertical white space character \V any character that is not a vertical white space character \w any "word" character \W any "non-word" character There is also the single sequence \N, which matches a non-newline char- acter. This is the same as the "." metacharacter when PCRE_DOTALL is not set. Perl also uses \N to match characters by name; PCRE does not support this. Each pair of lower and upper case escape sequences partitions the com- plete set of characters into two disjoint sets. Any given character matches one, and only one, of each pair. The sequences can appear both inside and outside character classes. They each match one character of the appropriate type. If the current matching point is at the end of the subject string, all of them fail, because there is no character to match. For compatibility with Perl, \s did not used to match the VT character (code 11), which made it different from the the POSIX "space" class. However, Perl added VT at release 5.18, and PCRE followed suit at release 8.34. The default \s characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space (32), which are defined as white space in the "C" locale. This list may vary if locale-specific matching is taking place. For example, in some locales the "non-breaking space" character (\xA0) is recognized as white space, and in others the VT character is not. A "word" character is an underscore or any character that is a letter or digit. By default, the definition of letters and digits is con- trolled by PCRE's low-valued character tables, and may vary if locale- specific matching is taking place (see "Locale support" in the pcreapi page). For example, in a French locale such as "fr_FR" in Unix-like systems, or "french" in Windows, some character codes greater than 127 are used for accented letters, and these are then matched by \w. The use of locales with Unicode is discouraged. By default, characters whose code points are greater than 127 never match \d, \s, or \w, and always match \D, \S, and \W, although this may vary for characters in the range 128-255 when locale-specific matching is happening. These escape sequences retain their original meanings from before Unicode support was available, mainly for efficiency rea- sons. If PCRE is compiled with Unicode property support, and the PCRE_UCP option is set, the behaviour is changed so that Unicode prop- erties are used to determine character types, as follows: \d any character that matches \p{Nd} (decimal digit) \s any character that matches \p{Z} or \h or \v \w any character that matches \p{L} or \p{N}, plus underscore The upper case escapes match the inverse sets of characters. Note that \d matches only decimal digits, whereas \w matches any Unicode digit, as well as any Unicode letter, and underscore. Note also that PCRE_UCP affects \b, and \B because they are defined in terms of \w and \W. Matching these sequences is noticeably slower when PCRE_UCP is set. The sequences \h, \H, \v, and \V are features that were added to Perl at release 5.10. In contrast to the other sequences, which match only ASCII characters by default, these always match certain high-valued code points, whether or not PCRE_UCP is set. The horizontal space char- acters are: U+0009 Horizontal tab (HT) U+0020 Space U+00A0 Non-break space U+1680 Ogham space mark U+180E Mongolian vowel separator U+2000 En quad U+2001 Em quad U+2002 En space U+2003 Em space U+2004 Three-per-em space U+2005 Four-per-em space U+2006 Six-per-em space U+2007 Figure space U+2008 Punctuation space U+2009 Thin space U+200A Hair space U+202F Narrow no-break space U+205F Medium mathematical space U+3000 Ideographic space The vertical space characters are: U+000A Linefeed (LF) U+000B Vertical tab (VT) U+000C Form feed (FF) U+000D Carriage return (CR) U+0085 Next line (NEL) U+2028 Line separator U+2029 Paragraph separator In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are relevant. Newline sequences Outside a character class, by default, the escape sequence \R matches any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the following: (?>\r\n|\n|\x0b|\f|\r|\x85) This is an example of an "atomic group", details of which are given below. This particular group matches either the two-character sequence CR followed by LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car- riage return, U+000D), or NEL (next line, U+0085). The two-character sequence is treated as a single unit that cannot be split. In other modes, two additional characters whose codepoints are greater than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- rator, U+2029). Unicode character property support is not needed for these characters to be recognized. It is possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched. (BSR is an abbrevation for "backslash R".) This can be made the default when PCRE is built; if this is the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. It is also possible to specify these settings by starting a pattern string with one of the following sequences: (*BSR_ANYCRLF) CR, LF, or CRLF only (*BSR_UNICODE) any Unicode newline sequence These override the default and the options given to the compiling func- tion, but they can themselves be overridden by options given to a matching function. Note that these special settings, which are not Perl-compatible, are recognized only at the very start of a pattern, and that they must be in upper case. If more than one of them is present, the last one is used. They can be combined with a change of newline convention; for example, a pattern can start with: (*ANY)(*BSR_ANYCRLF) They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or (*UCP) special sequences. Inside a character class, \R is treated as an unrecognized escape sequence, and so matches the letter "R" by default, but causes an error if PCRE_EXTRA is set. Unicode character properties When PCRE is built with Unicode character property support, three addi- tional escape sequences that match characters with specific properties are available. When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing characters whose codepoints are less than 256, but they do work in this mode. The extra escape sequences are: \p{xx} a character with the xx property \P{xx} a character without the xx property \X a Unicode extended grapheme cluster The property names represented by xx above are limited to the Unicode script names, the general category properties, "Any", which matches any character (including newline), and some special PCRE properties (described in the next section). Other Perl properties such as "InMu- sicalSymbols" are not currently supported by PCRE. Note that \P{Any} does not match any characters, so always causes a match failure. Sets of Unicode characters are defined as belonging to certain scripts. A character from one of these sets can be matched using a script name. For example: \p{Greek} \P{Han} Those that are not part of an identified script are lumped together as "Common". The current list of scripts is: Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali, Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Car- ian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cunei- form, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hiero- glyphs, Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip- tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Lin- ear_A, Linear_B, Lisu, Lycian, Lydian, Mahajani, Malayalam, Mandaic, Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician, Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Sha- vian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi. Each character has exactly one Unicode general category property, spec- ified by a two-letter abbreviation. For compatibility with Perl, nega- tion can be specified by including a circumflex between the opening brace and the property name. For example, \p{^Lu} is the same as \P{Lu}. If only one letter is specified with \p or \P, it includes all the gen- eral category properties that start with that letter. In this case, in the absence of negation, the curly brackets in the escape sequence are optional; these two examples have the same effect: \p{L} \pL The following general category property codes are supported: C Other Cc Control Cf Format Cn Unassigned Co Private use Cs Surrogate L Letter Ll Lower case letter Lm Modifier letter Lo Other letter Lt Title case letter Lu Upper case letter M Mark Mc Spacing mark Me Enclosing mark Mn Non-spacing mark N Number Nd Decimal number Nl Letter number No Other number P Punctuation Pc Connector punctuation Pd Dash punctuation Pe Close punctuation Pf Final punctuation Pi Initial punctuation Po Other punctuation Ps Open punctuation S Symbol Sc Currency symbol Sk Modifier symbol Sm Mathematical symbol So Other symbol Z Separator Zl Line separator Zp Paragraph separator Zs Space separator The special property L& is also supported: it matches a character that has the Lu, Ll, or Lt property, in other words, a letter that is not classified as a modifier or "other". The Cs (Surrogate) property applies only to characters in the range U+D800 to U+DFFF. Such characters are not valid in Unicode strings and so cannot be tested by PCRE, unless UTF validity checking has been turned off (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and PCRE_NO_UTF32_CHECK in the pcreapi page). Perl does not support the Cs property. The long synonyms for property names that Perl supports (such as \p{Letter}) are not supported by PCRE, nor is it permitted to prefix any of these properties with "Is". No character that is in the Unicode table has the Cn (unassigned) prop- erty. Instead, this property is assumed for any code point that is not in the Unicode table. Specifying caseless matching does not affect these escape sequences. For example, \p{Lu} always matches only upper case letters. This is different from the behaviour of current versions of Perl. Matching characters by Unicode property is not fast, because PCRE has to do a multistage table lookup in order to find a character's prop- erty. That is why the traditional escape sequences such as \d and \w do not use Unicode properties in PCRE by default, though you can make them do so by setting the PCRE_UCP option or by starting the pattern with (*UCP). Extended grapheme clusters The \X escape matches any number of Unicode characters that form an "extended grapheme cluster", and treats the sequence as an atomic group (see below). Up to and including release 8.31, PCRE matched an ear- lier, simpler definition that was equivalent to (?>\PM\pM*) That is, it matched a character without the "mark" property, followed by zero or more characters with the "mark" property. Characters with the "mark" property are typically non-spacing accents that affect the preceding character. This simple definition was extended in Unicode to include more compli- cated kinds of composite character by giving each character a grapheme breaking property, and creating rules that use these properties to define the boundaries of extended grapheme clusters. In releases of PCRE later than 8.31, \X matches one of these clusters. \X always matches at least one character. Then it decides whether to add additional characters according to the following rules for ending a cluster: 1. End at the end of the subject string. 2. Do not end between CR and LF; otherwise end after any control char- acter. 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters are of five types: L, V, T, LV, and LVT. An L character may be followed by an L, V, LV, or LVT character; an LV or V character may be followed by a V or T character; an LVT or T character may be follwed only by a T character. 4. Do not end before extending characters or spacing marks. Characters with the "mark" property always have the "extend" grapheme breaking property. 5. Do not end after prepend characters. 6. Otherwise, end the cluster. PCRE's additional properties As well as the standard Unicode properties described above, PCRE sup- ports four more that make it possible to convert traditional escape sequences such as \w and \s to use Unicode properties. PCRE uses these non-standard, non-Perl properties internally when PCRE_UCP is set. How- ever, they may also be used explicitly. These properties are: Xan Any alphanumeric character Xps Any POSIX space character Xsp Any Perl space character Xwd Any Perl "word" character Xan matches characters that have either the L (letter) or the N (num- ber) property. Xps matches the characters tab, linefeed, vertical tab, form feed, or carriage return, and any other character that has the Z (separator) property. Xsp is the same as Xps; it used to exclude ver- tical tab, for Perl compatibility, but Perl changed, and so PCRE fol- lowed at release 8.34. Xwd matches the same characters as Xan, plus underscore. There is another non-standard property, Xuc, which matches any charac- ter that can be represented by a Universal Character Name in C++ and other programming languages. These are the characters $, @, ` (grave accent), and all characters with Unicode code points greater than or equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit. Note that the Xuc property does not match these sequences but the char- acters that they represent.) Resetting the match start The escape sequence \K causes any previously matched characters not to be included in the final matched sequence. For example, the pattern: foo\Kbar matches "foobar", but reports that it has matched "bar". This feature is similar to a lookbehind assertion (described below). However, in this case, the part of the subject before the real match does not have to be of fixed length, as lookbehind assertions do. The use of \K does not interfere with the setting of captured substrings. For example, when the pattern (foo)\Kbar matches "foobar", the first substring is still set to "foo". Perl documents that the use of \K within assertions is "not well defined". In PCRE, \K is acted upon when it occurs inside positive assertions, but is ignored in negative assertions. Note that when a pattern such as (?=ab\K) matches, the reported start of the match can be greater than the end of the match. Simple assertions The final use of backslash is for certain simple assertions. An asser- tion specifies a condition that has to be met at a particular point in a match, without consuming any characters from the subject string. The use of subpatterns for more complicated assertions is described below. The backslashed assertions are: \b matches at a word boundary \B matches when not at a word boundary \A matches at the start of the subject \Z matches at the end of the subject also matches before a newline at the end of the subject \z matches only at the end of the subject \G matches at the first matching position in the subject Inside a character class, \b has a different meaning; it matches the backspace character. If any other of these assertions appears in a character class, by default it matches the corresponding literal char- acter (for example, \B matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid escape sequence" error is gener- ated instead. A word boundary is a position in the subject string where the current character and the previous character do not both match \w or \W (i.e. one matches \w and the other matches \W), or the start or end of the string if the first or last character matches \w, respectively. In a UTF mode, the meanings of \w and \W can be changed by setting the PCRE_UCP option. When this is done, it also affects \b and \B. Neither PCRE nor Perl has a separate "start of word" or "end of word" metase- quence. However, whatever follows \b normally determines which it is. For example, the fragment \ba matches "a" at the start of a word. The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described in the next section) in that they only ever match at the very start and end of the subject string, whatever options are set. Thus, they are independent of multiline mode. These three asser- tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the circumflex and dollar metacharacters. However, if the startoffset argument of pcre_exec() is non-zero, indi- cating that matching is to start at a point other than the beginning of the subject, \A can never match. The difference between \Z and \z is that \Z matches before a newline at the end of the string as well as at the very end, whereas \z matches only at the end. The \G assertion is true only when the current matching position is at the start point of the match, as specified by the startoffset argument of pcre_exec(). It differs from \A when the value of startoffset is non-zero. By calling pcre_exec() multiple times with appropriate argu- ments, you can mimic Perl's /g option, and it is in this kind of imple- mentation where \G can be useful. Note, however, that PCRE's interpretation of \G, as the start of the current match, is subtly different from Perl's, which defines it as the end of the previous match. In Perl, these can be different when the previously matched string was empty. Because PCRE does just one match at a time, it cannot reproduce this behaviour. If all the alternatives of a pattern begin with \G, the expression is anchored to the starting match position, and the "anchored" flag is set in the compiled regular expression. CIRCUMFLEX AND DOLLAR The circumflex and dollar metacharacters are zero-width assertions. That is, they test for a particular condition being true without con- suming any characters from the subject string. Outside a character class, in the default matching mode, the circumflex character is an assertion that is true only if the current matching point is at the start of the subject string. If the startoffset argu- ment of pcre_exec() is non-zero, circumflex can never match if the PCRE_MULTILINE option is unset. Inside a character class, circumflex has an entirely different meaning (see below). Circumflex need not be the first character of the pattern if a number of alternatives are involved, but it should be the first thing in each alternative in which it appears if the pattern is ever to match that branch. If all possible alternatives start with a circumflex, that is, if the pattern is constrained to match only at the start of the sub- ject, it is said to be an "anchored" pattern. (There are also other constructs that can cause a pattern to be anchored.) The dollar character is an assertion that is true only if the current matching point is at the end of the subject string, or immediately before a newline at the end of the string (by default). Note, however, that it does not actually match the newline. Dollar need not be the last character of the pattern if a number of alternatives are involved, but it should be the last item in any branch in which it appears. Dol- lar has no special meaning in a character class. The meaning of dollar can be changed so that it matches only at the very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This does not affect the \Z assertion. The meanings of the circumflex and dollar characters are changed if the PCRE_MULTILINE option is set. When this is the case, a circumflex matches immediately after internal newlines as well as at the start of the subject string. It does not match after a newline that ends the string. A dollar matches before any newlines in the string, as well as at the very end, when PCRE_MULTILINE is set. When newline is specified as the two-character sequence CRLF, isolated CR and LF characters do not indicate newlines. For example, the pattern /^abc$/ matches the subject string "def\nabc" (where \n represents a newline) in multiline mode, but not otherwise. Consequently, patterns that are anchored in single line mode because all branches start with ^ are not anchored in multiline mode, and a match for circumflex is possible when the startoffset argument of pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. Note that the sequences \A, \Z, and \z can be used to match the start and end of the subject in both modes, and if all branches of a pattern start with \A it is always anchored, whether or not PCRE_MULTILINE is set. FULL STOP (PERIOD, DOT) AND \N Outside a character class, a dot in the pattern matches any one charac- ter in the subject string except (by default) a character that signi- fies the end of a line. When a line ending is defined as a single character, dot never matches that character; when the two-character sequence CRLF is used, dot does not match CR if it is immediately followed by LF, but otherwise it matches all characters (including isolated CRs and LFs). When any Uni- code line endings are being recognized, dot does not match CR or LF or any of the other line ending characters. The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL option is set, a dot matches any one character, without exception. If the two-character sequence CRLF is present in the subject string, it takes two dots to match it. The handling of dot is entirely independent of the handling of circum- flex and dollar, the only relationship being that they both involve newlines. Dot has no special meaning in a character class. The escape sequence \N behaves like a dot, except that it is not affected by the PCRE_DOTALL option. In other words, it matches any character except one that signifies the end of a line. Perl also uses \N to match characters by name; PCRE does not support this. MATCHING A SINGLE DATA UNIT Outside a character class, the escape sequence \C matches any one data unit, whether or not a UTF mode is set. In the 8-bit library, one data unit is one byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is a 32-bit unit. Unlike a dot, \C always matches line-ending characters. The feature is provided in Perl in order to match individual bytes in UTF-8 mode, but it is unclear how it can use- fully be used. Because \C breaks up characters into individual data units, matching one unit with \C in a UTF mode means that the rest of the string may start with a malformed UTF character. This has undefined results, because PCRE assumes that it is dealing with valid UTF strings (and by default it checks this at the start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or PCRE_NO_UTF32_CHECK option is used). PCRE does not allow \C to appear in lookbehind assertions (described below) in a UTF mode, because this would make it impossible to calcu- late the length of the lookbehind. In general, the \C escape sequence is best avoided. However, one way of using it that avoids the problem of malformed UTF characters is to use a lookahead to check the length of the next character, as in this pat- tern, which could be used with a UTF-8 string (ignore white space and line breaks): (?| (?=[\x00-\x7f])(\C) | (?=[\x80-\x{7ff}])(\C)(\C) | (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) A group that starts with (?| resets the capturing parentheses numbers in each alternative (see "Duplicate Subpattern Numbers" below). The assertions at the start of each branch check the next UTF-8 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The character's individual bytes are then captured by the appropriate num- ber of groups. SQUARE BRACKETS AND CHARACTER CLASSES An opening square bracket introduces a character class, terminated by a closing square bracket. A closing square bracket on its own is not spe- cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square bracket causes a compile-time error. If a closing square bracket is required as a member of the class, it should be the first data character in the class (after an initial circumflex, if present) or escaped with a backslash. A character class matches a single character in the subject. In a UTF mode, the character may be more than one data unit long. A matched character must be in the set of characters defined by the class, unless the first character in the class definition is a circumflex, in which case the subject character must not be in the set defined by the class. If a circumflex is actually required as a member of the class, ensure it is not the first character, or escape it with a backslash. For example, the character class [aeiou] matches any lower case vowel, while [^aeiou] matches any character that is not a lower case vowel. Note that a circumflex is just a convenient notation for specifying the characters that are in the class by enumerating those that are not. A class that starts with a circumflex is not an assertion; it still con- sumes a character from the subject string, and therefore it fails if the current pointer is at the end of the string. In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff) can be included in a class as a literal string of data units, or by using the \x{ escaping mechanism. When caseless matching is set, any letters in a class represent both their upper case and lower case versions, so for example, a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a caseful version would. In a UTF mode, PCRE always understands the concept of case for characters whose values are less than 128, so caseless matching is always possible. For characters with higher values, the concept of case is supported if PCRE is compiled with Unicode property support, but not otherwise. If you want to use caseless matching in a UTF mode for characters 128 and above, you must ensure that PCRE is compiled with Unicode property support as well as with UTF support. Characters that might indicate line breaks are never treated in any special way when matching character classes, whatever line-ending sequence is in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class such as [^a] always matches one of these characters. The minus (hyphen) character can be used to specify a range of charac- ters in a character class. For example, [d-m] matches any letter between d and m, inclusive. If a minus character is required in a class, it must be escaped with a backslash or appear in a position where it cannot be interpreted as indicating a range, typically as the first or last character in the class, or immediately after a range. For example, [b-d-z] matches letters in the range b to d, a hyphen charac- ter, or z. It is not possible to have the literal character "]" as the end charac- ter of a range. A pattern such as [W-]46] is interpreted as a class of two characters ("W" and "-") followed by a literal string "46]", so it would match "W46]" or "-46]". However, if the "]" is escaped with a backslash it is interpreted as the end of range, so [W-\]46] is inter- preted as a class containing a range followed by two other characters. The octal or hexadecimal representation of "]" can also be used to end a range. An error is generated if a POSIX character class (see below) or an escape sequence other than one that defines a single character appears at a point where a range ending character is expected. For example, [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not. Ranges operate in the collating sequence of character values. They can also be used for characters specified numerically, for example [\000-\037]. Ranges can include any characters that are valid for the current mode. If a range that includes letters is used when caseless matching is set, it matches the letters in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character tables for a French locale are in use, [\xc8-\xcb] matches accented E characters in both cases. In UTF modes, PCRE supports the concept of case for characters with values greater than 128 only when it is compiled with Unicode property support. The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, \w, and \W may appear in a character class, and add the characters that they match to the class. For example, [\dABCDEF] matches any hexadeci- mal digit. In UTF modes, the PCRE_UCP option affects the meanings of \d, \s, \w and their upper case partners, just as it does when they appear outside a character class, as described in the section entitled "Generic character types" above. The escape sequence \b has a different meaning inside a character class; it matches the backspace character. The sequences \B, \N, \R, and \X are not special inside a character class. Like any other unrecognized escape sequences, they are treated as the literal characters "B", "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is set. A circumflex can conveniently be used with the upper case character types to specify a more restricted set of characters than the matching lower case type. For example, the class [^\W_] matches any letter or digit, but not underscore, whereas [\w] includes underscore. A positive character class should be read as "something OR something OR ..." and a negative class as "NOT something AND NOT something AND NOT ...". The only metacharacters that are recognized in character classes are backslash, hyphen (only where it can be interpreted as specifying a range), circumflex (only at the start), opening square bracket (only when it can be interpreted as introducing a POSIX class name, or for a special compatibility feature - see the next two sections), and the terminating closing square bracket. However, escaping other non- alphanumeric characters does no harm. POSIX CHARACTER CLASSES Perl supports the POSIX notation for character classes. This uses names enclosed by [: and :] within the enclosing square brackets. PCRE also supports this notation. For example, [01[:alpha:]%] matches "0", "1", any alphabetic character, or "%". The supported class names are: alnum letters and digits alpha letters ascii character codes 0 - 127 blank space or tab only cntrl control characters digit decimal digits (same as \d) graph printing characters, excluding space lower lower case letters print printing characters, including space punct printing characters, excluding letters and digits and space space white space (the same as \s from PCRE 8.34) upper upper case letters word "word" characters (same as \w) xdigit hexadecimal digits The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and space (32). If locale-specific matching is taking place, the list of space characters may be different; there may be fewer or more of them. "Space" used to be different to \s, which did not include VT, for Perl compatibility. However, Perl changed at release 5.18, and PCRE followed at release 8.34. "Space" and \s now match the same set of characters. The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 5.8. Another Perl extension is negation, which is indicated by a ^ character after the colon. For example, [12[:^digit:]] matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not supported, and an error is given if they are encountered. By default, characters with values greater than 128 do not match any of the POSIX character classes. However, if the PCRE_UCP option is passed to pcre_compile(), some of the classes are changed so that Unicode character properties are used. This is achieved by replacing certain POSIX classes by other sequences, as follows: [:alnum:] becomes \p{Xan} [:alpha:] becomes \p{L} [:blank:] becomes \h [:digit:] becomes \p{Nd} [:lower:] becomes \p{Ll} [:space:] becomes \p{Xps} [:upper:] becomes \p{Lu} [:word:] becomes \p{Xwd} Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX classes are handled specially in UCP mode: [:graph:] This matches characters that have glyphs that mark the page when printed. In Unicode property terms, it matches all char- acters with the L, M, N, P, S, or Cf properties, except for: U+061C Arabic Letter Mark U+180E Mongolian Vowel Separator U+2066 - U+2069 Various "isolate"s [:print:] This matches the same characters as [:graph:] plus space characters that are not controls, that is, characters with the Zs property. [:punct:] This matches all characters that have the Unicode P (punctua- tion) property, plus those characters whose code points are less than 128 that have the S (Symbol) property. The other POSIX classes are unchanged, and match only characters with code points less than 128. COMPATIBILITY FEATURE FOR WORD BOUNDARIES In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of word". PCRE treats these items as follows: [[:<:]] is converted to \b(?=\w) [[:>:]] is converted to \b(?<=\w) Only these exact character sequences are recognized. A sequence such as [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is not compatible with Perl. It is provided to help migrations from other environments, and is best not used in any new patterns. Note that \b matches at the start and the end of a word (see "Simple asser- tions" above), and in a Perl-style pattern the preceding or following character normally shows which is wanted, without the need for the assertions that are used above in order to give exactly the POSIX be- haviour. VERTICAL BAR Vertical bar characters are used to separate alternative patterns. For example, the pattern gilbert|sullivan matches either "gilbert" or "sullivan". Any number of alternatives may appear, and an empty alternative is permitted (matching the empty string). The matching process tries each alternative in turn, from left to right, and the first one that succeeds is used. If the alternatives are within a subpattern (defined below), "succeeds" means matching the rest of the main pattern as well as the alternative in the subpattern. INTERNAL OPTION SETTING The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED options (which are Perl-compatible) can be changed from within the pattern by a sequence of Perl option letters enclosed between "(?" and ")". The option letters are i for PCRE_CASELESS m for PCRE_MULTILINE s for PCRE_DOTALL x for PCRE_EXTENDED For example, (?im) sets caseless, multiline matching. It is also possi- ble to unset these options by preceding the letter with a hyphen, and a combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also permitted. If a letter appears both before and after the hyphen, the option is unset. The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be changed in the same way as the Perl-compatible options by using the characters J, U and X respectively. When one of these option changes occurs at top level (that is, not inside subpattern parentheses), the change applies to the remainder of the pattern that follows. An option change within a subpattern (see below for a description of subpatterns) affects only that part of the subpattern that follows it, so (a(?i)b)c matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). By this means, options can be made to have different settings in different parts of the pattern. Any changes made in one alternative do carry on into subsequent branches within the same subpattern. For example, (a(?i)b|c) matches "ab", "aB", "c", and "C", even though when matching "C" the first branch is abandoned before the option setting. This is because the effects of option settings happen at compile time. There would be some very weird behaviour otherwise. Note: There are other PCRE-specific options that can be set by the application when the compiling or matching functions are called. In some cases the pattern can contain special leading sequences such as (*CRLF) to override what the application has set or what has been defaulted. Details are given in the section entitled "Newline sequences" above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading sequences that can be used to set UTF and Unicode prop- erty modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP options, respectively. The (*UTF) sequence is a generic version that can be used with any of the libraries. How- ever, the application can set the PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences. SUBPATTERNS Subpatterns are delimited by parentheses (round brackets), which can be nested. Turning part of a pattern into a subpattern does two things: 1. It localizes a set of alternatives. For example, the pattern cat(aract|erpillar|) matches "cataract", "caterpillar", or "cat". Without the parentheses, it would match "cataract", "erpillar" or an empty string. 2. It sets up the subpattern as a capturing subpattern. This means that, when the whole pattern matches, that portion of the subject string that matched the subpattern is passed back to the caller via the ovector argument of the matching function. (This applies only to the traditional matching functions; the DFA matching functions do not sup- port capturing.) Opening parentheses are counted from left to right (starting from 1) to obtain numbers for the capturing subpatterns. For example, if the string "the red king" is matched against the pattern the ((red|white) (king|queen)) the captured substrings are "red king", "red", and "king", and are num- bered 1, 2, and 3, respectively. The fact that plain parentheses fulfil two functions is not always helpful. There are often times when a grouping subpattern is required without a capturing requirement. If an opening parenthesis is followed by a question mark and a colon, the subpattern does not do any captur- ing, and is not counted when computing the number of any subsequent capturing subpatterns. For example, if the string "the white queen" is matched against the pattern the ((?:red|white) (king|queen)) the captured substrings are "white queen" and "queen", and are numbered 1 and 2. The maximum number of capturing subpatterns is 65535. As a convenient shorthand, if any option settings are required at the start of a non-capturing subpattern, the option letters may appear between the "?" and the ":". Thus the two patterns (?i:saturday|sunday) (?:(?i)saturday|sunday) match exactly the same set of strings. Because alternative branches are tried from left to right, and options are not reset until the end of the subpattern is reached, an option setting in one branch does affect subsequent branches, so the above patterns match "SUNDAY" as well as "Saturday". DUPLICATE SUBPATTERN NUMBERS Perl 5.10 introduced a feature whereby each alternative in a subpattern uses the same numbers for its capturing parentheses. Such a subpattern starts with (?| and is itself a non-capturing subpattern. For example, consider this pattern: (?|(Sat)ur|(Sun))day Because the two alternatives are inside a (?| group, both sets of cap- turing parentheses are numbered one. Thus, when the pattern matches, you can look at captured substring number one, whichever alternative matched. This construct is useful when you want to capture part, but not all, of one of a number of alternatives. Inside a (?| group, paren- theses are numbered as usual, but the number is reset at the start of each branch. The numbers of any capturing parentheses that follow the subpattern start after the highest number used in any branch. The fol- lowing example is taken from the Perl documentation. The numbers under- neath show in which buffer the captured content will be stored. # before ---------------branch-reset----------- after / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x # 1 2 2 3 2 3 4 A back reference to a numbered subpattern uses the most recent value that is set for that number by any subpattern. The following pattern matches "abcabc" or "defdef": /(?|(abc)|(def))\1/ In contrast, a subroutine call to a numbered subpattern always refers to the first one in the pattern with the given number. The following pattern matches "abcabc" or "defabc": /(?|(abc)|(def))(?1)/ If a condition test for a subpattern's having matched refers to a non- unique number, the test is true if any of the subpatterns of that num- ber have matched. An alternative approach to using this "branch reset" feature is to use duplicate named subpatterns, as described in the next section. NAMED SUBPATTERNS Identifying capturing parentheses by number is simple, but it can be very hard to keep track of the numbers in complicated regular expres- sions. Furthermore, if an expression is modified, the numbers may change. To help with this difficulty, PCRE supports the naming of sub- patterns. This feature was not added to Perl until release 5.10. Python had the feature earlier, and PCRE introduced it at release 4.0, using the Python syntax. PCRE now supports both the Perl and the Python syn- tax. Perl allows identically numbered subpatterns to have different names, but PCRE does not. In PCRE, a subpattern can be named in one of three ways: (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing parentheses from other parts of the pattern, such as back references, recursion, and conditions, can be made by name as well as by number. Names consist of up to 32 alphanumeric characters and underscores, but must start with a non-digit. Named capturing parentheses are still allocated numbers as well as names, exactly as if the names were not present. The PCRE API provides function calls for extracting the name- to-number translation table from a compiled pattern. There is also a convenience function for extracting a captured substring by name. By default, a name must be unique within a pattern, but it is possible to relax this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate names are also always permitted for subpatterns with the same number, set up as described in the previous section.) Dupli- cate names can be useful for patterns where only one instance of the named parentheses can match. Suppose you want to match the name of a weekday, either as a 3-letter abbreviation or as the full name, and in both cases you want to extract the abbreviation. This pattern (ignoring the line breaks) does the job: (?<DN>Mon|Fri|Sun)(?:day)?| (?<DN>Tue)(?:sday)?| (?<DN>Wed)(?:nesday)?| (?<DN>Thu)(?:rsday)?| (?<DN>Sat)(?:urday)? There are five capturing substrings, but only one is ever set after a match. (An alternative way of solving this problem is to use a "branch reset" subpattern, as described in the previous section.) The convenience function for extracting the data by name returns the substring for the first (and in this example, the only) subpattern of that name that matched. This saves searching to find which numbered subpattern it was. If you make a back reference to a non-unique named subpattern from elsewhere in the pattern, the subpatterns to which the name refers are checked in the order in which they appear in the overall pattern. The first one that is set is used for the reference. For example, this pat- tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo": (?:(?<n>foo)|(?<n>bar))\k<n> If you make a subroutine call to a non-unique named subpattern, the one that corresponds to the first occurrence of the name is used. In the absence of duplicate numbers (see the previous section) this is the one with the lowest number. If you use a named reference in a condition test (see the section about conditions below), either to check whether a subpattern has matched, or to check for recursion, all subpatterns with the same name are tested. If the condition is true for any one of them, the overall condition is true. This is the same behaviour as testing by number. For further details of the interfaces for handling named subpatterns, see the pcreapi documentation. Warning: You cannot use different names to distinguish between two sub- patterns with the same number because PCRE uses only the numbers when matching. For this reason, an error is given at compile time if differ- ent names are given to subpatterns with the same number. However, you can always give the same name to subpatterns with the same number, even when PCRE_DUPNAMES is not set. REPETITION Repetition is specified by quantifiers, which can follow any of the following items: a literal data character the dot metacharacter the \C escape sequence the \X escape sequence the \R escape sequence an escape such as \d or \pL that matches a single character a character class a back reference (see next section) a parenthesized subpattern (including assertions) a subroutine call to a subpattern (recursive or otherwise) The general repetition quantifier specifies a minimum and maximum num- ber of permitted matches, by giving the two numbers in curly brackets (braces), separated by a comma. The numbers must be less than 65536, and the first must be less than or equal to the second. For example: z{2,4} matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special character. If the second number is omitted, but the comma is present, there is no upper limit; if the second number and the comma are both omitted, the quantifier specifies an exact number of required matches. Thus [aeiou]{3,} matches at least 3 successive vowels, but may match many more, while \d{8} matches exactly 8 digits. An opening curly bracket that appears in a position where a quantifier is not allowed, or one that does not match the syntax of a quantifier, is taken as a literal character. For exam- ple, {,6} is not a quantifier, but a literal string of four characters. In UTF modes, quantifiers apply to characters rather than to individual data units. Thus, for example, \x{100}{2} matches two characters, each of which is represented by a two-byte sequence in a UTF-8 string. Simi- larly, \X{3} matches three Unicode extended grapheme clusters, each of which may be several data units long (and they may be of different lengths). The quantifier {0} is permitted, causing the expression to behave as if the previous item and the quantifier were not present. This may be use- ful for subpatterns that are referenced as subroutines from elsewhere in the pattern (but see also the section entitled "Defining subpatterns for use by reference only" below). Items other than subpatterns that have a {0} quantifier are omitted from the compiled pattern. For convenience, the three most common quantifiers have single-charac- ter abbreviations: * is equivalent to {0,} + is equivalent to {1,} ? is equivalent to {0,1} It is possible to construct infinite loops by following a subpattern that can match no characters with a quantifier that has no upper limit, for example: (a?)* Earlier versions of Perl and PCRE used to give an error at compile time for such patterns. However, because there are cases where this can be useful, such patterns are now accepted, but if any repetition of the subpattern does in fact match no characters, the loop is forcibly bro- ken. By default, the quantifiers are "greedy", that is, they match as much as possible (up to the maximum number of permitted times), without causing the rest of the pattern to fail. The classic example of where this gives problems is in trying to match comments in C programs. These appear between /* and */ and within the comment, individual * and / characters may appear. An attempt to match C comments by applying the pattern /\*.*\*/ to the string /* first comment */ not comment /* second comment */ fails, because it matches the entire string owing to the greediness of the .* item. However, if a quantifier is followed by a question mark, it ceases to be greedy, and instead matches the minimum number of times possible, so the pattern /\*.*?\*/ does the right thing with the C comments. The meaning of the various quantifiers is not otherwise changed, just the preferred number of matches. Do not confuse this use of question mark with its use as a quantifier in its own right. Because it has two uses, it can sometimes appear doubled, as in \d??\d which matches one digit by preference, but can match two if that is the only way the rest of the pattern matches. If the PCRE_UNGREEDY option is set (an option that is not available in Perl), the quantifiers are not greedy by default, but individual ones can be made greedy by following them with a question mark. In other words, it inverts the default behaviour. When a parenthesized subpattern is quantified with a minimum repeat count that is greater than 1 or with a limited maximum, more memory is required for the compiled pattern, in proportion to the size of the minimum or maximum. If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv- alent to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is implicitly anchored, because whatever follows will be tried against every character position in the subject string, so there is no point in retrying the overall match at any position after the first. PCRE normally treats such a pattern as though it were preceded by \A. In cases where it is known that the subject string contains no new- lines, it is worth setting PCRE_DOTALL in order to obtain this opti- mization, or alternatively using ^ to indicate anchoring explicitly. However, there are some cases where the optimization cannot be used. When .* is inside capturing parentheses that are the subject of a back reference elsewhere in the pattern, a match at the start may fail where a later one succeeds. Consider, for example: (.*)abc\1 If the subject is "xyz123abc123" the match point is the fourth charac- ter. For this reason, such a pattern is not implicitly anchored. Another case where implicit anchoring is not applied is when the lead- ing .* is inside an atomic group. Once again, a match at the start may fail where a later one succeeds. Consider this pattern: (?>.*?a)b It matches "ab" in the subject "aab". The use of the backtracking con- trol verbs (*PRUNE) and (*SKIP) also disable this optimization. When a capturing subpattern is repeated, the value captured is the sub- string that matched the final iteration. For example, after (tweedle[dume]{3}\s*)+ has matched "tweedledum tweedledee" the value of the captured substring is "tweedledee". However, if there are nested capturing subpatterns, the corresponding captured values may have been set in previous itera- tions. For example, after /(a|(b))+/ matches "aba" the value of the second captured substring is "b". ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure of what follows normally causes the repeated item to be re-evaluated to see if a different number of repeats allows the rest of the pattern to match. Sometimes it is useful to prevent this, either to change the nature of the match, or to cause it fail earlier than it otherwise might, when the author of the pattern knows there is no point in carrying on. Consider, for example, the pattern \d+foo when applied to the subject line 123456bar After matching all 6 digits and then failing to match "foo", the normal action of the matcher is to try again with only 5 digits matching the \d+ item, and then with 4, and so on, before ultimately failing. "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides the means for specifying that once a subpattern has matched, it is not to be re-evaluated in this way. If we use atomic grouping for the previous example, the matcher gives up immediately on failing to match "foo" the first time. The notation is a kind of special parenthesis, starting with (?> as in this example: (?>\d+)foo This kind of parenthesis "locks up" the part of the pattern it con- tains once it has matched, and a failure further into the pattern is prevented from backtracking into it. Backtracking past it to previous items, however, works as normal. An alternative description is that a subpattern of this type matches the string of characters that an identical standalone pattern would match, if anchored at the current point in the subject string. Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as the above example can be thought of as a maximizing repeat that must swallow everything it can. So, while both \d+ and \d+? are pre- pared to adjust the number of digits they match in order to make the rest of the pattern match, (?>\d+) can only match an entire sequence of digits. Atomic groups in general can of course contain arbitrarily complicated subpatterns, and can be nested. However, when the subpattern for an atomic group is just a single repeated item, as in the example above, a simpler notation, called a "possessive quantifier" can be used. This consists of an additional + character following a quantifier. Using this notation, the previous example can be rewritten as \d++foo Note that a possessive quantifier can be used with an entire group, for example: (abc|xyz){2,3}+ Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY option is ignored. They are a convenient notation for the simpler forms of atomic group. However, there is no difference in the meaning of a possessive quantifier and the equivalent atomic group, though there may be a performance difference; possessive quantifiers should be slightly faster. The possessive quantifier syntax is an extension to the Perl 5.8 syn- tax. Jeffrey Friedl originated the idea (and the name) in the first edition of his book. Mike McCloskey liked it, so implemented it when he built Sun's Java package, and PCRE copied it from there. It ultimately found its way into Perl at release 5.10. PCRE has an optimization that automatically "possessifies" certain sim- ple pattern constructs. For example, the sequence A+B is treated as A++B because there is no point in backtracking into a sequence of A's when B must follow. When a pattern contains an unlimited repeat inside a subpattern that can itself be repeated an unlimited number of times, the use of an atomic group is the only way to avoid some failing matches taking a very long time indeed. The pattern (\D+|<\d+>)*[!?] matches an unlimited number of substrings that either consist of non- digits, or digits enclosed in <>, followed by either ! or ?. When it matches, it runs quickly. However, if it is applied to aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa it takes a long time before reporting failure. This is because the string can be divided between the internal \D+ repeat and the external * repeat in a large number of ways, and all have to be tried. (The example uses [!?] rather than a single character at the end, because both PCRE and Perl have an optimization that allows for fast failure when a single character is used. They remember the last single charac- ter that is required for a match, and fail early if it is not present in the string.) If the pattern is changed so that it uses an atomic group, like this: ((?>\D+)|<\d+>)*[!?] sequences of non-digits cannot be broken, and failure happens quickly. BACK REFERENCES Outside a character class, a backslash followed by a digit greater than 0 (and possibly further digits) is a back reference to a capturing sub- pattern earlier (that is, to its left) in the pattern, provided there have been that many previous capturing left parentheses. However, if the decimal number following the backslash is less than 10, it is always taken as a back reference, and causes an error only if there are not that many capturing left parentheses in the entire pat- tern. In other words, the parentheses that are referenced need not be to the left of the reference for numbers less than 10. A "forward back reference" of this type can make sense when a repetition is involved and the subpattern to the right has participated in an earlier itera- tion. It is not possible to have a numerical "forward back reference" to a subpattern whose number is 10 or more using this syntax because a sequence such as \50 is interpreted as a character defined in octal. See the subsection entitled "Non-printing characters" above for further details of the handling of digits following a backslash. There is no such problem when named parentheses are used. A back reference to any subpattern is possible using named parentheses (see below). Another way of avoiding the ambiguity inherent in the use of digits following a backslash is to use the \g escape sequence. This escape must be followed by an unsigned number or a negative number, optionally enclosed in braces. These examples are all identical: (ring), \1 (ring), \g1 (ring), \g{1} An unsigned number specifies an absolute reference without the ambigu- ity that is present in the older syntax. It is also useful when literal digits follow the reference. A negative number is a relative reference. Consider this example: (abc(def)ghi)\g{-1} The sequence \g{-1} is a reference to the most recently started captur- ing subpattern before \g, that is, is it equivalent to \2 in this exam- ple. Similarly, \g{-2} would be equivalent to \1. The use of relative references can be helpful in long patterns, and also in patterns that are created by joining together fragments that contain references within themselves. A back reference matches whatever actually matched the capturing sub- pattern in the current subject string, rather than anything matching the subpattern itself (see "Subpatterns as subroutines" below for a way of doing that). So the pattern (sens|respons)e and \1ibility matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If caseful matching is in force at the time of the back reference, the case of letters is relevant. For exam- ple, ((?i)rah)\s+\1 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original capturing subpattern is matched caselessly. There are several different ways of writing back references to named subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified back reference syntax, in which \g can be used for both numeric and named references, is also supported. We could rewrite the above example in any of the following ways: (?<p1>(?i)rah)\s+\k<p1> (?'p1'(?i)rah)\s+\k{p1} (?P<p1>(?i)rah)\s+(?P=p1) (?<p1>(?i)rah)\s+\g{p1} A subpattern that is referenced by name may appear in the pattern before or after the reference. There may be more than one back reference to the same subpattern. If a subpattern has not actually been used in a particular match, any back references to it always fail by default. For example, the pattern (a|(bc))\2 always fails if it starts to match "a" rather than "bc". However, if the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer- ence to an unset value matches an empty string. Because there may be many capturing parentheses in a pattern, all dig- its following a backslash are taken as part of a potential back refer- ence number. If the pattern continues with a digit character, some delimiter must be used to terminate the back reference. If the PCRE_EXTENDED option is set, this can be white space. Otherwise, the \g{ syntax or an empty comment (see "Comments" below) can be used. Recursive back references A back reference that occurs inside the parentheses to which it refers fails when the subpattern is first used, so, for example, (a\1) never matches. However, such references can be useful inside repeated sub- patterns. For example, the pattern (a|b\1)+ matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- ation of the subpattern, the back reference matches the character string corresponding to the previous iteration. In order for this to work, the pattern must be such that the first iteration does not need to match the back reference. This can be done using alternation, as in the example above, or by a quantifier with a minimum of zero. Back references of this type cause the group that they reference to be treated as an atomic group. Once the whole group has been matched, a subsequent matching failure cannot cause backtracking into the middle of the group. ASSERTIONS An assertion is a test on the characters following or preceding the current matching point that does not actually consume any characters. The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described above. More complicated assertions are coded as subpatterns. There are two kinds: those that look ahead of the current position in the subject string, and those that look behind it. An assertion subpattern is matched in the normal way, except that it does not cause the current matching position to be changed. Assertion subpatterns are not capturing subpatterns. If such an asser- tion contains capturing subpatterns within it, these are counted for the purposes of numbering the capturing subpatterns in the whole pat- tern. However, substring capturing is carried out only for positive assertions. (Perl sometimes, but not always, does do capturing in nega- tive assertions.) WARNING: If a positive assertion containing one or more capturing sub- patterns succeeds, but failure to match later in the pattern causes backtracking over this assertion, the captures within the assertion are reset only if no higher numbered captures are already set. This is, unfortunately, a fundamental limitation of the current implementation, and as PCRE1 is now in maintenance-only status, it is unlikely ever to change. For compatibility with Perl, assertion subpatterns may be repeated; though it makes no sense to assert the same thing several times, the side effect of capturing parentheses may occasionally be useful. In practice, there only three cases: (1) If the quantifier is {0}, the assertion is never obeyed during matching. However, it may contain internal capturing parenthesized groups that are called from elsewhere via the subroutine mechanism. (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it were {0,1}. At run time, the rest of the pattern match is tried with and without the assertion, the order depending on the greed- iness of the quantifier. (3) If the minimum repetition is greater than zero, the quantifier is ignored. The assertion is obeyed just once when encountered during matching. Lookahead assertions Lookahead assertions start with (?= for positive assertions and (?! for negative assertions. For example, \w+(?=;) matches a word followed by a semicolon, but does not include the semi- colon in the match, and foo(?!bar) matches any occurrence of "foo" that is not followed by "bar". Note that the apparently similar pattern (?!foo)bar does not find an occurrence of "bar" that is preceded by something other than "foo"; it finds any occurrence of "bar" whatsoever, because the assertion (?!foo) is always true when the next three characters are "bar". A lookbehind assertion is needed to achieve the other effect. If you want to force a matching failure at some point in a pattern, the most convenient way to do it is with (?!) because an empty string always matches, so an assertion that requires there not to be an empty string must always fail. The backtracking control verb (*FAIL) or (*F) is a synonym for (?!). Lookbehind assertions Lookbehind assertions start with (?<= for positive assertions and (?<! for negative assertions. For example, (?<!foo)bar does find an occurrence of "bar" that is not preceded by "foo". The contents of a lookbehind assertion are restricted such that all the strings it matches must have a fixed length. However, if there are sev- eral top-level alternatives, they do not all have to have the same fixed length. Thus (?<=bullock|donkey) is permitted, but (?<!dogs?|cats?) causes an error at compile time. Branches that match different length strings are permitted only at the top level of a lookbehind assertion. This is an extension compared with Perl, which requires all branches to match the same length of string. An assertion such as (?<=ab(c|de)) is not permitted, because its single top-level branch can match two different lengths, but it is acceptable to PCRE if rewritten to use two top-level branches: (?<=abc|abde) In some cases, the escape sequence \K (see above) can be used instead of a lookbehind assertion to get round the fixed-length restriction. The implementation of lookbehind assertions is, for each alternative, to temporarily move the current position back by the fixed length and then try to match. If there are insufficient characters before the cur- rent position, the assertion fails. In a UTF mode, PCRE does not allow the \C escape (which matches a sin- gle data unit even in a UTF mode) to appear in lookbehind assertions, because it makes it impossible to calculate the length of the lookbe- hind. The \X and \R escapes, which can match different numbers of data units, are also not permitted. "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long as the subpattern matches a fixed-length string. Recursion, however, is not supported. Possessive quantifiers can be used in conjunction with lookbehind assertions to specify efficient matching of fixed-length strings at the end of subject strings. Consider a simple pattern such as abcd$ when applied to a long string that does not match. Because matching proceeds from left to right, PCRE will look for each "a" in the subject and then see if what follows matches the rest of the pattern. If the pattern is specified as ^.*abcd$ the initial .* matches the entire string at first, but when this fails (because there is no following "a"), it backtracks to match all but the last character, then all but the last two characters, and so on. Once again the search for "a" covers the entire string, from right to left, so we are no better off. However, if the pattern is written as ^.*+(?<=abcd) there can be no backtracking for the .*+ item; it can match only the entire string. The subsequent lookbehind assertion does a single test on the last four characters. If it fails, the match fails immediately. For long strings, this approach makes a significant difference to the processing time. Using multiple assertions Several assertions (of any sort) may occur in succession. For example, (?<=\d{3})(?<!999)foo matches "foo" preceded by three digits that are not "999". Notice that each of the assertions is applied independently at the same point in the subject string. First there is a check that the previous three characters are all digits, and then there is a check that the same three characters are not "999". This pattern does not match "foo" pre- ceded by six characters, the first of which are digits and the last three of which are not "999". For example, it doesn't match "123abc- foo". A pattern to do that is (?<=\d{3}...)(?<!999)foo This time the first assertion looks at the preceding six characters, checking that the first three are digits, and then the second assertion checks that the preceding three characters are not "999". Assertions can be nested in any combination. For example, (?<=(?<!foo)bar)baz matches an occurrence of "baz" that is preceded by "bar" which in turn is not preceded by "foo", while (?<=\d{3}(?!999)...)foo is another pattern that matches "foo" preceded by three digits and any three characters that are not "999". CONDITIONAL SUBPATTERNS It is possible to cause the matching process to obey a subpattern con- ditionally or to choose between two alternative subpatterns, depending on the result of an assertion, or whether a specific capturing subpat- tern has already been matched. The two possible forms of conditional subpattern are: (?(condition)yes-pattern) (?(condition)yes-pattern|no-pattern) If the condition is satisfied, the yes-pattern is used; otherwise the no-pattern (if present) is used. If there are more than two alterna- tives in the subpattern, a compile-time error occurs. Each of the two alternatives may itself contain nested subpatterns of any form, includ- ing conditional subpatterns; the restriction to two alternatives applies only at the level of the condition. This pattern fragment is an example where the alternatives are complex: (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) There are four kinds of condition: references to subpatterns, refer- ences to recursion, a pseudo-condition called DEFINE, and assertions. Checking for a used subpattern by number If the text between the parentheses consists of a sequence of digits, the condition is true if a capturing subpattern of that number has pre- viously matched. If there is more than one capturing subpattern with the same number (see the earlier section about duplicate subpattern numbers), the condition is true if any of them have matched. An alter- native notation is to precede the digits with a plus or minus sign. In this case, the subpattern number is relative rather than absolute. The most recently opened parentheses can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside loops it can also make sense to refer to subsequent groups. The next parentheses to be opened can be referenced as (?(+1), and so on. (The value zero in any of these forms is not used; it provokes a compile-time error.) Consider the following pattern, which contains non-significant white space to make it more readable (assume the PCRE_EXTENDED option) and to divide it into three parts for ease of discussion: ( \( )? [^()]+ (?(1) \) ) The first part matches an optional opening parenthesis, and if that character is present, sets it as the first captured substring. The sec- ond part matches one or more characters that are not parentheses. The third part is a conditional subpattern that tests whether or not the first set of parentheses matched. If they did, that is, if subject started with an opening parenthesis, the condition is true, and so the yes-pattern is executed and a closing parenthesis is required. Other- wise, since no-pattern is not present, the subpattern matches nothing. In other words, this pattern matches a sequence of non-parentheses, optionally enclosed in parentheses. If you were embedding this pattern in a larger one, you could use a relative reference: ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... This makes the fragment independent of the parentheses in the larger pattern. Checking for a used subpattern by name Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used subpattern by name. For compatibility with earlier versions of PCRE, which had this facility before Perl, the syntax (?(name)...) is also recognized. Rewriting the above example to use a named subpattern gives this: (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) If the name used in a condition of this kind is a duplicate, the test is applied to all subpatterns of the same name, and is true if any one of them has matched. Checking for pattern recursion If the condition is the string (R), and there is no subpattern with the name R, the condition is true if a recursive call to the whole pattern or any subpattern has been made. If digits or a name preceded by amper- sand follow the letter R, for example: (?(R3)...) or (?(R&name)...) the condition is true if the most recent recursion is into a subpattern whose number or name is given. This condition does not check the entire recursion stack. If the name used in a condition of this kind is a duplicate, the test is applied to all subpatterns of the same name, and is true if any one of them is the most recent recursion. At "top level", all these recursion test conditions are false. The syntax for recursive patterns is described below. Defining subpatterns for use by reference only If the condition is the string (DEFINE), and there is no subpattern with the name DEFINE, the condition is always false. In this case, there may be only one alternative in the subpattern. It is always skipped if control reaches this point in the pattern; the idea of DEFINE is that it can be used to define subroutines that can be refer- enced from elsewhere. (The use of subroutines is described below.) For example, a pattern to match an IPv4 address such as "192.168.23.245" could be written like this (ignore white space and line breaks): (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) \b (?&byte) (\.(?&byte)){3} \b The first part of the pattern is a DEFINE group inside which a another group named "byte" is defined. This matches an individual component of an IPv4 address (a number less than 256). When matching takes place, this part of the pattern is skipped because DEFINE acts like a false condition. The rest of the pattern uses references to the named group to match the four dot-separated components of an IPv4 address, insist- ing on a word boundary at each end. Assertion conditions If the condition is not in any of the above formats, it must be an assertion. This may be a positive or negative lookahead or lookbehind assertion. Consider this pattern, again containing non-significant white space, and with the two alternatives on the second line: (?(?=[^a-z]*[a-z]) \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) The condition is a positive lookahead assertion that matches an optional sequence of non-letters followed by a letter. In other words, it tests for the presence of at least one letter in the subject. If a letter is found, the subject is matched against the first alternative; otherwise it is matched against the second. This pattern matches strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. COMMENTS There are two ways of including comments in patterns that are processed by PCRE. In both cases, the start of the comment must not be in a char- acter class, nor in the middle of any other sequence of related charac- ters such as (?: or a subpattern name or number. The characters that make up a comment play no part in the pattern matching. The sequence (?# marks the start of a comment that continues up to the next closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED option is set, an unescaped # character also introduces a comment, which in this case continues to immediately after the next newline character or character sequence in the pattern. Which charac- ters are interpreted as newlines is controlled by the options passed to a compiling function or by a special sequence at the start of the pat- tern, as described in the section entitled "Newline conventions" above. Note that the end of this type of comment is a literal newline sequence in the pattern; escape sequences that happen to represent a newline do not count. For example, consider this pattern when PCRE_EXTENDED is set, and the default newline convention is in force: abc #comment \n still comment On encountering the # character, pcre_compile() skips along, looking for a newline in the pattern. The sequence \n is still literal at this stage, so it does not terminate the comment. Only an actual character with the code value 0x0a (the default newline) does so. RECURSIVE PATTERNS Consider the problem of matching a string in parentheses, allowing for unlimited nested parentheses. Without the use of recursion, the best that can be done is to use a pattern that matches up to some fixed depth of nesting. It is not possible to handle an arbitrary nesting depth. For some time, Perl has provided a facility that allows regular expres- sions to recurse (amongst other things). It does this by interpolating Perl code in the expression at run time, and the code can refer to the expression itself. A Perl pattern using code interpolation to solve the parentheses problem can be created like this: $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; The (?p{...}) item interpolates Perl code at run time, and in this case refers recursively to the pattern in which it appears. Obviously, PCRE cannot support the interpolation of Perl code. Instead, it supports special syntax for recursion of the entire pattern, and also for individual subpattern recursion. After its introduction in PCRE and Python, this kind of recursion was subsequently introduced into Perl at release 5.10. A special item that consists of (? followed by a number greater than zero and a closing parenthesis is a recursive subroutine call of the subpattern of the given number, provided that it occurs inside that subpattern. (If not, it is a non-recursive subroutine call, which is described in the next section.) The special item (?R) or (?0) is a recursive call of the entire regular expression. This PCRE pattern solves the nested parentheses problem (assume the PCRE_EXTENDED option is set so that white space is ignored): \( ( [^()]++ | (?R) )* \) First it matches an opening parenthesis. Then it matches any number of substrings which can either be a sequence of non-parentheses, or a recursive match of the pattern itself (that is, a correctly parenthe- sized substring). Finally there is a closing parenthesis. Note the use of a possessive quantifier to avoid backtracking into sequences of non- parentheses. If this were part of a larger pattern, you would not want to recurse the entire pattern, so instead you could use this: ( \( ( [^()]++ | (?1) )* \) ) We have put the pattern into parentheses, and caused the recursion to refer to them instead of the whole pattern. In a larger pattern, keeping track of parenthesis numbers can be tricky. This is made easier by the use of relative references. Instead of (?1) in the pattern above you can write (?-2) to refer to the second most recently opened parentheses preceding the recursion. In other words, a negative number counts capturing parentheses leftwards from the point at which it is encountered. It is also possible to refer to subsequently opened parentheses, by writing references such as (?+2). However, these cannot be recursive because the reference is not inside the parentheses that are refer- enced. They are always non-recursive subroutine calls, as described in the next section. An alternative approach is to use named parentheses instead. The Perl syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We could rewrite the above example as follows: (?<pn> \( ( [^()]++ | (?&pn) )* \) ) If there is more than one subpattern with the same name, the earliest one is used. This particular example pattern that we have been looking at contains nested unlimited repeats, and so the use of a possessive quantifier for matching strings of non-parentheses is important when applying the pat- tern to strings that do not match. For example, when this pattern is applied to (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() it yields "no match" quickly. However, if a possessive quantifier is not used, the match runs for a very long time indeed because there are so many different ways the + and * repeats can carve up the subject, and all have to be tested before failure can be reported. At the end of a match, the values of capturing parentheses are those from the outermost level. If you want to obtain intermediate values, a callout function can be used (see below and the pcrecallout documenta- tion). If the pattern above is matched against (ab(cd)ef) the value for the inner capturing parentheses (numbered 2) is "ef", which is the last value taken on at the top level. If a capturing sub- pattern is not matched at the top level, its final captured value is unset, even if it was (temporarily) set at a deeper level during the matching process. If there are more than 15 capturing parentheses in a pattern, PCRE has to obtain extra memory to store data during a recursion, which it does by using pcre_malloc, freeing it via pcre_free afterwards. If no memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. Do not confuse the (?R) item with the condition (R), which tests for recursion. Consider this pattern, which matches text in angle brack- ets, allowing for arbitrary nesting. Only digits are allowed in nested brackets (that is, when recursing), whereas any characters are permit- ted at the outer level. < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > In this pattern, (?(R) is the start of a conditional subpattern, with two different alternatives for the recursive and non-recursive cases. The (?R) item is the actual recursive call. Differences in recursion processing between PCRE and Perl Recursion processing in PCRE differs from Perl in two important ways. In PCRE (like Python, but unlike Perl), a recursive subpattern call is always treated as an atomic group. That is, once it has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure. This can be illustrated by the following pattern, which purports to match a palin- dromic string that contains an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"): ^(.|(.)(?1)\2)$ The idea is that it either matches a single character, or two identical characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE it does not if the pattern is longer than three characters. Consider the subject string "abcba": At the top level, the first character is matched, but as it is not at the end of the string, the first alternative fails; the second alterna- tive is taken and the recursion kicks in. The recursive call to subpat- tern 1 successfully matches the next character ("b"). (Note that the beginning and end of line tests are not part of the recursion). Back at the top level, the next character ("c") is compared with what subpattern 2 matched, which was "a". This fails. Because the recursion is treated as an atomic group, there are now no backtracking points, and so the entire match fails. (Perl is able, at this point, to re- enter the recursion and try the second alternative.) However, if the pattern is written with the alternatives in the other order, things are different: ^((.)(?1)\2|.)$ This time, the recursing alternative is tried first, and continues to recurse until it runs out of characters, at which point the recursion fails. But this time we do have another alternative to try at the higher level. That is the big difference: in the previous case the remaining alternative is at a deeper recursion level, which PCRE cannot use. To change the pattern so that it matches all palindromic strings, not just those with an odd number of characters, it is tempting to change the pattern to this: ^((.)(?1)\2|.?)$ Again, this works in Perl, but not in PCRE, and for the same reason. When a deeper recursion has matched a single character, it cannot be entered again in order to match an empty string. The solution is to separate the two cases, and write out the odd and even cases as alter- natives at the higher level: ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) If you want to match typical palindromic phrases, the pattern has to ignore all non-word characters, which can be done like this: ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ If run with the PCRE_CASELESS option, this pattern matches phrases such as "A man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note the use of the possessive quantifier *+ to avoid backtrack- ing into sequences of non-word characters. Without this, PCRE takes a great deal longer (ten times or more) to match typical phrases, and Perl takes so long that you think it has gone into a loop. WARNING: The palindrome-matching patterns above work only if the sub- ject string does not start with a palindrome that is shorter than the entire string. For example, although "abcba" is correctly matched, if the subject is "ababa", PCRE finds the palindrome "aba" at the start, then fails at top level because the end of the string does not follow. Once again, it cannot jump back into the recursion to try other alter- natives, so the entire match fails. The second way in which PCRE and Perl differ in their recursion pro- cessing is in the handling of captured values. In Perl, when a subpat- tern is called recursively or as a subpattern (see the next section), it has no access to any values that were captured outside the recur- sion, whereas in PCRE these values can be referenced. Consider this pattern: ^(.)(\1|a(?2)) In PCRE, this pattern matches "bab". The first capturing parentheses match "b", then in the second group, when the back reference \1 fails to match "b", the second alternative matches "a" and then recurses. In the recursion, \1 does now match "b" and so the whole match succeeds. In Perl, the pattern fails to match because inside the recursive call \1 cannot access the externally set value. SUBPATTERNS AS SUBROUTINES If the syntax for a recursive subpattern call (either by number or by name) is used outside the parentheses to which it refers, it operates like a subroutine in a programming language. The called subpattern may be defined before or after the reference. A numbered reference can be absolute or relative, as in these examples: (...(absolute)...)...(?2)... (...(relative)...)...(?-1)... (...(?+1)...(relative)... An earlier example pointed out that the pattern (sens|respons)e and \1ibility matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If instead the pattern (sens|respons)e and (?1)ibility is used, it does match "sense and responsibility" as well as the other two strings. Another example is given in the discussion of DEFINE above. All subroutine calls, whether recursive or not, are always treated as atomic groups. That is, once a subroutine has matched some of the sub- ject string, it is never re-entered, even if it contains untried alter- natives and there is a subsequent matching failure. Any capturing parentheses that are set during the subroutine call revert to their previous values afterwards. Processing options such as case-independence are fixed when a subpat- tern is defined, so if it is used as a subroutine, such options cannot be changed for different calls. For example, consider this pattern: (abc)(?i:(?-1)) It matches "abcabc". It does not match "abcABC" because the change of processing option does not affect the called subpattern. ONIGURUMA SUBROUTINE SYNTAX For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative syntax for referencing a subpattern as a subroutine, possibly recursively. Here are two of the examples used above, rewrit- ten using this syntax: (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) (sens|respons)e and \g'1'ibility PCRE supports an extension to Oniguruma: if a number is preceded by a plus or a minus sign it is taken as a relative reference. For example: (abc)(?i:\g<-1>) Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a back reference; the latter is a subroutine call. CALLOUTS Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl code to be obeyed in the middle of matching a regular expression. This makes it possible, amongst other things, to extract different sub- strings that match the same pair of parentheses when there is a repeti- tion. PCRE provides a similar feature, but of course it cannot obey arbitrary Perl code. The feature is called "callout". The caller of PCRE provides an external function by putting its entry point in the global variable pcre_callout (8-bit library) or pcre[16|32]_callout (16-bit or 32-bit library). By default, this variable contains NULL, which disables all calling out. Within a regular expression, (?C) indicates the points at which the external function is to be called. If you want to identify different callout points, you can put a number less than 256 after the letter C. The default value is zero. For example, this pattern has two callout points: (?C1)abc(?C2)def If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, call- outs are automatically installed before each item in the pattern. They are all numbered 255. If there is a conditional group in the pattern whose condition is an assertion, an additional callout is inserted just before the condition. An explicit callout may also be set at this posi- tion, as in this example: (?(?C9)(?=a)abc|def) Note that this applies only to assertion conditions, not to other types of condition. During matching, when PCRE reaches a callout point, the external func- tion is called. It is provided with the number of the callout, the position in the pattern, and, optionally, one item of data originally supplied by the caller of the matching function. The callout function may cause matching to proceed, to backtrack, or to fail altogether. By default, PCRE implements a number of optimizations at compile time and matching time, and one side-effect is that sometimes callouts are skipped. If you need all possible callouts to happen, you need to set options that disable the relevant optimizations. More details, and a complete description of the interface to the callout function, are given in the pcrecallout documentation. BACKTRACKING CONTROL Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which are still described in the Perl documentation as "experimental and subject to change or removal in a future version of Perl". It goes on to say: "Their usage in production code should be noted to avoid problems during upgrades." The same remarks apply to the PCRE features described in this section. The new verbs make use of what was previously invalid syntax: an open- ing parenthesis followed by an asterisk. They are generally of the form (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving differently depending on whether or not a name is present. A name is any sequence of characters that does not include a closing parenthesis. The maximum length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit libraries. If the name is empty, that is, if the closing parenthesis immediately follows the colon, the effect is as if the colon were not there. Any number of these verbs may occur in a pattern. Since these verbs are specifically related to backtracking, most of them can be used only when the pattern is to be matched using one of the traditional matching functions, because these use a backtracking algorithm. With the exception of (*FAIL), which behaves like a failing negative assertion, the backtracking control verbs cause an error if encountered by a DFA matching function. The behaviour of these verbs in repeated groups, assertions, and in subpatterns called as subroutines (whether or not recursively) is docu- mented below. Optimizations that affect backtracking verbs PCRE contains some optimizations that are used to speed up matching by running some checks at the start of each match attempt. For example, it may know the minimum length of matching subject, or that a particular character must be present. When one of these optimizations bypasses the running of a match, any included backtracking verbs will not, of course, be processed. You can suppress the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com- pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT). There is more discussion of this option in the section entitled "Option bits for pcre_exec()" in the pcreapi documentation. Experiments with Perl suggest that it too has similar optimizations, sometimes leading to anomalous results. Verbs that act immediately The following verbs act as soon as they are encountered. They may not be followed by a name. (*ACCEPT) This verb causes the match to end successfully, skipping the remainder of the pattern. However, when it is inside a subpattern that is called as a subroutine, only that subpattern is ended successfully. Matching then continues at the outer level. If (*ACCEPT) in triggered in a posi- tive assertion, the assertion succeeds; in a negative assertion, the assertion fails. If (*ACCEPT) is inside capturing parentheses, the data so far is cap- tured. For example: A((?:A|B(*ACCEPT)|C)D) This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- tured by the outer parentheses. (*FAIL) or (*F) This verb causes a matching failure, forcing backtracking to occur. It is equivalent to (?!) but easier to read. The Perl documentation notes that it is probably useful only when combined with (?{}) or (??{}). Those are, of course, Perl features that are not present in PCRE. The nearest equivalent is the callout feature, as for example in this pat- tern: a+(?C)(*FAIL) A match with the string "aaaa" always fails, but the callout is taken before each backtrack happens (in this example, 10 times). Recording which path was taken There is one verb whose main purpose is to track how a match was arrived at, though it also has a secondary use in conjunction with advancing the match starting point (see (*SKIP) below). (*MARK:NAME) or (*:NAME) A name is always required with this verb. There may be as many instances of (*MARK) as you like in a pattern, and their names do not have to be unique. When a match succeeds, the name of the last-encountered (*MARK:NAME), (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the caller as described in the section entitled "Extra data for pcre_exec()" in the pcreapi documentation. Here is an example of pcretest output, where the /K modifier requests the retrieval and out- putting of (*MARK) data: re> /X(*MARK:A)Y|X(*MARK:B)Z/K data> XY 0: XY MK: A XZ 0: XZ MK: B The (*MARK) name is tagged with "MK:" in this output, and in this exam- ple it indicates which of the two alternatives matched. This is a more efficient way of obtaining this information than putting each alterna- tive in its own capturing parentheses. If a verb with a name is encountered in a positive assertion that is true, the name is recorded and passed back if it is the last-encoun- tered. This does not happen for negative assertions or failing positive assertions. After a partial match or a failed match, the last encountered name in the entire match process is returned. For example: re> /X(*MARK:A)Y|X(*MARK:B)Z/K data> XP No match, mark = B Note that in this unanchored example the mark is retained from the match attempt that started at the letter "X" in the subject. Subsequent match attempts starting at "P" and then with an empty string do not get as far as the (*MARK) item, but nevertheless do not reset it. If you are interested in (*MARK) values after failed matches, you should probably set the PCRE_NO_START_OPTIMIZE option (see above) to ensure that the match is always attempted. Verbs that act after backtracking The following verbs do nothing when they are encountered. Matching con- tinues with what follows, but if there is no subsequent match, causing a backtrack to the verb, a failure is forced. That is, backtracking cannot pass to the left of the verb. However, when one of these verbs appears inside an atomic group or an assertion that is true, its effect is confined to that group, because once the group has been matched, there is never any backtracking into it. In this situation, backtrack- ing can "jump back" to the left of the entire atomic group or asser- tion. (Remember also, as stated above, that this localization also applies in subroutine calls.) These verbs differ in exactly what kind of failure occurs when back- tracking reaches them. The behaviour described below is what happens when the verb is not in a subroutine or an assertion. Subsequent sec- tions cover these special cases. (*COMMIT) This verb, which may not be followed by a name, causes the whole match to fail outright if there is a later matching failure that causes back- tracking to reach it. Even if the pattern is unanchored, no further attempts to find a match by advancing the starting point take place. If (*COMMIT) is the only backtracking verb that is encountered, once it has been passed pcre_exec() is committed to finding a match at the cur- rent starting point, or not at all. For example: a+(*COMMIT)b This matches "xxaab" but not "aacaab". It can be thought of as a kind of dynamic anchor, or "I've started, so I must finish." The name of the most recently passed (*MARK) in the path is passed back when (*COMMIT) forces a match failure. If there is more than one backtracking verb in a pattern, a different one that follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a match does not always guarantee that a match must be at this starting point. Note that (*COMMIT) at the start of a pattern is not the same as an anchor, unless PCRE's start-of-match optimizations are turned off, as shown in this output from pcretest: re> /(*COMMIT)abc/ data> xyzabc 0: abc data> xyzabc\Y No match For this pattern, PCRE knows that any match must start with "a", so the optimization skips along the subject to "a" before applying the pattern to the first set of data. The match attempt then succeeds. In the sec- ond set of data, the escape sequence \Y is interpreted by the pcretest program. It causes the PCRE_NO_START_OPTIMIZE option to be set when pcre_exec() is called. This disables the optimization that skips along to the first character. The pattern is now applied starting at "x", and so the (*COMMIT) causes the match to fail without trying any other starting points. (*PRUNE) or (*PRUNE:NAME) This verb causes the match to fail at the current starting position in the subject if there is a later matching failure that causes backtrack- ing to reach it. If the pattern is unanchored, the normal "bumpalong" advance to the next starting character then happens. Backtracking can occur as usual to the left of (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but if there is no match to the right, backtracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alternative to an atomic group or possessive quan- tifier, but there are some uses of (*PRUNE) that cannot be expressed in any other way. In an anchored pattern (*PRUNE) has the same effect as (*COMMIT). The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is remembered for passing back to the caller. However, (*SKIP:NAME) searches only for names set with (*MARK). (*SKIP) This verb, when given without a name, is like (*PRUNE), except that if the pattern is unanchored, the "bumpalong" advance is not to the next character, but to the position in the subject where (*SKIP) was encoun- tered. (*SKIP) signifies that whatever text was matched leading up to it cannot be part of a successful match. Consider: a+(*SKIP)b If the subject is "aaaac...", after the first match attempt fails (starting at the first character in the string), the starting point skips on to start the next attempt at "c". Note that a possessive quan- tifer does not have the same effect as this example; although it would suppress backtracking during the first match attempt, the second attempt would start at the second character instead of skipping on to "c". (*SKIP:NAME) When (*SKIP) has an associated name, its behaviour is modified. When it is triggered, the previous path through the pattern is searched for the most recent (*MARK) that has the same name. If one is found, the "bumpalong" advance is to the subject position that corresponds to that (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a matching name is found, the (*SKIP) is ignored. Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME). (*THEN) or (*THEN:NAME) This verb causes a skip to the next innermost alternative when back- tracking reaches it. That is, it cancels any further backtracking within the current alternative. Its name comes from the observation that it can be used for a pattern-based if-then-else block: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... If the COND1 pattern matches, FOO is tried (and possibly further items after the end of the group if FOO succeeds); on failure, the matcher skips to the second alternative and tries COND2, without backtracking into COND1. If that succeeds and BAR fails, COND3 is tried. If subse- quently BAZ fails, there are no more alternatives, so there is a back- track to whatever came before the entire group. If (*THEN) is not inside an alternation, it acts like (*PRUNE). The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is remembered for passing back to the caller. However, (*SKIP:NAME) searches only for names set with (*MARK). A subpattern that does not contain a | character is just a part of the enclosing alternative; it is not a nested alternation with only one alternative. The effect of (*THEN) extends beyond such a subpattern to the enclosing alternative. Consider this pattern, where A, B, etc. are complex pattern fragments that do not contain any | characters at this level: A (B(*THEN)C) | D If A and B are matched, but there is a failure in C, matching does not backtrack into A; instead it moves to the next alternative, that is, D. However, if the subpattern containing (*THEN) is given an alternative, it behaves differently: A (B(*THEN)C | (*FAIL)) | D The effect of (*THEN) is now confined to the inner subpattern. After a failure in C, matching moves to (*FAIL), which causes the whole subpat- tern to fail because there are no more alternatives to try. In this case, matching does now backtrack into A. Note that a conditional subpattern is not considered as having two alternatives, because only one is ever used. In other words, the | character in a conditional subpattern has a different meaning. Ignoring white space, consider: ^.*? (?(?=a) a | b(*THEN)c ) If the subject is "ba", this pattern does not match. Because .*? is ungreedy, it initially matches zero characters. The condition (?=a) then fails, the character "b" is matched, but "c" is not. At this point, matching does not backtrack to .*? as might perhaps be expected from the presence of the | character. The conditional subpattern is part of the single alternative that comprises the whole pattern, and so the match fails. (If there was a backtrack into .*?, allowing it to match "b", the match would succeed.) The verbs just described provide four different "strengths" of control when subsequent matching fails. (*THEN) is the weakest, carrying on the match at the next alternative. (*PRUNE) comes next, failing the match at the current starting position, but allowing an advance to the next character (for an unanchored pattern). (*SKIP) is similar, except that the advance may be more than one character. (*COMMIT) is the strongest, causing the entire match to fail. More than one backtracking verb If more than one backtracking verb is present in a pattern, the one that is backtracked onto first acts. For example, consider this pat- tern, where A, B, etc. are complex pattern fragments: (A(*COMMIT)B(*THEN)C|ABD) If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes the next alternative (ABD) to be tried. This behaviour is consistent, but is not always the same as Perl's. It means that if two or more backtracking verbs appear in succession, all the the last of them has no effect. Consider this example: ...(*COMMIT)(*PRUNE)... If there is a matching failure to the right, backtracking onto (*PRUNE) causes it to be triggered, and its action is taken. There can never be a backtrack onto (*COMMIT). Backtracking verbs in repeated groups PCRE differs from Perl in its handling of backtracking verbs in repeated groups. For example, consider: /(a(*COMMIT)b)+ac/ If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in the second repeat of the group acts. Backtracking verbs in assertions (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack. (*ACCEPT) in a positive assertion causes the assertion to succeed with- out any further processing. In a negative assertion, (*ACCEPT) causes the assertion to fail without any further processing. The other backtracking verbs are not treated specially if they appear in a positive assertion. In particular, (*THEN) skips to the next alternative in the innermost enclosing group that has alternations, whether or not this is within the assertion. Negative assertions are, however, different, in order to ensure that changing a positive assertion into a negative assertion changes its result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a neg- ative assertion to be true, without considering any further alternative branches in the assertion. Backtracking into (*THEN) causes it to skip to the next enclosing alternative within the assertion (the normal be- haviour), but if the assertion does not have such an alternative, (*THEN) behaves like (*PRUNE). Backtracking verbs in subroutines These behaviours occur whether or not the subpattern is called recur- sively. Perl's treatment of subroutines is different in some cases. (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces an immediate backtrack. (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to succeed without any further processing. Matching then contin- ues after the subroutine call. (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause the subroutine match to fail. (*THEN) skips to the next alternative in the innermost enclosing group within the subpattern that has alternatives. If there is no such group within the subpattern, (*THEN) causes the subroutine match to fail. SEE ALSO pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3), pcre16(3), pcre32(3). AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 23 October 2016 Copyright (c) 1997-2016 University of Cambridge. ------------------------------------------------------------------------------ PCRESYNTAX(3) Library Functions Manual PCRESYNTAX(3) NAME PCRE - Perl-compatible regular expressions PCRE REGULAR EXPRESSION SYNTAX SUMMARY The full syntax and semantics of the regular expressions that are sup- ported by PCRE are described in the pcrepattern documentation. This document contains a quick-reference summary of the syntax. QUOTING \x where x is non-alphanumeric is a literal x \Q...\E treat enclosed characters as literal CHARACTERS \a alarm, that is, the BEL character (hex 07) \cx "control-x", where x is any ASCII character \e escape (hex 1B) \f form feed (hex 0C) \n newline (hex 0A) \r carriage return (hex 0D) \t tab (hex 09) \0dd character with octal code 0dd \ddd character with octal code ddd, or backreference \o{ddd..} character with octal code ddd.. \xhh character with hex code hh \x{hhh..} character with hex code hhh.. Note that \0dd is always an octal code, and that \8 and \9 are the lit- eral characters "8" and "9". CHARACTER TYPES . any character except newline; in dotall mode, any character whatsoever \C one data unit, even in UTF mode (best avoided) \d a decimal digit \D a character that is not a decimal digit \h a horizontal white space character \H a character that is not a horizontal white space character \N a character that is not a newline \p{xx} a character with the xx property \P{xx} a character without the xx property \R a newline sequence \s a white space character \S a character that is not a white space character \v a vertical white space character \V a character that is not a vertical white space character \w a "word" character \W a "non-word" character \X a Unicode extended grapheme cluster By default, \d, \s, and \w match only ASCII characters, even in UTF-8 mode or in the 16- bit and 32-bit libraries. However, if locale-spe- cific matching is happening, \s and \w may also match characters with code points in the range 128-255. If the PCRE_UCP option is set, the behaviour of these escape sequences is changed to use Unicode proper- ties and they match many more characters. GENERAL CATEGORY PROPERTIES FOR \p and \P C Other Cc Control Cf Format Cn Unassigned Co Private use Cs Surrogate L Letter Ll Lower case letter Lm Modifier letter Lo Other letter Lt Title case letter Lu Upper case letter L& Ll, Lu, or Lt M Mark Mc Spacing mark Me Enclosing mark Mn Non-spacing mark N Number Nd Decimal number Nl Letter number No Other number P Punctuation Pc Connector punctuation Pd Dash punctuation Pe Close punctuation Pf Final punctuation Pi Initial punctuation Po Other punctuation Ps Open punctuation S Symbol Sc Currency symbol Sk Modifier symbol Sm Mathematical symbol So Other symbol Z Separator Zl Line separator Zp Paragraph separator Zs Space separator PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P Xan Alphanumeric: union of properties L and N Xps POSIX space: property Z or tab, NL, VT, FF, CR Xsp Perl space: property Z or tab, NL, VT, FF, CR Xuc Univerally-named character: one that can be represented by a Universal Character Name Xwd Perl word: property Xan or underscore Perl and POSIX space are now the same. Perl added VT to its space char- acter set at release 5.18 and PCRE changed at release 8.34. SCRIPT NAMES FOR \p AND \P Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali, Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Car- ian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cunei- form, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hiero- glyphs, Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip- tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Lin- ear_A, Linear_B, Lisu, Lycian, Lydian, Mahajani, Malayalam, Mandaic, Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician, Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Sha- vian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi. CHARACTER CLASSES [...] positive character class [^...] negative character class [x-y] range (can be used for hex characters) [[:xxx:]] positive POSIX named set [[:^xxx:]] negative POSIX named set alnum alphanumeric alpha alphabetic ascii 0-127 blank space or tab cntrl control character digit decimal digit graph printing, excluding space lower lower case letter print printing, including space punct printing, excluding alphanumeric space white space upper upper case letter word same as \w xdigit hexadecimal digit In PCRE, POSIX character set names recognize only ASCII characters by default, but some of them use Unicode properties if PCRE_UCP is set. You can use \Q...\E inside a character class. QUANTIFIERS ? 0 or 1, greedy ?+ 0 or 1, possessive ?? 0 or 1, lazy * 0 or more, greedy *+ 0 or more, possessive *? 0 or more, lazy + 1 or more, greedy ++ 1 or more, possessive +? 1 or more, lazy {n} exactly n {n,m} at least n, no more than m, greedy {n,m}+ at least n, no more than m, possessive {n,m}? at least n, no more than m, lazy {n,} n or more, greedy {n,}+ n or more, possessive {n,}? n or more, lazy ANCHORS AND SIMPLE ASSERTIONS \b word boundary \B not a word boundary ^ start of subject also after internal newline in multiline mode \A start of subject $ end of subject also before newline at end of subject also before internal newline in multiline mode \Z end of subject also before newline at end of subject \z end of subject \G first matching position in subject MATCH POINT RESET \K reset start of match \K is honoured in positive assertions, but ignored in negative ones. ALTERNATION expr|expr|expr... CAPTURING (...) capturing group (?<name>...) named capturing group (Perl) (?'name'...) named capturing group (Perl) (?P<name>...) named capturing group (Python) (?:...) non-capturing group (?|...) non-capturing group; reset group numbers for capturing groups in each alternative ATOMIC GROUPS (?>...) atomic, non-capturing group COMMENT (?#....) comment (not nestable) OPTION SETTING (?i) caseless (?J) allow duplicate names (?m) multiline (?s) single line (dotall) (?U) default ungreedy (lazy) (?x) extended (ignore white space) (?-...) unset option(s) The following are recognized only at the very start of a pattern or after one of the newline or \R options with similar syntax. More than one of them may appear. (*LIMIT_MATCH=d) set the match limit to d (decimal number) (*LIMIT_RECURSION=d) set the recursion limit to d (decimal number) (*NO_AUTO_POSSESS) no auto-possessification (PCRE_NO_AUTO_POSSESS) (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE) (*UTF8) set UTF-8 mode: 8-bit library (PCRE_UTF8) (*UTF16) set UTF-16 mode: 16-bit library (PCRE_UTF16) (*UTF32) set UTF-32 mode: 32-bit library (PCRE_UTF32) (*UTF) set appropriate UTF mode for the library in use (*UCP) set PCRE_UCP (use Unicode properties for \d etc) Note that LIMIT_MATCH and LIMIT_RECURSION can only reduce the value of the limits set by the caller of pcre_exec(), not increase them. NEWLINE CONVENTION These are recognized only at the very start of the pattern or after option settings with a similar syntax. (*CR) carriage return only (*LF) linefeed only (*CRLF) carriage return followed by linefeed (*ANYCRLF) all three of the above (*ANY) any Unicode newline sequence WHAT \R MATCHES These are recognized only at the very start of the pattern or after option setting with a similar syntax. (*BSR_ANYCRLF) CR, LF, or CRLF (*BSR_UNICODE) any Unicode newline sequence LOOKAHEAD AND LOOKBEHIND ASSERTIONS (?=...) positive look ahead (?!...) negative look ahead (?<=...) positive look behind (?<!...) negative look behind Each top-level branch of a look behind must be of a fixed length. BACKREFERENCES \n reference by number (can be ambiguous) \gn reference by number \g{n} reference by number \g{-n} relative reference by number \k<name> reference by name (Perl) \k'name' reference by name (Perl) \g{name} reference by name (Perl) \k{name} reference by name (.NET) (?P=name) reference by name (Python) SUBROUTINE REFERENCES (POSSIBLY RECURSIVE) (?R) recurse whole pattern (?n) call subpattern by absolute number (?+n) call subpattern by relative number (?-n) call subpattern by relative number (?&name) call subpattern by name (Perl) (?P>name) call subpattern by name (Python) \g<name> call subpattern by name (Oniguruma) \g'name' call subpattern by name (Oniguruma) \g<n> call subpattern by absolute number (Oniguruma) \g'n' call subpattern by absolute number (Oniguruma) \g<+n> call subpattern by relative number (PCRE extension) \g'+n' call subpattern by relative number (PCRE extension) \g<-n> call subpattern by relative number (PCRE extension) \g'-n' call subpattern by relative number (PCRE extension) CONDITIONAL PATTERNS (?(condition)yes-pattern) (?(condition)yes-pattern|no-pattern) (?(n)... absolute reference condition (?(+n)... relative reference condition (?(-n)... relative reference condition (?(<name>)... named reference condition (Perl) (?('name')... named reference condition (Perl) (?(name)... named reference condition (PCRE) (?(R)... overall recursion condition (?(Rn)... specific group recursion condition (?(R&name)... specific recursion condition (?(DEFINE)... define subpattern for reference (?(assert)... assertion condition BACKTRACKING CONTROL The following act immediately they are reached: (*ACCEPT) force successful match (*FAIL) force backtrack; synonym (*F) (*MARK:NAME) set name to be passed back; synonym (*:NAME) The following act only when a subsequent match failure causes a back- track to reach them. They all force a match failure, but they differ in what happens afterwards. Those that advance the start-of-match point do so only if the pattern is not anchored. (*COMMIT) overall failure, no advance of starting point (*PRUNE) advance to next starting character (*PRUNE:NAME) equivalent to (*MARK:NAME)(*PRUNE) (*SKIP) advance to current matching position (*SKIP:NAME) advance to position corresponding to an earlier (*MARK:NAME); if not found, the (*SKIP) is ignored (*THEN) local failure, backtrack to next alternation (*THEN:NAME) equivalent to (*MARK:NAME)(*THEN) CALLOUTS (?C) callout (?Cn) callout with data n SEE ALSO pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3). AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 08 January 2014 Copyright (c) 1997-2014 University of Cambridge. ------------------------------------------------------------------------------ PCREUNICODE(3) Library Functions Manual PCREUNICODE(3) NAME PCRE - Perl-compatible regular expressions UTF-8, UTF-16, UTF-32, AND UNICODE PROPERTY SUPPORT As well as UTF-8 support, PCRE also supports UTF-16 (from release 8.30) and UTF-32 (from release 8.32), by means of two additional libraries. They can be built as well as, or instead of, the 8-bit library. UTF-8 SUPPORT In order process UTF-8 strings, you must build PCRE's 8-bit library with UTF support, and, in addition, you must call pcre_compile() with the PCRE_UTF8 option flag, or the pattern must start with the sequence (*UTF8) or (*UTF). When either of these is the case, both the pattern and any subject strings that are matched against it are treated as UTF-8 strings instead of strings of individual 1-byte characters. UTF-16 AND UTF-32 SUPPORT In order process UTF-16 or UTF-32 strings, you must build PCRE's 16-bit or 32-bit library with UTF support, and, in addition, you must call pcre16_compile() or pcre32_compile() with the PCRE_UTF16 or PCRE_UTF32 option flag, as appropriate. Alternatively, the pattern must start with the sequence (*UTF16), (*UTF32), as appropriate, or (*UTF), which can be used with either library. When UTF mode is set, both the pattern and any subject strings that are matched against it are treated as UTF-16 or UTF-32 strings instead of strings of individual 16-bit or 32-bit characters. UTF SUPPORT OVERHEAD If you compile PCRE with UTF support, but do not use it at run time, the library will be a bit bigger, but the additional run time overhead is limited to testing the PCRE_UTF[8|16|32] flag occasionally, so should not be very big. UNICODE PROPERTY SUPPORT If PCRE is built with Unicode character property support (which implies UTF support), the escape sequences \p{..}, \P{..}, and \X can be used. The available properties that can be tested are limited to the general category properties such as Lu for an upper case letter or Nd for a decimal number, the Unicode script names such as Arabic or Han, and the derived properties Any and L&. Full lists is given in the pcrepattern and pcresyntax documentation. Only the short names for properties are supported. For example, \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported. Furthermore, in Perl, many properties may optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE does not support this. Validity of UTF-8 strings When you set the PCRE_UTF8 flag, the byte strings passed as patterns and subjects are (by default) checked for validity on entry to the rel- evant functions. The entire string is checked before any other process- ing takes place. From release 7.3 of PCRE, the check is according the rules of RFC 3629, which are themselves derived from the Unicode speci- fication. Earlier releases of PCRE followed the rules of RFC 2279, which allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current check allows only values in the range U+0 to U+10FFFF, exclud- ing the surrogate area. (From release 8.33 the so-called "non-charac- ter" code points are no longer excluded because Unicode corrigendum #9 makes it clear that they should not be.) Characters in the "Surrogate Area" of Unicode are reserved for use by UTF-16, where they are used in pairs to encode codepoints with values greater than 0xFFFF. The code points that are encoded by UTF-16 pairs are available independently in the UTF-8 and UTF-32 encodings. (In other words, the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8 and UTF-32.) If an invalid UTF-8 string is passed to PCRE, an error return is given. At compile time, the only additional information is the offset to the first byte of the failing character. The run-time functions pcre_exec() and pcre_dfa_exec() also pass back this information, as well as a more detailed reason code if the caller has provided memory in which to do this. In some situations, you may already know that your strings are valid, and therefore want to skip these checks in order to improve perfor- mance, for example in the case of a long subject string that is being scanned repeatedly. If you set the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that the pattern or subject it is given (respectively) contains only valid UTF-8 codes. In this case, it does not diagnose an invalid UTF-8 string. Note that passing PCRE_NO_UTF8_CHECK to pcre_compile() just disables the check for the pattern; it does not also apply to subject strings. If you want to disable the check for a subject string you must pass this option to pcre_exec() or pcre_dfa_exec(). If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, the result is undefined and your program may crash. Validity of UTF-16 strings When you set the PCRE_UTF16 flag, the strings of 16-bit data units that are passed as patterns and subjects are (by default) checked for valid- ity on entry to the relevant functions. Values other than those in the surrogate range U+D800 to U+DFFF are independent code points. Values in the surrogate range must be used in pairs in the correct manner. If an invalid UTF-16 string is passed to PCRE, an error return is given. At compile time, the only additional information is the offset to the first data unit of the failing character. The run-time functions pcre16_exec() and pcre16_dfa_exec() also pass back this information, as well as a more detailed reason code if the caller has provided memory in which to do this. In some situations, you may already know that your strings are valid, and therefore want to skip these checks in order to improve perfor- mance. If you set the PCRE_NO_UTF16_CHECK flag at compile time or at run time, PCRE assumes that the pattern or subject it is given (respec- tively) contains only valid UTF-16 sequences. In this case, it does not diagnose an invalid UTF-16 string. However, if an invalid string is passed, the result is undefined. Validity of UTF-32 strings When you set the PCRE_UTF32 flag, the strings of 32-bit data units that are passed as patterns and subjects are (by default) checked for valid- ity on entry to the relevant functions. This check allows only values in the range U+0 to U+10FFFF, excluding the surrogate area U+D800 to U+DFFF. If an invalid UTF-32 string is passed to PCRE, an error return is given. At compile time, the only additional information is the offset to the first data unit of the failing character. The run-time functions pcre32_exec() and pcre32_dfa_exec() also pass back this information, as well as a more detailed reason code if the caller has provided memory in which to do this. In some situations, you may already know that your strings are valid, and therefore want to skip these checks in order to improve perfor- mance. If you set the PCRE_NO_UTF32_CHECK flag at compile time or at run time, PCRE assumes that the pattern or subject it is given (respec- tively) contains only valid UTF-32 sequences. In this case, it does not diagnose an invalid UTF-32 string. However, if an invalid string is passed, the result is undefined. General comments about UTF modes 1. Codepoints less than 256 can be specified in patterns by either braced or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3). Larger values have to use braced sequences. 2. Octal numbers up to \777 are recognized, and in UTF-8 mode they match two-byte characters for values greater than \177. 3. Repeat quantifiers apply to complete UTF characters, not to individ- ual data units, for example: \x{100}{3}. 4. The dot metacharacter matches one UTF character instead of a single data unit. 5. The escape sequence \C can be used to match a single byte in UTF-8 mode, or a single 16-bit data unit in UTF-16 mode, or a single 32-bit data unit in UTF-32 mode, but its use can lead to some strange effects because it breaks up multi-unit characters (see the description of \C in the pcrepattern documentation). The use of \C is not supported in the alternative matching function pcre[16|32]_dfa_exec(), nor is it supported in UTF mode by the JIT optimization of pcre[16|32]_exec(). If JIT optimization is requested for a UTF pattern that contains \C, it will not succeed, and so the matching will be carried out by the normal interpretive function. 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test characters of any code value, but, by default, the characters that PCRE recognizes as digits, spaces, or word characters remain the same set as in non-UTF mode, all with values less than 256. This remains true even when PCRE is built to include Unicode property support, because to do otherwise would slow down PCRE in many common cases. Note in particular that this applies to \b and \B, because they are defined in terms of \w and \W. If you really want to test for a wider sense of, say, "digit", you can use explicit Unicode property tests such as \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that the character escapes work is changed so that Unicode properties are used to determine which characters match. There are more details in the sec- tion on generic character types in the pcrepattern documentation. 7. Similarly, characters that match the POSIX named character classes are all low-valued characters, unless the PCRE_UCP option is set. 8. However, the horizontal and vertical white space matching escapes (\h, \H, \v, and \V) do match all the appropriate Unicode characters, whether or not PCRE_UCP is set. 9. Case-insensitive matching applies only to characters whose values are less than 128, unless PCRE is built with Unicode property support. A few Unicode characters such as Greek sigma have more than two code- points that are case-equivalent. Up to and including PCRE release 8.31, only one-to-one case mappings were supported, but later releases (with Unicode property support) do treat as case-equivalent all versions of characters such as Greek sigma. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 27 February 2013 Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------ PCREJIT(3) Library Functions Manual PCREJIT(3) NAME PCRE - Perl-compatible regular expressions PCRE JUST-IN-TIME COMPILER SUPPORT Just-in-time compiling is a heavyweight optimization that can greatly speed up pattern matching. However, it comes at the cost of extra pro- cessing before the match is performed. Therefore, it is of most benefit when the same pattern is going to be matched many times. This does not necessarily mean many calls of a matching function; if the pattern is not anchored, matching attempts may take place many times at various positions in the subject, even for a single call. Therefore, if the subject string is very long, it may still pay to use JIT for one-off matches. JIT support applies only to the traditional Perl-compatible matching function. It does not apply when the DFA matching function is being used. The code for this support was written by Zoltan Herczeg. 8-BIT, 16-BIT AND 32-BIT SUPPORT JIT support is available for all of the 8-bit, 16-bit and 32-bit PCRE libraries. To keep this documentation simple, only the 8-bit interface is described in what follows. If you are using the 16-bit library, sub- stitute the 16-bit functions and 16-bit structures (for example, pcre16_jit_stack instead of pcre_jit_stack). If you are using the 32-bit library, substitute the 32-bit functions and 32-bit structures (for example, pcre32_jit_stack instead of pcre_jit_stack). AVAILABILITY OF JIT SUPPORT JIT support is an optional feature of PCRE. The "configure" option --enable-jit (or equivalent CMake option) must be set when PCRE is built if you want to use JIT. The support is limited to the following hardware platforms: ARM v5, v7, and Thumb2 Intel x86 32-bit and 64-bit MIPS 32-bit Power PC 32-bit and 64-bit SPARC 32-bit (experimental) If --enable-jit is set on an unsupported platform, compilation fails. A program that is linked with PCRE 8.20 or later can tell if JIT sup- port is available by calling pcre_config() with the PCRE_CONFIG_JIT option. The result is 1 when JIT is available, and 0 otherwise. How- ever, a simple program does not need to check this in order to use JIT. The normal API is implemented in a way that falls back to the interpre- tive code if JIT is not available. For programs that need the best pos- sible performance, there is also a "fast path" API that is JIT-spe- cific. If your program may sometimes be linked with versions of PCRE that are older than 8.20, but you want to use JIT when it is available, you can test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such as PCRE_CONFIG_JIT, for compile-time control of your code. SIMPLE USE OF JIT You have to do two things to make use of the JIT support in the sim- plest way: (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for each compiled pattern, and pass the resulting pcre_extra block to pcre_exec(). (2) Use pcre_free_study() to free the pcre_extra block when it is no longer needed, instead of just freeing it yourself. This ensures that any JIT data is also freed. For a program that may be linked with pre-8.20 versions of PCRE, you can insert #ifndef PCRE_STUDY_JIT_COMPILE #define PCRE_STUDY_JIT_COMPILE 0 #endif so that no option is passed to pcre_study(), and then use something like this to free the study data: #ifdef PCRE_CONFIG_JIT pcre_free_study(study_ptr); #else pcre_free(study_ptr); #endif PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for complete matches. If you want to run partial matches using the PCRE_PARTIAL_HARD or PCRE_PARTIAL_SOFT options of pcre_exec(), you should set one or both of the following options in addition to, or instead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study(): PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE The JIT compiler generates different optimized code for each of the three modes (normal, soft partial, hard partial). When pcre_exec() is called, the appropriate code is run if it is available. Otherwise, the pattern is matched using interpretive code. In some circumstances you may need to call additional functions. These are described in the section entitled "Controlling the JIT stack" below. If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are ignored, and no JIT data is created. Otherwise, the compiled pattern is passed to the JIT compiler, which turns it into machine code that exe- cutes much faster than the normal interpretive code. When pcre_exec() is passed a pcre_extra block containing a pointer to JIT code of the appropriate mode (normal or hard/soft partial), it obeys that code instead of running the interpreter. The result is identical, but the compiled JIT code runs much faster. There are some pcre_exec() options that are not supported for JIT exe- cution. There are also some pattern items that JIT cannot handle. Details are given below. In both cases, execution automatically falls back to the interpretive code. If you want to know whether JIT was actually used for a particular match, you should arrange for a JIT callback function to be set up as described in the section entitled "Controlling the JIT stack" below, even if you do not need to supply a non-default JIT stack. Such a callback function is called whenever JIT code is about to be obeyed. If the execution options are not right for JIT execution, the callback function is not obeyed. If the JIT compiler finds an unsupported item, no JIT data is gener- ated. You can find out if JIT execution is available after studying a pattern by calling pcre_fullinfo() with the PCRE_INFO_JIT option. A result of 1 means that JIT compilation was successful. A result of 0 means that JIT support is not available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE etc., or the JIT compiler was not able to handle the pattern. Once a pattern has been studied, with or without JIT, it can be used as many times as you like for matching different subject strings. UNSUPPORTED OPTIONS AND PATTERN ITEMS The only pcre_exec() options that are supported for JIT execution are PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NO_UTF32_CHECK, PCRE_NOT- BOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PAR- TIAL_HARD, and PCRE_PARTIAL_SOFT. The only unsupported pattern items are \C (match a single data unit) when running in a UTF mode, and a callout immediately before an asser- tion condition in a conditional group. RETURN VALUES FROM JIT EXECUTION When a pattern is matched using JIT execution, the return values are the same as those given by the interpretive pcre_exec() code, with the addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used for the JIT stack was insufficient. See "Control- ling the JIT stack" below for a discussion of JIT stack usage. For com- patibility with the interpretive pcre_exec() code, no more than two- thirds of the ovector argument is used for passing back captured sub- strings. The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a very large pattern tree goes on for too long, as it is in the same circumstance when JIT is not used, but the details of exactly what is counted are not the same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT execution. SAVING AND RESTORING COMPILED PATTERNS The code that is generated by the JIT compiler is architecture-spe- cific, and is also position dependent. For those reasons it cannot be saved (in a file or database) and restored later like the bytecode and other data of a compiled pattern. Saving and restoring compiled pat- terns is not something many people do. More detail about this facility is given in the pcreprecompile documentation. It should be possible to run pcre_study() on a saved and restored pattern, and thereby recreate the JIT data, but because JIT compilation uses significant resources, it is probably not worth doing this; you might as well recompile the original pattern. CONTROLLING THE JIT STACK When the compiled JIT code runs, it needs a block of memory to use as a stack. By default, it uses 32K on the machine stack. However, some large or complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT is given when there is not enough stack. Three functions are provided for managing blocks of memory for use as JIT stacks. There is further discussion about the use of JIT stacks in the section entitled "JIT stack FAQ" below. The pcre_jit_stack_alloc() function creates a JIT stack. Its arguments are a starting size and a maximum size, and it returns a pointer to an opaque structure of type pcre_jit_stack, or NULL if there is an error. The pcre_jit_stack_free() function can be used to free a stack that is no longer needed. (For the technically minded: the address space is allocated by mmap or VirtualAlloc.) JIT uses far less memory for recursion than the interpretive code, and a maximum stack size of 512K to 1M should be more than enough for any pattern. The pcre_assign_jit_stack() function specifies which stack JIT code should use. Its arguments are as follows: pcre_extra *extra pcre_jit_callback callback void *data The extra argument must be the result of studying a pattern with PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the other two options: (1) If callback is NULL and data is NULL, an internal 32K block on the machine stack is used. (2) If callback is NULL and data is not NULL, data must be a valid JIT stack, the result of calling pcre_jit_stack_alloc(). (3) If callback is not NULL, it must point to a function that is called with data as an argument at the start of matching, in order to set up a JIT stack. If the return from the callback function is NULL, the internal 32K stack is used; otherwise the return value must be a valid JIT stack, the result of calling pcre_jit_stack_alloc(). A callback function is obeyed whenever JIT code is about to be run; it is not obeyed when pcre_exec() is called with options that are incom- patible for JIT execution. A callback function can therefore be used to determine whether a match operation was executed by JIT or by the interpreter. You may safely use the same JIT stack for more than one pattern (either by assigning directly or by callback), as long as the patterns are all matched sequentially in the same thread. In a multithread application, if you do not specify a JIT stack, or if you assign or pass back NULL from a callback, that is thread-safe, because each thread has its own machine stack. However, if you assign or pass back a non-NULL JIT stack, this must be a different stack for each thread so that the application is thread-safe. Strictly speaking, even more is allowed. You can assign the same non- NULL stack to any number of patterns as long as they are not used for matching by multiple threads at the same time. For example, you can assign the same stack to all compiled patterns, and use a global mutex in the callback to wait until the stack is available for use. However, this is an inefficient solution, and not recommended. This is a suggestion for how a multithreaded program that needs to set up non-default JIT stacks might operate: During thread initalization thread_local_var = pcre_jit_stack_alloc(...) During thread exit pcre_jit_stack_free(thread_local_var) Use a one-line callback function return thread_local_var All the functions described in this section do nothing if JIT is not available, and pcre_assign_jit_stack() does nothing unless the extra argument is non-NULL and points to a pcre_extra block that is the result of a successful study with PCRE_STUDY_JIT_COMPILE etc. JIT STACK FAQ (1) Why do we need JIT stacks? PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where the local data of the current node is pushed before checking its child nodes. Allocating real machine stack on some platforms is diffi- cult. For example, the stack chain needs to be updated every time if we extend the stack on PowerPC. Although it is possible, its updating time overhead decreases performance. So we do the recursion in memory. (2) Why don't we simply allocate blocks of memory with malloc()? Modern operating systems have a nice feature: they can reserve an address space instead of allocating memory. We can safely allocate mem- ory pages inside this address space, so the stack could grow without moving memory data (this is important because of pointers). Thus we can allocate 1M address space, and use only a single memory page (usually 4K) if that is enough. However, we can still grow up to 1M anytime if needed. (3) Who "owns" a JIT stack? The owner of the stack is the user program, not the JIT studied pattern or anything else. The user program must ensure that if a stack is used by pcre_exec(), (that is, it is assigned to the pattern currently run- ning), that stack must not be used by any other threads (to avoid over- writing the same memory area). The best practice for multithreaded pro- grams is to allocate a stack for each thread, and return this stack through the JIT callback function. (4) When should a JIT stack be freed? You can free a JIT stack at any time, as long as it will not be used by pcre_exec() again. When you assign the stack to a pattern, only a pointer is set. There is no reference counting or any other magic. You can free the patterns and stacks in any order, anytime. Just do not call pcre_exec() with a pattern pointing to an already freed stack, as that will cause SEGFAULT. (Also, do not free a stack currently used by pcre_exec() in another thread). You can also replace the stack for a pattern at any time. You can even free the previous stack before assigning a replacement. (5) Should I allocate/free a stack every time before/after calling pcre_exec()? No, because this is too costly in terms of resources. However, you could implement some clever idea which release the stack if it is not used in let's say two minutes. The JIT callback can help to achieve this without keeping a list of the currently JIT studied patterns. (6) OK, the stack is for long term memory allocation. But what happens if a pattern causes stack overflow with a stack of 1M? Is that 1M kept until the stack is freed? Especially on embedded sytems, it might be a good idea to release mem- ory sometimes without freeing the stack. There is no API for this at the moment. Probably a function call which returns with the currently allocated memory for any stack and another which allows releasing mem- ory (shrinking the stack) would be a good idea if someone needs this. (7) This is too much of a headache. Isn't there any better solution for JIT stack handling? No, thanks to Windows. If POSIX threads were used everywhere, we could throw out this complicated API. EXAMPLE CODE This is a single-threaded example that specifies a JIT stack without using a callback. int rc; int ovector[30]; pcre *re; pcre_extra *extra; pcre_jit_stack *jit_stack; re = pcre_compile(pattern, 0, &error, &erroffset, NULL); /* Check for errors */ extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error); jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024); /* Check for error (NULL) */ pcre_assign_jit_stack(extra, NULL, jit_stack); rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30); /* Check results */ pcre_free(re); pcre_free_study(extra); pcre_jit_stack_free(jit_stack); JIT FAST PATH API Because the API described above falls back to interpreted execution when JIT is not available, it is convenient for programs that are writ- ten for general use in many environments. However, calling JIT via pcre_exec() does have a performance impact. Programs that are written for use where JIT is known to be available, and which need the best possible performance, can instead use a "fast path" API to call JIT execution directly instead of calling pcre_exec() (obviously only for patterns that have been successfully studied by JIT). The fast path function is called pcre_jit_exec(), and it takes exactly the same arguments as pcre_exec(), plus one additional argument that must point to a JIT stack. The JIT stack arrangements described above do not apply. The return values are the same as for pcre_exec(). When you call pcre_exec(), as well as testing for invalid options, a number of other sanity checks are performed on the arguments. For exam- ple, if the subject pointer is NULL, or its length is negative, an immediate error is given. Also, unless PCRE_NO_UTF[8|16|32] is set, a UTF subject string is tested for validity. In the interests of speed, these checks do not happen on the JIT fast path, and if invalid data is passed, the result is undefined. Bypassing the sanity checks and the pcre_exec() wrapping can give speedups of more than 10%. SEE ALSO pcreapi(3) AUTHOR Philip Hazel (FAQ by Zoltan Herczeg) University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 17 March 2013 Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------ PCREPARTIAL(3) Library Functions Manual PCREPARTIAL(3) NAME PCRE - Perl-compatible regular expressions PARTIAL MATCHING IN PCRE In normal use of PCRE, if the subject string that is passed to a match- ing function matches as far as it goes, but is too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where it might be helpful to distinguish this case from other cases in which there is no match. Consider, for example, an application where a human is required to type in data for a field with specific formatting requirements. An example might be a date in the form ddmmmyy, defined by this pattern: ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$ If the application sees the user's keystrokes one by one, and can check that what has been typed so far is potentially valid, it is able to raise an error as soon as a mistake is made, by beeping and not reflecting the character that has been typed, for example. This immedi- ate feedback is likely to be a better user interface than a check that is delayed until the entire string has been entered. Partial matching can also be useful when the subject string is very long and is not all available at once. PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and PCRE_PARTIAL_HARD options, which can be set when calling any of the matching functions. For backwards compatibility, PCRE_PARTIAL is a syn- onym for PCRE_PARTIAL_SOFT. The essential difference between the two options is whether or not a partial match is preferred to an alterna- tive complete match, though the details differ between the two types of matching function. If both options are set, PCRE_PARTIAL_HARD takes precedence. If you want to use partial matching with just-in-time optimized code, you must call pcre_study(), pcre16_study() or pcre32_study() with one or both of these options: PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non- partial matches on the same pattern. If the appropriate JIT study mode has not been set for a match, the interpretive matching code is used. Setting a partial matching option disables two of PCRE's standard opti- mizations. PCRE remembers the last literal data unit in a pattern, and abandons matching immediately if it is not present in the subject string. This optimization cannot be used for a subject string that might match only partially. If the pattern was studied, PCRE knows the minimum length of a matching string, and does not bother to run the matching function on shorter strings. This optimization is also dis- abled for partial matching. PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec() A partial match occurs during a call to pcre_exec() or pcre[16|32]_exec() when the end of the subject string is reached suc- cessfully, but matching cannot continue because more characters are needed. However, at least one character in the subject must have been inspected. This character need not form part of the final matched string; lookbehind assertions and the \K escape sequence provide ways of inspecting characters before the start of a matched substring. The requirement for inspecting at least one character exists because an empty string can always be matched; without such a restriction there would always be a partial match of an empty string at the end of the subject. If there are at least two slots in the offsets vector when a partial match is returned, the first slot is set to the offset of the earliest character that was inspected. For convenience, the second offset points to the end of the subject so that a substring can easily be identified. If there are at least three slots in the offsets vector, the third slot is set to the offset of the character where matching started. For the majority of patterns, the contents of the first and third slots will be the same. However, for patterns that contain lookbehind asser- tions, or begin with \b or \B, characters before the one where matching started may have been inspected while carrying out the match. For exam- ple, consider this pattern: /(?<=abc)123/ This pattern matches "123", but only if it is preceded by "abc". If the subject string is "xyzabc12", the first two offsets after a partial match are for the substring "abc12", because all these characters were inspected. However, the third offset is set to 6, because that is the offset where matching began. What happens when a partial match is identified depends on which of the two partial matching options are set. PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec() If PCRE_PARTIAL_SOFT is set when pcre_exec() or pcre[16|32]_exec() identifies a partial match, the partial match is remembered, but match- ing continues as normal, and other alternatives in the pattern are tried. If no complete match can be found, PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. This option is "soft" because it prefers a complete match over a par- tial match. All the various matching items in a pattern behave as if the subject string is potentially complete. For example, \z, \Z, and $ match at the end of the subject, as normal, and for \b and \B the end of the subject is treated as a non-alphanumeric. If there is more than one partial match, the first one that was found provides the data that is returned. Consider this pattern: /123\w+X|dogY/ If this is matched against the subject string "abc123dog", both alter- natives fail to match, but the end of the subject is reached during matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9, identifying "123dog" as the first partial match that was found. (In this example, there are two partial matches, because "dog" on its own partially matches the second alternative.) PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec() If PCRE_PARTIAL_HARD is set for pcre_exec() or pcre[16|32]_exec(), PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without continuing to search for possible complete matches. This option is "hard" because it prefers an earlier partial match over a later com- plete match. For this reason, the assumption is made that the end of the supplied subject string may not be the true end of the available data, and so, if \z, \Z, \b, \B, or $ are encountered at the end of the subject, the result is PCRE_ERROR_PARTIAL, provided that at least one character in the subject has been inspected. Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16 subject strings are checked for validity. Normally, an invalid sequence causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the special case of a truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when PCRE_PARTIAL_HARD is set. Comparing hard and soft partial matching The difference between the two partial matching options can be illus- trated by a pattern such as: /dog(sbody)?/ This matches either "dog" or "dogsbody", greedily (that is, it prefers the longer string if possible). If it is matched against the string "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand, if the pattern is made ungreedy the result is dif- ferent: /dog(sbody)??/ In this case the result is always a complete match because that is found first, and matching never continues after finding a complete match. It might be easier to follow this explanation by thinking of the two patterns like this: /dog(sbody)?/ is the same as /dogsbody|dog/ /dog(sbody)??/ is the same as /dog|dogsbody/ The second pattern will never match "dogsbody", because it will always find the shorter match first. PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec() The DFA functions move along the subject string character by character, without backtracking, searching for all possible matches simultane- ously. If the end of the subject is reached before the end of the pat- tern, there is the possibility of a partial match, again provided that at least one character has been inspected. When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there have been no complete matches. Otherwise, the complete matches are returned. However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any complete matches. The portion of the string that was inspected when the longest partial match was found is set as the first matching string, provided there are at least two slots in the offsets vector. Because the DFA functions always search for all possible matches, and there is no difference between greedy and ungreedy repetition, their behaviour is different from the standard functions when PCRE_PAR- TIAL_HARD is set. Consider the string "dog" matched against the ungreedy pattern shown above: /dog(sbody)??/ Whereas the standard functions stop as soon as they find the complete match for "dog", the DFA functions also find the partial match for "dogsbody", and so return that when PCRE_PARTIAL_HARD is set. PARTIAL MATCHING AND WORD BOUNDARIES If a pattern ends with one of sequences \b or \B, which test for word boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter- intuitive results. Consider this pattern: /\bcat\b/ This matches "cat", provided there is a word boundary at either end. If the subject string is "the cat", the comparison of the final "t" with a following character cannot take place, so a partial match is found. However, normal matching carries on, and \b matches at the end of the subject when the last character is a letter, so a complete match is found. The result, therefore, is not PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because then the partial match takes precedence. FORMERLY RESTRICTED PATTERNS For releases of PCRE prior to 8.00, because of the way certain internal optimizations were implemented in the pcre_exec() function, the PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with all patterns. From release 8.00 onwards, the restrictions no longer apply, and partial matching with can be requested for any pat- tern. Items that were formerly restricted were repeated single characters and repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not conform to the restrictions, pcre_exec() returned the error code PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled pattern can be used for partial matching now always returns 1. EXAMPLE OF PARTIAL MATCHING USING PCRETEST If the escape sequence \P is present in a pcretest data line, the PCRE_PARTIAL_SOFT option is used for the match. Here is a run of pcretest that uses the date example quoted above: re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 25jun04\P 0: 25jun04 1: jun data> 25dec3\P Partial match: 23dec3 data> 3ju\P Partial match: 3ju data> 3juj\P No match data> j\P No match The first data string is matched completely, so pcretest shows the matched substrings. The remaining four strings do not match the com- plete pattern, but the first two are partial matches. Similar output is obtained if DFA matching is used. If the escape sequence \P is present more than once in a pcretest data line, the PCRE_PARTIAL_HARD option is set for the match. MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec() When a partial match has been found using a DFA matching function, it is possible to continue the match by providing additional subject data and calling the function again with the same compiled regular expres- sion, this time setting the PCRE_DFA_RESTART option. You must pass the same working space as before, because this is where details of the pre- vious partial match are stored. Here is an example using pcretest, using the \R escape sequence to set the PCRE_DFA_RESTART option (\D specifies the use of the DFA matching function): re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 23ja\P\D Partial match: 23ja data> n05\R\D 0: n05 The first call has "23ja" as the subject, and requests partial match- ing; the second call has "n05" as the subject for the continued (restarted) match. Notice that when the match is complete, only the last part is shown; PCRE does not retain the previously partially- matched string. It is up to the calling program to do that if it needs to. That means that, for an unanchored pattern, if a continued match fails, it is not possible to try again at a new starting point. All this facility is capable of doing is continuing with the previous match attempt. In the previous example, if the second set of data is "ug23" the result is no match, even though there would be a match for "aug23" if the entire string were given at once. Depending on the application, this may or may not be what you want. The only way to allow for start- ing again at the next character is to retain the matched part of the subject and try a new complete match. You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with PCRE_DFA_RESTART to continue partial matching over multiple segments. This facility can be used to pass very long subject strings to the DFA matching functions. MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec() From release 8.00, the standard matching functions can also be used to do multi-segment matching. Unlike the DFA functions, it is not possible to restart the previous match with a new segment of data. Instead, new data must be added to the previous subject string, and the entire match re-run, starting from the point where the partial match occurred. Ear- lier data can be discarded. It is best to use PCRE_PARTIAL_HARD in this situation, because it does not treat the end of a segment as the end of the subject when matching \z, \Z, \b, \B, and $. Consider an unanchored pattern that matches dates: re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ data> The date is 23ja\P\P Partial match: 23ja At this stage, an application could discard the text preceding "23ja", add on text from the next segment, and call the matching function again. Unlike the DFA matching functions, the entire matching string must always be available, and the complete matching process occurs for each call, so more memory and more processing time is needed. Note: If the pattern contains lookbehind assertions, or \K, or starts with \b or \B, the string that is returned for a partial match includes characters that precede the start of what would be returned for a com- plete match, because it contains all the characters that were inspected during the partial match. ISSUES WITH MULTI-SEGMENT MATCHING Certain types of pattern may give problems with multi-segment matching, whichever matching function is used. 1. If the pattern contains a test for the beginning of a line, you need to pass the PCRE_NOTBOL option when the subject string for any call does start at the beginning of a line. There is also a PCRE_NOTEOL option, but in practice when doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL. 2. Lookbehind assertions that have already been obeyed are catered for in the offsets that are returned for a partial match. However a lookbe- hind assertion later in the pattern could require even earlier charac- ters to be inspected. You can handle this case by using the PCRE_INFO_MAXLOOKBEHIND option of the pcre_fullinfo() or pcre[16|32]_fullinfo() functions to obtain the length of the longest lookbehind in the pattern. This length is given in characters, not bytes. If you always retain at least that many characters before the partially matched string, all should be well. (Of course, near the start of the subject, fewer characters may be present; in that case all characters should be retained.) From release 8.33, there is a more accurate way of deciding which char- acters to retain. Instead of subtracting the length of the longest lookbehind from the earliest inspected character (offsets[0]), the match start position (offsets[2]) should be used, and the next match attempt started at the offsets[2] character by setting the startoffset argument of pcre_exec() or pcre_dfa_exec(). For example, if the pattern "(?<=123)abc" is partially matched against the string "xx123a", the three offset values returned are 2, 6, and 5. This indicates that the matching process that gave a partial match started at offset 5, but the characters "123a" were all inspected. The maximum lookbehind for that pattern is 3, so taking that away from 5 shows that we need only keep "123a", and the next match attempt can be started at offset 3 (that is, at "a") when further characters have been added. When the match start is not the earliest inspected character, pcretest shows it explicitly: re> "(?<=123)abc" data> xx123a\P\P Partial match at offset 5: 123a 3. Because a partial match must always contain at least one character, what might be considered a partial match of an empty string actually gives a "no match" result. For example: re> /c(?<=abc)x/ data> ab\P No match If the next segment begins "cx", a match should be found, but this will only happen if characters from the previous segment are retained. For this reason, a "no match" result should be interpreted as "partial match of an empty string" when the pattern contains lookbehinds. 4. Matching a subject string that is split into multiple segments may not always produce exactly the same result as matching over one single long string, especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and Word Boundaries" above describes an issue that arises if the pattern ends with \b or \B. Another kind of difference may occur when there are multiple matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result is given only when there are no completed matches. This means that as soon as the shortest match has been found, continuation to a new subject segment is no longer possi- ble. Consider again this pcretest example: re> /dog(sbody)?/ data> dogsb\P 0: dog data> do\P\D Partial match: do data> gsb\R\P\D 0: g data> dogsbody\D 0: dogsbody 1: dog The first data line passes the string "dogsb" to a standard matching function, setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string "dog" is a complete match. Similarly, when the subject is presented to a DFA matching function in several parts ("do" and "gsb" being the first two) the match stops when "dog" has been found, and it is not possible to continue. On the other hand, if "dogsbody" is presented as a single string, a DFA matching function finds both matches. Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching multi-segment data. The example above then behaves differ- ently: re> /dog(sbody)?/ data> dogsb\P\P Partial match: dogsb data> do\P\D Partial match: do data> gsb\R\P\P\D Partial match: gsb 5. Patterns that contain alternatives at the top level which do not all start with the same pattern item may not work as expected when PCRE_DFA_RESTART is used. For example, consider this pattern: 1234|3789 If the first part of the subject is "ABC123", a partial match of the first alternative is found at offset 3. There is no partial match for the second alternative, because such a match does not start at the same point in the subject string. Attempting to continue with the string "7890" does not yield a match because only those alternatives that match at one point in the subject are remembered. The problem arises because the start of the second alternative matches within the first alternative. There is no problem with anchored patterns or patterns such as: 1234|ABCD where no string can be a partial match for both alternatives. This is not a problem if a standard matching function is used, because the entire match has to be rerun each time: re> /1234|3789/ data> ABC123\P\P Partial match: 123 data> 1237890 0: 3789 Of course, instead of using PCRE_DFA_RESTART, the same technique of re- running the entire match can also be used with the DFA matching func- tions. Another possibility is to work with two buffers. If a partial match at offset n in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on the second buffer, you can then try a new match starting at offset n+1 in the first buffer. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 02 July 2013 Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------ PCREPRECOMPILE(3) Library Functions Manual PCREPRECOMPILE(3) NAME PCRE - Perl-compatible regular expressions SAVING AND RE-USING PRECOMPILED PCRE PATTERNS If you are running an application that uses a large number of regular expression patterns, it may be useful to store them in a precompiled form instead of having to compile them every time the application is run. If you are not using any private character tables (see the pcre_maketables() documentation), this is relatively straightforward. If you are using private tables, it is a little bit more complicated. However, if you are using the just-in-time optimization feature, it is not possible to save and reload the JIT data. If you save compiled patterns to a file, you can copy them to a differ- ent host and run them there. If the two hosts have different endianness (byte order), you should run the pcre[16|32]_pat- tern_to_host_byte_order() function on the new host before trying to match the pattern. The matching functions return PCRE_ERROR_BADENDIAN- NESS if they detect a pattern with the wrong endianness. Compiling regular expressions with one version of PCRE for use with a different version is not guaranteed to work and may cause crashes, and saving and restoring a compiled pattern loses any JIT optimization data. SAVING A COMPILED PATTERN The value returned by pcre[16|32]_compile() points to a single block of memory that holds the compiled pattern and associated data. You can find the length of this block in bytes by calling pcre[16|32]_fullinfo() with an argument of PCRE_INFO_SIZE. You can then save the data in any appropriate manner. Here is sample code for the 8-bit library that compiles a pattern and writes it to a file. It assumes that the variable fd refers to a file that is open for output: int erroroffset, rc, size; char *error; pcre *re; re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL); if (re == NULL) { ... handle errors ... } rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size); if (rc < 0) { ... handle errors ... } rc = fwrite(re, 1, size, fd); if (rc != size) { ... handle errors ... } In this example, the bytes that comprise the compiled pattern are copied exactly. Note that this is binary data that may contain any of the 256 possible byte values. On systems that make a distinction between binary and non-binary data, be sure that the file is opened for binary output. If you want to write more than one pattern to a file, you will have to devise a way of separating them. For binary data, preceding each pat- tern with its length is probably the most straightforward approach. Another possibility is to write out the data in hexadecimal instead of binary, one pattern to a line. Saving compiled patterns in a file is only one possible way of storing them for later use. They could equally well be saved in a database, or in the memory of some daemon process that passes them via sockets to the processes that want them. If the pattern has been studied, it is also possible to save the normal study data in a similar way to the compiled pattern itself. However, if the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre- ated cannot be saved because it is too dependent on the current envi- ronment. When studying generates additional information, pcre[16|32]_study() returns a pointer to a pcre[16|32]_extra data block. Its format is defined in the section on matching a pattern in the pcreapi documentation. The study_data field points to the binary study data, and this is what you must save (not the pcre[16|32]_extra block itself). The length of the study data can be obtained by calling pcre[16|32]_fullinfo() with an argument of PCRE_INFO_STUDYSIZE. Remem- ber to check that pcre[16|32]_study() did return a non-NULL value before trying to save the study data. RE-USING A PRECOMPILED PATTERN Re-using a precompiled pattern is straightforward. Having reloaded it into main memory, called pcre[16|32]_pattern_to_host_byte_order() if necessary, you pass its pointer to pcre[16|32]_exec() or pcre[16|32]_dfa_exec() in the usual way. However, if you passed a pointer to custom character tables when the pattern was compiled (the tableptr argument of pcre[16|32]_compile()), you must now pass a similar pointer to pcre[16|32]_exec() or pcre[16|32]_dfa_exec(), because the value saved with the compiled pat- tern will obviously be nonsense. A field in a pcre[16|32]_extra() block is used to pass this data, as described in the section on matching a pattern in the pcreapi documentation. Warning: The tables that pcre_exec() and pcre_dfa_exec() use must be the same as those that were used when the pattern was compiled. If this is not the case, the behaviour is undefined. If you did not provide custom character tables when the pattern was compiled, the pointer in the compiled pattern is NULL, which causes the matching functions to use PCRE's internal tables. Thus, you do not need to take any special action at run time in this case. If you saved study data with the compiled pattern, you need to create your own pcre[16|32]_extra data block and set the study_data field to point to the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA bit in the flags field to indicate that study data is present. Then pass the pcre[16|32]_extra block to the matching function in the usual way. If the pattern was studied for just-in-time optimization, that data cannot be saved, and so is lost by a save/restore cycle. COMPATIBILITY WITH DIFFERENT PCRE RELEASES In general, it is safest to recompile all saved patterns when you update to a new PCRE release, though not all updates actually require this. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 12 November 2013 Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------ PCREPERFORM(3) Library Functions Manual PCREPERFORM(3) NAME PCRE - Perl-compatible regular expressions PCRE PERFORMANCE Two aspects of performance are discussed below: memory usage and pro- cessing time. The way you express your pattern as a regular expression can affect both of them. COMPILED PATTERN MEMORY USAGE Patterns are compiled by PCRE into a reasonably efficient interpretive code, so that most simple patterns do not use much memory. However, there is one case where the memory usage of a compiled pattern can be unexpectedly large. If a parenthesized subpattern has a quantifier with a minimum greater than 1 and/or a limited maximum, the whole subpattern is repeated in the compiled code. For example, the pattern (abc|def){2,4} is compiled as if it were (abc|def)(abc|def)((abc|def)(abc|def)?)? (Technical aside: It is done this way so that backtrack points within each of the repetitions can be independently maintained.) For regular expressions whose quantifiers use only small numbers, this is not usually a problem. However, if the numbers are large, and par- ticularly if such repetitions are nested, the memory usage can become an embarrassment. For example, the very simple pattern ((ab){1,1000}c){1,3} uses 51K bytes when compiled using the 8-bit library. When PCRE is com- piled with its default internal pointer size of two bytes, the size limit on a compiled pattern is 64K data units, and this is reached with the above pattern if the outer repetition is increased from 3 to 4. PCRE can be compiled to use larger internal pointers and thus handle larger compiled patterns, but it is better to try to rewrite your pat- tern to use less memory if you can. One way of reducing the memory usage for such patterns is to make use of PCRE's "subroutine" facility. Re-writing the above pattern as ((ab)(?2){0,999}c)(?1){0,2} reduces the memory requirements to 18K, and indeed it remains under 20K even with the outer repetition increased to 100. However, this pattern is not exactly equivalent, because the "subroutine" calls are treated as atomic groups into which there can be no backtracking if there is a subsequent matching failure. Therefore, PCRE cannot do this kind of rewriting automatically. Furthermore, there is a noticeable loss of speed when executing the modified pattern. Nevertheless, if the atomic grouping is not a problem and the loss of speed is acceptable, this kind of rewriting will allow you to process patterns that PCRE cannot otherwise handle. STACK USAGE AT RUN TIME When pcre_exec() or pcre[16|32]_exec() is used for matching, certain kinds of pattern can cause it to use large amounts of the process stack. In some environments the default process stack is quite small, and if it runs out the result is often SIGSEGV. This issue is probably the most frequently raised problem with PCRE. Rewriting your pattern can often help. The pcrestack documentation discusses this issue in detail. PROCESSING TIME Certain items in regular expression patterns are processed more effi- ciently than others. It is more efficient to use a character class like [aeiou] than a set of single-character alternatives such as (a|e|i|o|u). In general, the simplest construction that provides the required behaviour is usually the most efficient. Jeffrey Friedl's book contains a lot of useful general discussion about optimizing regular expressions for efficient performance. This document contains a few observations about PCRE. Using Unicode character properties (the \p, \P, and \X escapes) is slow, because PCRE has to use a multi-stage table lookup whenever it needs a character's property. If you can find an alternative pattern that does not use character properties, it will probably be faster. By default, the escape sequences \b, \d, \s, and \w, and the POSIX character classes such as [:alpha:] do not use Unicode properties, partly for backwards compatibility, and partly for performance reasons. However, you can set PCRE_UCP if you want Unicode character properties to be used. This can double the matching time for items such as \d, when matched with a traditional matching function; the performance loss is less with a DFA matching function, and in both cases there is not much difference for \b. When a pattern begins with .* not in parentheses, or in parentheses that are not the subject of a backreference, and the PCRE_DOTALL option is set, the pattern is implicitly anchored by PCRE, since it can match only at the start of a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization, because the . metacharacter does not then match a newline, and if the subject string contains new- lines, the pattern may match from the character immediately following one of them instead of from the very start. For example, the pattern .*second matches the subject "first\nand second" (where \n stands for a newline character), with the match starting at the seventh character. In order to do this, PCRE has to retry the match starting after every newline in the subject. If you are using such a pattern with subject strings that do not con- tain newlines, the best performance is obtained by setting PCRE_DOTALL, or starting the pattern with ^.* or ^.*? to indicate explicit anchor- ing. That saves PCRE from having to scan along the subject looking for a newline to restart at. Beware of patterns that contain nested indefinite repeats. These can take a long time to run when applied to a string that does not match. Consider the pattern fragment ^(a+)* This can match "aaaa" in 16 different ways, and this number increases very rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + repeats can match different numbers of times.) When the remainder of the pattern is such that the entire match is going to fail, PCRE has in principle to try every possible variation, and this can take an extremely long time, even for relatively short strings. An optimization catches some of the more simple cases such as (a+)*b where a literal character follows. Before embarking on the standard matching procedure, PCRE checks that there is a "b" later in the sub- ject string, and if there is not, it fails the match immediately. How- ever, when there is no following literal this optimization cannot be used. You can see the difference by comparing the behaviour of (a+)*\d with the pattern above. The former gives a failure almost instantly when applied to a whole line of "a" characters, whereas the latter takes an appreciable time with strings longer than about 20 characters. In many cases, the solution to this kind of performance issue is to use an atomic group or a possessive quantifier. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 25 August 2012 Copyright (c) 1997-2012 University of Cambridge. ------------------------------------------------------------------------------ PCREPOSIX(3) Library Functions Manual PCREPOSIX(3) NAME PCRE - Perl-compatible regular expressions. SYNOPSIS #include <pcreposix.h> int regcomp(regex_t *preg, const char *pattern, int cflags); int regexec(regex_t *preg, const char *string, size_t nmatch, regmatch_t pmatch[], int eflags); size_t regerror(int errcode, const regex_t *preg, char *errbuf, size_t errbuf_size); void regfree(regex_t *preg); DESCRIPTION This set of functions provides a POSIX-style API for the PCRE regular expression 8-bit library. See the pcreapi documentation for a descrip- tion of PCRE's native API, which contains much additional functional- ity. There is no POSIX-style wrapper for PCRE's 16-bit and 32-bit library. The functions described here are just wrapper functions that ultimately call the PCRE native API. Their prototypes are defined in the pcreposix.h header file, and on Unix systems the library itself is called pcreposix.a, so can be accessed by adding -lpcreposix to the command for linking an application that uses them. Because the POSIX functions call the native ones, it is also necessary to add -lpcre. I have implemented only those POSIX option bits that can be reasonably mapped to PCRE native options. In addition, the option REG_EXTENDED is defined with the value zero. This has no effect, but since programs that are written to the POSIX interface often use it, this makes it easier to slot in PCRE as a replacement library. Other POSIX options are not even defined. There are also some other options that are not defined by POSIX. These have been added at the request of users who want to make use of certain PCRE-specific features via the POSIX calling interface. When PCRE is called via these functions, it is only the API that is POSIX-like in style. The syntax and semantics of the regular expres- sions themselves are still those of Perl, subject to the setting of various PCRE options, as described below. "POSIX-like in style" means that the API approximates to the POSIX definition; it is not fully POSIX-compatible, and in multi-byte encoding domains it is probably even less compatible. The header for these functions is supplied as pcreposix.h to avoid any potential clash with other POSIX libraries. It can, of course, be renamed or aliased as regex.h, which is the "correct" name. It provides two structure types, regex_t for compiled internal forms, and reg- match_t for returning captured substrings. It also defines some con- stants whose names start with "REG_"; these are used for setting options and identifying error codes. COMPILING A PATTERN The function regcomp() is called to compile a pattern into an internal form. The pattern is a C string terminated by a binary zero, and is passed in the argument pattern. The preg argument is a pointer to a regex_t structure that is used as a base for storing information about the compiled regular expression. The argument cflags is either zero, or contains one or more of the bits defined by the following macros: REG_DOTALL The PCRE_DOTALL option is set when the regular expression is passed for compilation to the native function. Note that REG_DOTALL is not part of the POSIX standard. REG_ICASE The PCRE_CASELESS option is set when the regular expression is passed for compilation to the native function. REG_NEWLINE The PCRE_MULTILINE option is set when the regular expression is passed for compilation to the native function. Note that this does not mimic the defined POSIX behaviour for REG_NEWLINE (see the following sec- tion). REG_NOSUB The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is passed for compilation to the native function. In addition, when a pat- tern that is compiled with this flag is passed to regexec() for match- ing, the nmatch and pmatch arguments are ignored, and no captured strings are returned. REG_UCP The PCRE_UCP option is set when the regular expression is passed for compilation to the native function. This causes PCRE to use Unicode properties when matchine \d, \w, etc., instead of just recognizing ASCII values. Note that REG_UTF8 is not part of the POSIX standard. REG_UNGREEDY The PCRE_UNGREEDY option is set when the regular expression is passed for compilation to the native function. Note that REG_UNGREEDY is not part of the POSIX standard. REG_UTF8 The PCRE_UTF8 option is set when the regular expression is passed for compilation to the native function. This causes the pattern itself and all data strings used for matching it to be treated as UTF-8 strings. Note that REG_UTF8 is not part of the POSIX standard. In the absence of these flags, no options are passed to the native function. This means the the regex is compiled with PCRE default semantics. In particular, the way it handles newline characters in the subject string is the Perl way, not the POSIX way. Note that setting PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE. It does not affect the way newlines are matched by . (they are not) or by a negative class such as [^a] (they are). The yield of regcomp() is zero on success, and non-zero otherwise. The preg structure is filled in on success, and one member of the structure is public: re_nsub contains the number of capturing subpatterns in the regular expression. Various error codes are defined in the header file. NOTE: If the yield of regcomp() is non-zero, you must not attempt to use the contents of the preg structure. If, for example, you pass it to regexec(), the result is undefined and your program is likely to crash. MATCHING NEWLINE CHARACTERS This area is not simple, because POSIX and Perl take different views of things. It is not possible to get PCRE to obey POSIX semantics, but then PCRE was never intended to be a POSIX engine. The following table lists the different possibilities for matching newline characters in PCRE: Default Change with . matches newline no PCRE_DOTALL newline matches [^a] yes not changeable $ matches \n at end yes PCRE_DOLLARENDONLY $ matches \n in middle no PCRE_MULTILINE ^ matches \n in middle no PCRE_MULTILINE This is the equivalent table for POSIX: Default Change with . matches newline yes REG_NEWLINE newline matches [^a] yes REG_NEWLINE $ matches \n at end no REG_NEWLINE $ matches \n in middle no REG_NEWLINE ^ matches \n in middle no REG_NEWLINE PCRE's behaviour is the same as Perl's, except that there is no equiva- lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is no way to stop newline from matching [^a]. The default POSIX newline handling can be obtained by setting PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE behave exactly as for the REG_NEWLINE action. MATCHING A PATTERN The function regexec() is called to match a compiled pattern preg against a given string, which is by default terminated by a zero byte (but see REG_STARTEND below), subject to the options in eflags. These can be: REG_NOTBOL The PCRE_NOTBOL option is set when calling the underlying PCRE matching function. REG_NOTEMPTY The PCRE_NOTEMPTY option is set when calling the underlying PCRE match- ing function. Note that REG_NOTEMPTY is not part of the POSIX standard. However, setting this option can give more POSIX-like behaviour in some situations. REG_NOTEOL The PCRE_NOTEOL option is set when calling the underlying PCRE matching function. REG_STARTEND The string is considered to start at string + pmatch[0].rm_so and to have a terminating NUL located at string + pmatch[0].rm_eo (there need not actually be a NUL at that location), regardless of the value of nmatch. This is a BSD extension, compatible with but not specified by IEEE Standard 1003.2 (POSIX.2), and should be used with caution in software intended to be portable to other systems. Note that a non-zero rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location of the string, not how it is matched. If the pattern was compiled with the REG_NOSUB flag, no data about any matched strings is returned. The nmatch and pmatch arguments of regexec() are ignored. If the value of nmatch is zero, or if the value pmatch is NULL, no data about any matched strings is returned. Otherwise,the portion of the string that was matched, and also any cap- tured substrings, are returned via the pmatch argument, which points to an array of nmatch structures of type regmatch_t, containing the mem- bers rm_so and rm_eo. These contain the offset to the first character of each substring and the offset to the first character after the end of each substring, respectively. The 0th element of the vector relates to the entire portion of string that was matched; subsequent elements relate to the capturing subpatterns of the regular expression. Unused entries in the array have both structure members set to -1. A successful match yields a zero return; various error codes are defined in the header file, of which REG_NOMATCH is the "expected" failure code. ERROR MESSAGES The regerror() function maps a non-zero errorcode from either regcomp() or regexec() to a printable message. If preg is not NULL, the error should have arisen from the use of that structure. A message terminated by a binary zero is placed in errbuf. The length of the message, including the zero, is limited to errbuf_size. The yield of the func- tion is the size of buffer needed to hold the whole message. MEMORY USAGE Compiling a regular expression causes memory to be allocated and asso- ciated with the preg structure. The function regfree() frees all such memory, after which preg may no longer be used as a compiled expres- sion. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 09 January 2012 Copyright (c) 1997-2012 University of Cambridge. ------------------------------------------------------------------------------ PCRECPP(3) Library Functions Manual PCRECPP(3) NAME PCRE - Perl-compatible regular expressions. SYNOPSIS OF C++ WRAPPER #include <pcrecpp.h> DESCRIPTION The C++ wrapper for PCRE was provided by Google Inc. Some additional functionality was added by Giuseppe Maxia. This brief man page was con- structed from the notes in the pcrecpp.h file, which should be con- sulted for further details. Note that the C++ wrapper supports only the original 8-bit PCRE library. There is no 16-bit or 32-bit support at present. MATCHING INTERFACE The "FullMatch" operation checks that supplied text matches a supplied pattern exactly. If pointer arguments are supplied, it copies matched sub-strings that match sub-patterns into them. Example: successful match pcrecpp::RE re("h.*o"); re.FullMatch("hello"); Example: unsuccessful match (requires full match): pcrecpp::RE re("e"); !re.FullMatch("hello"); Example: creating a temporary RE object: pcrecpp::RE("h.*o").FullMatch("hello"); You can pass in a "const char*" or a "string" for "text". The examples below tend to use a const char*. You can, as in the different examples above, store the RE object explicitly in a variable or use a temporary RE object. The examples below use one mode or the other arbitrarily. Either could correctly be used for any of these examples. You must supply extra pointer arguments to extract matched subpieces. Example: extracts "ruby" into "s" and 1234 into "i" int i; string s; pcrecpp::RE re("(\\w+):(\\d+)"); re.FullMatch("ruby:1234", &s, &i); Example: does not try to extract any extra sub-patterns re.FullMatch("ruby:1234", &s); Example: does not try to extract into NULL re.FullMatch("ruby:1234", NULL, &i); Example: integer overflow causes failure !re.FullMatch("ruby:1234567891234", NULL, &i); Example: fails because there aren't enough sub-patterns: !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s); Example: fails because string cannot be stored in integer !pcrecpp::RE("(.*)").FullMatch("ruby", &i); The provided pointer arguments can be pointers to any scalar numeric type, or one of: string (matched piece is copied to string) StringPiece (StringPiece is mutated to point to matched piece) T (where "bool T::ParseFrom(const char*, int)" exists) NULL (the corresponding matched sub-pattern is not copied) The function returns true iff all of the following conditions are sat- isfied: a. "text" matches "pattern" exactly; b. The number of matched sub-patterns is >= number of supplied pointers; c. The "i"th argument has a suitable type for holding the string captured as the "i"th sub-pattern. If you pass in void * NULL for the "i"th argument, or a non-void * NULL of the correct type, or pass fewer arguments than the number of sub-patterns, "i"th captured sub-pattern is ignored. CAVEAT: An optional sub-pattern that does not exist in the matched string is assigned the empty string. Therefore, the following will return false (because the empty string is not a valid number): int number; pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number); The matching interface supports at most 16 arguments per call. If you need more, consider using the more general interface pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch. NOTE: Do not use no_arg, which is used internally to mark the end of a list of optional arguments, as a placeholder for missing arguments, as this can lead to segfaults. QUOTING METACHARACTERS You can use the "QuoteMeta" operation to insert backslashes before all potentially meaningful characters in a string. The returned string, used as a regular expression, will exactly match the original string. Example: string quoted = RE::QuoteMeta(unquoted); Note that it's legal to escape a character even if it has no special meaning in a regular expression -- so this function does that. (This also makes it identical to the perl function of the same name; see "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes "1\.5\-2\.0\?". PARTIAL MATCHES You can use the "PartialMatch" operation when you want the pattern to match any substring of the text. Example: simple search for a string: pcrecpp::RE("ell").PartialMatch("hello"); Example: find first number in a string: int number; pcrecpp::RE re("(\\d+)"); re.PartialMatch("x*100 + 20", &number); assert(number == 100); UTF-8 AND THE MATCHING INTERFACE By default, pattern and text are plain text, one byte per character. The UTF8 flag, passed to the constructor, causes both pattern and string to be treated as UTF-8 text, still a byte stream but potentially multiple bytes per character. In practice, the text is likelier to be UTF-8 than the pattern, but the match returned may depend on the UTF8 flag, so always use it when matching UTF8 text. For example, "." will match one byte normally but with UTF8 set may match up to three bytes of a multi-byte character. Example: pcrecpp::RE_Options options; options.set_utf8(); pcrecpp::RE re(utf8_pattern, options); re.FullMatch(utf8_string); Example: using the convenience function UTF8(): pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8()); re.FullMatch(utf8_string); NOTE: The UTF8 flag is ignored if pcre was not configured with the --enable-utf8 flag. PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE PCRE defines some modifiers to change the behavior of the regular expression engine. The C++ wrapper defines an auxiliary class, RE_Options, as a vehicle to pass such modifiers to a RE class. Cur- rently, the following modifiers are supported: modifier description Perl corresponding PCRE_CASELESS case insensitive match /i PCRE_MULTILINE multiple lines match /m PCRE_DOTALL dot matches newlines /s PCRE_DOLLAR_ENDONLY $ matches only at end N/A PCRE_EXTRA strict escape parsing N/A PCRE_EXTENDED ignore white spaces /x PCRE_UTF8 handles UTF8 chars built-in PCRE_UNGREEDY reverses * and *? N/A PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*) (*) Both Perl and PCRE allow non capturing parentheses by means of the "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap- ture, while (ab|cd) does. For a full account on how each modifier works, please check the PCRE API reference page. For each modifier, there are two member functions whose name is made out of the modifier in lowercase, without the "PCRE_" prefix. For instance, PCRE_CASELESS is handled by bool caseless() which returns true if the modifier is set, and RE_Options & set_caseless(bool) which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can be accessed through the set_match_limit() and match_limit() member functions. Setting match_limit to a non-zero value will limit the exe- cution of pcre to keep it from doing bad things like blowing the stack or taking an eternity to return a result. A value of 5000 is good enough to stop stack blowup in a 2MB thread stack. Setting match_limit to zero disables match limiting. Alternatively, you can call match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to limit how much PCRE recurses. match_limit() limits the number of matches PCRE does; match_limit_recursion() limits the depth of internal recursion, and therefore the amount of stack that is used. Normally, to pass one or more modifiers to a RE class, you declare a RE_Options object, set the appropriate options, and pass this object to a RE constructor. Example: RE_Options opt; opt.set_caseless(true); if (RE("HELLO", opt).PartialMatch("hello world")) ... RE_options has two constructors. The default constructor takes no argu- ments and creates a set of flags that are off by default. The optional parameter option_flags is to facilitate transfer of legacy code from C programs. This lets you do RE(pattern, RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str); However, new code is better off doing RE(pattern, RE_Options().set_caseless(true).set_multiline(true)) .PartialMatch(str); If you are going to pass one of the most used modifiers, there are some convenience functions that return a RE_Options class with the appropri- ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(), and EXTENDED(). If you need to set several options at once, and you don't want to go through the pains of declaring a RE_Options object and setting several options, there is a parallel method that give you such ability on the fly. You can concatenate several set_xxxxx() member functions, since each of them returns a reference to its class object. For example, to pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one statement, you may write: RE(" ^ xyz \\s+ .* blah$", RE_Options() .set_caseless(true) .set_extended(true) .set_multiline(true)).PartialMatch(sometext); SCANNING TEXT INCREMENTALLY The "Consume" operation may be useful if you want to repeatedly match regular expressions at the front of a string and skip over them as they match. This requires use of the "StringPiece" type, which represents a sub-range of a real string. Like RE, StringPiece is defined in the pcrecpp namespace. Example: read lines of the form "var = value" from a string. string contents = ...; // Fill string somehow pcrecpp::StringPiece input(contents); // Wrap in a StringPiece string var; int value; pcrecpp::RE re("(\\w+) = (\\d+)\n"); while (re.Consume(&input, &var, &value)) { ...; } Each successful call to "Consume" will set "var/value", and also advance "input" so it points past the matched text. The "FindAndConsume" operation is similar to "Consume" but does not anchor your match at the beginning of the string. For example, you could extract all words from a string by repeatedly calling pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word) PARSING HEX/OCTAL/C-RADIX NUMBERS By default, if you pass a pointer to a numeric value, the corresponding text is interpreted as a base-10 number. You can instead wrap the pointer with a call to one of the operators Hex(), Octal(), or CRadix() to interpret the text in another base. The CRadix operator interprets C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to base-10. Example: int a, b, c, d; pcrecpp::RE re("(.*) (.*) (.*) (.*)"); re.FullMatch("100 40 0100 0x40", pcrecpp::Octal(&a), pcrecpp::Hex(&b), pcrecpp::CRadix(&c), pcrecpp::CRadix(&d)); will leave 64 in a, b, c, and d. REPLACING PARTS OF STRINGS You can replace the first match of "pattern" in "str" with "rewrite". Within "rewrite", backslash-escaped digits (\1 to \9) can be used to insert text matching corresponding parenthesized group from the pat- tern. \0 in "rewrite" refers to the entire matching text. For example: string s = "yabba dabba doo"; pcrecpp::RE("b+").Replace("d", &s); will leave "s" containing "yada dabba doo". The result is true if the pattern matches and a replacement occurs, false otherwise. GlobalReplace is like Replace except that it replaces all occurrences of the pattern in the string with the rewrite. Replacements are not subject to re-matching. For example: string s = "yabba dabba doo"; pcrecpp::RE("b+").GlobalReplace("d", &s); will leave "s" containing "yada dada doo". It returns the number of replacements made. Extract is like Replace, except that if the pattern matches, "rewrite" is copied into "out" (an additional argument) with substitutions. The non-matching portions of "text" are ignored. Returns true iff a match occurred and the extraction happened successfully; if no match occurs, the string is left unaffected. AUTHOR The C++ wrapper was contributed by Google Inc. Copyright (c) 2007 Google Inc. REVISION Last updated: 08 January 2012 ------------------------------------------------------------------------------ PCRESAMPLE(3) Library Functions Manual PCRESAMPLE(3) NAME PCRE - Perl-compatible regular expressions PCRE SAMPLE PROGRAM A simple, complete demonstration program, to get you started with using PCRE, is supplied in the file pcredemo.c in the PCRE distribution. A listing of this program is given in the pcredemo documentation. If you do not have a copy of the PCRE distribution, you can save this listing to re-create pcredemo.c. The demonstration program, which uses the original PCRE 8-bit library, compiles the regular expression that is its first argument, and matches it against the subject string in its second argument. No PCRE options are set, and default character tables are used. If matching succeeds, the program outputs the portion of the subject that matched, together with the contents of any captured substrings. If the -g option is given on the command line, the program then goes on to check for further matches of the same regular expression in the same subject string. The logic is a little bit tricky because of the possi- bility of matching an empty string. Comments in the code explain what is going on. If PCRE is installed in the standard include and library directories for your operating system, you should be able to compile the demonstra- tion program using this command: gcc -o pcredemo pcredemo.c -lpcre If PCRE is installed elsewhere, you may need to add additional options to the command line. For example, on a Unix-like system that has PCRE installed in /usr/local, you can compile the demonstration program using a command like this: gcc -o pcredemo -I/usr/local/include pcredemo.c \ -L/usr/local/lib -lpcre In a Windows environment, if you want to statically link the program against a non-dll pcre.a file, you must uncomment the line that defines PCRE_STATIC before including pcre.h, because otherwise the pcre_mal- loc() and pcre_free() exported functions will be declared __declspec(dllimport), with unwanted results. Once you have compiled and linked the demonstration program, you can run simple tests like this: ./pcredemo 'cat|dog' 'the cat sat on the mat' ./pcredemo -g 'cat|dog' 'the dog sat on the cat' Note that there is a much more comprehensive test program, called pcretest, which supports many more facilities for testing regular expressions and both PCRE libraries. The pcredemo program is provided as a simple coding example. If you try to run pcredemo when PCRE is not installed in the standard library directory, you may get an error like this on some operating systems (e.g. Solaris): ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or directory This is caused by the way shared library support works on those sys- tems. You need to add -R/usr/local/lib (for example) to the compile command to get round this problem. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 10 January 2012 Copyright (c) 1997-2012 University of Cambridge. ------------------------------------------------------------------------------ PCRELIMITS(3) Library Functions Manual PCRELIMITS(3) NAME PCRE - Perl-compatible regular expressions SIZE AND OTHER LIMITATIONS There are some size limitations in PCRE but it is hoped that they will never in practice be relevant. The maximum length of a compiled pattern is approximately 64K data units (bytes for the 8-bit library, 16-bit units for the 16-bit library, and 32-bit units for the 32-bit library) if PCRE is compiled with the default internal linkage size, which is 2 bytes for the 8-bit and 16-bit libraries, and 4 bytes for the 32-bit library. If you want to process regular expressions that are truly enormous, you can compile PCRE with an internal linkage size of 3 or 4 (when building the 16-bit or 32-bit library, 3 is rounded up to 4). See the README file in the source distribution and the pcrebuild documentation for details. In these cases the limit is substantially larger. However, the speed of execution is slower. All values in repeating quantifiers must be less than 65536. There is no limit to the number of parenthesized subpatterns, but there can be no more than 65535 capturing subpatterns. There is, however, a limit to the depth of nesting of parenthesized subpatterns of all kinds. This is imposed in order to limit the amount of system stack used at compile time. The limit can be specified when PCRE is built; the default is 250. There is a limit to the number of forward references to subsequent sub- patterns of around 200,000. Repeated forward references with fixed upper limits, for example, (?2){0,100} when subpattern number 2 is to the right, are included in the count. There is no limit to the number of backward references. The maximum length of name for a named subpattern is 32 characters, and the maximum number of named subpatterns is 10000. The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or (*THEN) verb is 255 for the 8-bit library and 65535 for the 16-bit and 32-bit libraries. The maximum length of a subject string is the largest positive number that an integer variable can hold. However, when using the traditional matching function, PCRE uses recursion to handle subpatterns and indef- inite repetition. This means that the available stack space may limit the size of a subject string that can be processed by certain patterns. For a discussion of stack issues, see the pcrestack documentation. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 05 November 2013 Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------ PCRESTACK(3) Library Functions Manual PCRESTACK(3) NAME PCRE - Perl-compatible regular expressions PCRE DISCUSSION OF STACK USAGE When you call pcre[16|32]_exec(), it makes use of an internal function called match(). This calls itself recursively at branch points in the pattern, in order to remember the state of the match so that it can back up and try a different alternative if the first one fails. As matching proceeds deeper and deeper into the tree of possibilities, the recursion depth increases. The match() function is also called in other circumstances, for example, whenever a parenthesized sub-pattern is entered, and in certain cases of repetition. Not all calls of match() increase the recursion depth; for an item such as a* it may be called several times at the same level, after matching different numbers of a's. Furthermore, in a number of cases where the result of the recursive call would immediately be passed back as the result of the current call (a "tail recursion"), the function is just restarted instead. The above comments apply when pcre[16|32]_exec() is run in its normal interpretive manner. If the pattern was studied with the PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was success- ful, and the options passed to pcre[16|32]_exec() were not incompati- ble, the matching process uses the JIT-compiled code instead of the match() function. In this case, the memory requirements are handled entirely differently. See the pcrejit documentation for details. The pcre[16|32]_dfa_exec() function operates in an entirely different way, and uses recursion only when there is a regular expression recur- sion or subroutine call in the pattern. This includes the processing of assertion and "once-only" subpatterns, which are handled like subrou- tine calls. Normally, these are never very deep, and the limit on the complexity of pcre[16|32]_dfa_exec() is controlled by the amount of workspace it is given. However, it is possible to write patterns with runaway infinite recursions; such patterns will cause pcre[16|32]_dfa_exec() to run out of stack. At present, there is no protection against this. The comments that follow do NOT apply to pcre[16|32]_dfa_exec(); they are relevant only for pcre[16|32]_exec() without the JIT optimization. Reducing pcre[16|32]_exec()'s stack usage Each time that match() is actually called recursively, it uses memory from the process stack. For certain kinds of pattern and data, very large amounts of stack may be needed, despite the recognition of "tail recursion". You can often reduce the amount of recursion, and there- fore the amount of stack used, by modifying the pattern that is being matched. Consider, for example, this pattern: ([^<]|<(?!inet))+ It matches from wherever it starts until it encounters "<inet" or the end of the data, and is the kind of pattern that might be used when processing an XML file. Each iteration of the outer parentheses matches either one character that is not "<" or a "<" that is not followed by "inet". However, each time a parenthesis is processed, a recursion occurs, so this formulation uses a stack frame for each matched charac- ter. For a long string, a lot of stack is required. Consider now this rewritten pattern, which matches exactly the same strings: ([^<]++|<(?!inet))+ This uses very much less stack, because runs of characters that do not contain "<" are "swallowed" in one item inside the parentheses. Recur- sion happens only when a "<" character that is not followed by "inet" is encountered (and we assume this is relatively rare). A possessive quantifier is used to stop any backtracking into the runs of non-"<" characters, but that is not related to stack usage. This example shows that one way of avoiding stack problems when match- ing long subject strings is to write repeated parenthesized subpatterns to match more than one character whenever possible. Compiling PCRE to use heap instead of stack for pcre[16|32]_exec() In environments where stack memory is constrained, you might want to compile PCRE to use heap memory instead of stack for remembering back- up points when pcre[16|32]_exec() is running. This makes it run a lot more slowly, however. Details of how to do this are given in the pcre- build documentation. When built in this way, instead of using the stack, PCRE obtains and frees memory by calling the functions that are pointed to by the pcre[16|32]_stack_malloc and pcre[16|32]_stack_free variables. By default, these point to malloc() and free(), but you can replace the pointers to cause PCRE to use your own functions. Since the block sizes are always the same, and are always freed in reverse order, it may be possible to implement customized memory handlers that are more efficient than the standard functions. Limiting pcre[16|32]_exec()'s stack usage You can set limits on the number of times that match() is called, both in total and recursively. If a limit is exceeded, pcre[16|32]_exec() returns an error code. Setting suitable limits should prevent it from running out of stack. The default values of the limits are very large, and unlikely ever to operate. They can be changed when PCRE is built, and they can also be set when pcre[16|32]_exec() is called. For details of these interfaces, see the pcrebuild documentation and the section on extra data for pcre[16|32]_exec() in the pcreapi documentation. As a very rough rule of thumb, you should reckon on about 500 bytes per recursion. Thus, if you want to limit your stack usage to 8Mb, you should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can support around 128000 recursions. In Unix-like environments, the pcretest test program has a command line option (-S) that can be used to increase the size of its stack. As long as the stack is large enough, another option (-M) can be used to find the smallest limits that allow a particular pattern to match a given subject string. This is done by calling pcre[16|32]_exec() repeatedly with different limits. Obtaining an estimate of stack usage The actual amount of stack used per recursion can vary quite a lot, depending on the compiler that was used to build PCRE and the optimiza- tion or debugging options that were set for it. The rule of thumb value of 500 bytes mentioned above may be larger or smaller than what is actually needed. A better approximation can be obtained by running this command: pcretest -m -C The -C option causes pcretest to output information about the options with which PCRE was compiled. When -m is also given (before -C), infor- mation about stack use is given in a line like this: Match recursion uses stack: approximate frame size = 640 bytes The value is approximate because some recursions need a bit more (up to perhaps 16 more bytes). If the above command is given when PCRE is compiled to use the heap instead of the stack for recursion, the value that is output is the size of each block that is obtained from the heap. Changing stack size in Unix-like systems In Unix-like environments, there is not often a problem with the stack unless very long strings are involved, though the default limit on stack size varies from system to system. Values from 8Mb to 64Mb are common. You can find your default limit by running the command: ulimit -s Unfortunately, the effect of running out of stack is often SIGSEGV, though sometimes a more explicit error message is given. You can nor- mally increase the limit on stack size by code such as this: struct rlimit rlim; getrlimit(RLIMIT_STACK, &rlim); rlim.rlim_cur = 100*1024*1024; setrlimit(RLIMIT_STACK, &rlim); This reads the current limits (soft and hard) using getrlimit(), then attempts to increase the soft limit to 100Mb using setrlimit(). You must do this before calling pcre[16|32]_exec(). Changing stack size in Mac OS X Using setrlimit(), as described above, should also work on Mac OS X. It is also possible to set a stack size when linking a program. There is a discussion about stack sizes in Mac OS X at this web site: http://developer.apple.com/qa/qa2005/qa1419.html. AUTHOR Philip Hazel University Computing Service Cambridge CB2 3QH, England. REVISION Last updated: 24 June 2012 Copyright (c) 1997-2012 University of Cambridge. ------------------------------------------------------------------------------