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diff --git a/doc/pcre.txt b/doc/pcre.txt index 698baa5..fdf0d6f 100644 --- a/doc/pcre.txt +++ b/doc/pcre.txt @@ -1,3 +1,4 @@ +----------------------------------------------------------------------------- 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 @@ -12,19 +13,19 @@ PCRE(3) PCRE(3) NAME PCRE - Perl-compatible regular expressions -DESCRIPTION +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. The current implementation of PCRE (release - 4.x) corresponds approximately with Perl 5.8, including support for - UTF-8 encoded strings. However, this support has to be explicitly - enabled; it is not the default. + 5.x) corresponds approximately with Perl 5.8, including support for + UTF-8 encoded strings and Unicode general category properties. However, + this support has to be explicitly enabled; it is not the default. - PCRE is written in C and released as a C library. However, a number of - people have written wrappers and interfaces of various kinds. A C++ - class is included in these contributions, which can be found in the - Contrib directory at the primary FTP site, which is: + PCRE is written in C and released as a C library. A number of people + have written wrappers and interfaces of various kinds. A C++ class is + included in these contributions, which can be found in the Contrib + directory at the primary FTP site, which is: ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre @@ -34,18 +35,19 @@ DESCRIPTION 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. Documentation about - building PCRE for various operating systems can be found in the README - file in the source distribution. + 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 file + in the source distribution. USER DOCUMENTATION - The user documentation for PCRE has been split up into a number of dif- - ferent sections. 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, all the sections are concate- - nated, for ease of searching. The sections are as follows: + 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, all the sections are concatenated, for ease + of searching. The sections are as follows: pcre this document pcreapi details of PCRE's native API @@ -53,12 +55,14 @@ USER DOCUMENTATION pcrecallout details of the callout feature pcrecompat discussion of Perl compatibility pcregrep description of the pcregrep command + pcrepartial details of the partial matching facility pcrepattern syntax and semantics of supported regular expressions pcreperform discussion of performance issues pcreposix the POSIX-compatible API + pcreprecompile details of saving and re-using precompiled patterns pcresample discussion of the sample program - pcretest the pcretest testing command + pcretest description of the pcretest testing command In addition, in the "man" and HTML formats, there is a short page for each library function, listing its arguments and results. @@ -74,7 +78,7 @@ LIMITATIONS process regular expressions that are truly enormous, you can compile PCRE with an internal linkage size of 3 or 4 (see the README file in the source distribution and the pcrebuild documentation for details). - If these cases the limit is substantially larger. However, the speed + In these cases the limit is substantially larger. However, the speed of execution will be slower. All values in repeating quantifiers must be less than 65536. The maxi- @@ -92,67 +96,83 @@ LIMITATIONS processed by certain patterns. -UTF-8 SUPPORT +UTF-8 AND UNICODE PROPERTY SUPPORT - Starting at release 3.3, PCRE has had some support for character - strings encoded in the UTF-8 format. For release 4.0 this has been - greatly extended to cover most common requirements. + From release 3.3, PCRE has had some support for character strings + encoded in the UTF-8 format. For release 4.0 this was greatly extended + to cover most common requirements, and in release 5.0 additional sup- + port for Unicode general category properties was added. - In order process UTF-8 strings, you must build PCRE to include UTF-8 - support in the code, and, in addition, you must call pcre_compile() - with the PCRE_UTF8 option flag. When you do this, both the pattern and - any subject strings that are matched against it are treated as UTF-8 + In order process UTF-8 strings, you must build PCRE to include UTF-8 + support in the code, and, in addition, you must call pcre_compile() + with the PCRE_UTF8 option flag. When you do this, both the pattern and + any subject strings that are matched against it are treated as UTF-8 strings instead of just strings of bytes. - If you compile PCRE with UTF-8 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_UTF8 flag in several places, so should + If you compile PCRE with UTF-8 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_UTF8 flag in several places, so should not be very large. + If PCRE is built with Unicode character property support (which implies + UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup- + ported. 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. A full list is given in the pcrepattern documen- + tation. The PCRE library is increased in size by about 90K when Unicode + property support is included. + The following comments apply when PCRE is running in UTF-8 mode: - 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and - subjects are checked for validity on entry to the relevant functions. + 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and + subjects are checked for validity on entry to the relevant functions. If an invalid UTF-8 string is passed, an error return is given. In some - situations, you may already know that your strings are valid, and + situations, you may already know that your strings are valid, and therefore want to skip these checks in order to improve performance. 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. If you pass an invalid UTF-8 string to PCRE when - PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program may + 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. If you pass an invalid UTF-8 string to PCRE when + PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program may crash. 2. In a pattern, the escape sequence \x{...}, where the contents of the - braces is a string of hexadecimal digits, is interpreted as a UTF-8 - character whose code number is the given hexadecimal number, for exam- - ple: \x{1234}. If a non-hexadecimal digit appears between the braces, + braces is a string of hexadecimal digits, is interpreted as a UTF-8 + character whose code number is the given hexadecimal number, for exam- + ple: \x{1234}. If a non-hexadecimal digit appears between the braces, the item is not recognized. This escape sequence can be used either as a literal, or within a character class. - 3. The original hexadecimal escape sequence, \xhh, matches a two-byte + 3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8 character if the value is greater than 127. - 4. Repeat quantifiers apply to complete UTF-8 characters, not to indi- + 4. Repeat quantifiers apply to complete UTF-8 characters, not to indi- vidual bytes, for example: \x{100}{3}. - 5. The dot metacharacter matches one UTF-8 character instead of a - single byte. + 5. The dot metacharacter matches one UTF-8 character instead of a sin- + gle byte. - 6. The escape sequence \C can be used to match a single byte in UTF-8 + 6. The escape sequence \C can be used to match a single byte in UTF-8 mode, but its use can lead to some strange effects. - 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly - test characters of any code value, but the characters that PCRE recog- - nizes as digits, spaces, or word characters remain the same set as - before, all with values less than 256. + 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly + test characters of any code value, but the characters that PCRE recog- + nizes as digits, spaces, or word characters remain the same set as + before, all with values less than 256. This remains true even when PCRE + includes Unicode property support, because to do otherwise would slow + down PCRE in many common cases. If you really want to test for a wider + sense of, say, "digit", you must use Unicode property tests such as + \p{Nd}. - 8. Case-insensitive matching applies only to characters whose values - are less than 256. PCRE does not support the notion of "case" for - higher-valued characters. + 8. Similarly, characters that match the POSIX named character classes + are all low-valued characters. - 9. PCRE does not support the use of Unicode tables and properties or - the Perl escapes \p, \P, and \X. + 9. Case-insensitive matching applies only to characters whose values + are less than 128, unless PCRE is built with Unicode property support. + Even when Unicode property support is available, PCRE still uses its + own character tables when checking the case of low-valued characters, + so as not to degrade performance. The Unicode property information is + used only for characters with higher values. AUTHOR @@ -162,8 +182,8 @@ AUTHOR Cambridge CB2 3QG, England. Phone: +44 1223 334714 -Last updated: 20 August 2003 -Copyright (c) 1997-2003 University of Cambridge. +Last updated: 09 September 2004 +Copyright (c) 1997-2004 University of Cambridge. ----------------------------------------------------------------------------- PCRE(3) PCRE(3) @@ -177,10 +197,10 @@ PCRE BUILD-TIME OPTIONS This document describes the optional features of PCRE that can be selected when the library is compiled. They are all selected, or dese- - lected, by providing options to the configure script which is run - before the make command. The complete list of options for configure - (which includes the standard ones such as the selection of the instal- - lation directory) can be obtained by running + lected, by providing options to the configure script that is run before + the make command. 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 @@ -204,6 +224,25 @@ UTF-8 SUPPORT function. +UNICODE CHARACTER PROPERTY SUPPORT + + UTF-8 support allows PCRE to process character values greater than 255 + in the strings that it handles. On its own, however, it does not pro- + vide any facilities for accessing the properties of such characters. 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-8 support, even if you have + not explicitly requested it. + + Including Unicode property support adds around 90K of tables to the + PCRE library, approximately doubling its size. Only the general cate- + gory properties such as Lu and Nd are supported. Details are given in + the pcrepattern documentation. + + CODE VALUE OF NEWLINE By default, PCRE treats character 10 (linefeed) as the newline charac- @@ -231,9 +270,9 @@ BUILDING SHARED AND STATIC LIBRARIES POSIX MALLOC USAGE - When PCRE 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 + When PCRE is called through the POSIX interface (see the pcreposix doc- + umentation), 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 function uses space on the stack, because this is faster than using malloc() for each call. @@ -247,13 +286,13 @@ POSIX MALLOC USAGE LIMITING PCRE RESOURCE USAGE - Internally, PCRE has a function called match() which it calls repeat- - edly (possibly recursively) when performing a matching operation. By - limiting the number of times this function may be called, 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 + Internally, PCRE has a function called match(), which it calls repeat- + edly (possibly recursively) when matching a pattern. 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 documentation. The default is 10 million, but + this can be changed by adding a setting such as --with-match-limit=500000 @@ -264,7 +303,7 @@ 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 two-byte values are used for these + nation metacharacter). By default, 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 enormous patterns, so it @@ -297,9 +336,9 @@ AVOIDING EXCESSIVE STACK USAGE --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 - management functions. Separate functions are provided because the usage - is very predictable: the block sizes requested are always the same, and + pcre_stack_malloc and pcre_stack_free variables to call memory manage- + ment functions. Separate functions are provided 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 the standard malloc() and free() functions. PCRE runs noticeably more @@ -309,15 +348,16 @@ AVOIDING EXCESSIVE STACK USAGE USING EBCDIC CODE PCRE assumes by default that it will run in an environment where the - character code is ASCII (or UTF-8, which is a superset of ASCII). PCRE - can, however, be compiled to run in an EBCDIC environment by adding + character code is ASCII (or Unicode, which is a superset of ASCII). + PCRE can, however, be compiled to run in an EBCDIC environment by + adding --enable-ebcdic to the configure command. -Last updated: 09 December 2003 -Copyright (c) 1997-2003 University of Cambridge. +Last updated: 09 September 2004 +Copyright (c) 1997-2004 University of Cambridge. ----------------------------------------------------------------------------- PCRE(3) PCRE(3) @@ -327,7 +367,7 @@ PCRE(3) PCRE(3) NAME PCRE - Perl-compatible regular expressions -SYNOPSIS OF PCRE API +PCRE NATIVE API #include <pcre.h> @@ -392,81 +432,95 @@ SYNOPSIS OF PCRE API int (*pcre_callout)(pcre_callout_block *); -PCRE API +PCRE API OVERVIEW PCRE has its own native API, which is described in this document. There is also a set of wrapper functions that correspond to the POSIX regular expression API. These are described in the pcreposix documentation. The native API function prototypes are defined in the header file - pcre.h, and on Unix systems the library itself is called libpcre.a, so - can be accessed by adding -lpcre to the command for linking an applica- - tion which calls it. 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. + pcre.h, and on Unix systems the 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 num- + bers for the library. Applications can use these to include support + for different releases of PCRE. The functions pcre_compile(), pcre_study(), and pcre_exec() are used for compiling and matching regular expressions. A sample program that - demonstrates the simplest way of using them is given in the file pcre- - demo.c. The pcresample documentation describes how to run it. + demonstrates the simplest way of using them is provided in the file + called pcredemo.c in the source distribution. The pcresample documenta- + tion describes how to run it. - There are convenience functions for extracting captured substrings from - a matched subject string. They are: + In addition to the main compiling and matching functions, there are + convenience functions for extracting captured substrings from a matched + subject string. They are: pcre_copy_substring() pcre_copy_named_substring() pcre_get_substring() pcre_get_named_substring() pcre_get_substring_list() + pcre_get_stringnumber() 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 (optionally) to build a set of - character tables in the current locale for passing to pcre_compile(). + The function pcre_maketables() is used to build a set of character + tables in the current locale for passing to pcre_compile() or + pcre_exec(). This is an optional facility that is provided for spe- + cialist 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; pcre_info() is an obsolete version which returns only - some of the available information, but is retained for backwards com- - patibility. The function pcre_version() returns a pointer to a string + The function pcre_fullinfo() is used to find out information about a + compiled pattern; pcre_info() is an obsolete version that returns only + some of the available information, but is retained for backwards com- + patibility. The function pcre_version() returns a pointer to a string containing the version of PCRE and its date of release. - The global variables pcre_malloc and pcre_free initially contain the - entry points of the standard malloc() and free() functions respec- + 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 + 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. This is a non-standard way - of building PCRE, for use in environments 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 + 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. This is a non-standard way + of building PCRE, for use in environments 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 are always called - in a stack-like manner (last obtained, first freed), and always for + in a stack-like manner (last obtained, first freed), and always for memory blocks of the same size. 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 + 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. MULTITHREADING - The PCRE functions can be used in multi-threading applications, with + 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 function pointed to by pcre_callout, are shared by all threads. - The compiled form of a regular expression is not altered during match- + 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. +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. + + CHECKING BUILD-TIME OPTIONS int pcre_config(int what, void *where); @@ -486,42 +540,47 @@ CHECKING BUILD-TIME OPTIONS The output is an integer that is set to one if UTF-8 support is avail- able; otherwise it is set to zero. + 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_NEWLINE - The output is an integer that is set to the value of the code that is - used for the newline character. It is either linefeed (10) or carriage - return (13), and should normally be the standard character for your + The output is an integer that is set to the value of the code that is + used for the newline character. It is either linefeed (10) or carriage + return (13), and should normally be the standard character for your operating system. PCRE_CONFIG_LINK_SIZE - The output is an integer that contains the number of bytes used for + The output is an integer that contains the number of bytes used for internal linkage in compiled regular expressions. The value is 2, 3, or - 4. Larger values allow larger regular expressions to be compiled, at - the expense of slower matching. The default value of 2 is sufficient - for all but the most massive patterns, since it allows the compiled + 4. Larger values allow larger regular expressions to be compiled, at + the expense of slower matching. 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. 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 + 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_MATCH_LIMIT The output is an integer that gives the default limit for the number of - internal matching function calls in a pcre_exec() execution. Further + internal matching function calls 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 is - implemented by recursive function calls that use the stack to remember + The output is an integer that is set to one if internal recursion 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 + 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. @@ -531,29 +590,29 @@ COMPILING A PATTERN const char **errptr, int *erroffset, const unsigned char *tableptr); - - The function pcre_compile() 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. 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 when + The function pcre_compile() 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 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 when it is no longer required. - Although the compiled code of a PCRE regex is relocatable, that is, it + 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 contains a copy of the tableptr argument, - which is an address (see below). + fully relocatable, because it may contain a copy of the tableptr argu- + ment, which is an address (see below). The options argument contains independent bits that affect the compila- - tion. It should be zero if no options are required. Some of the - options, in particular, those that are compatible with Perl, can also - be set and unset from within the pattern (see the detailed description - of regular expressions in the pcrepattern documentation). For these - options, the contents of the options argument specifies their initial - settings at the start of compilation and execution. The PCRE_ANCHORED - option can be set at the time of matching as well as at compile time. + tion. 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, can also be set and unset from within the + pattern (see the detailed description in the pcrepattern documenta- + tion). For these options, the contents of the options argument speci- + fies their initial settings at the start of compilation and execution. + The PCRE_ANCHORED option 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 @@ -564,11 +623,14 @@ COMPILING A PATTERN given. If the final argument, tableptr, is NULL, PCRE uses a default set of - character tables which are built when it is compiled, using the default - C locale. Otherwise, tableptr must be the result of a call to - pcre_maketables(). See the section on locale support below. - - This code fragment shows a typical straightforward call to pcre_com- + 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(), unless another table + pointer is passed to it. For more discussion, see the section on locale + support below. + + This code fragment shows a typical straightforward call to pcre_com- pile(): pcre *re; @@ -581,21 +643,30 @@ COMPILING A PATTERN &erroffset, /* for error offset */ NULL); /* use default character tables */ - The following option bits are defined: + 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 - which is being searched (the "subject string"). This effect can also be + 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_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. + changed within a pattern by a (?i) option setting. When running in + UTF-8 mode, case support for high-valued characters is available only + when PCRE is built with Unicode character property support. PCRE_DOLLAR_ENDONLY @@ -645,11 +716,11 @@ COMPILING A PATTERN PCRE_MULTILINE By default, PCRE treats the subject string as consisting of a single - "line" of characters (even if it actually contains several newlines). - The "start of line" metacharacter (^) matches only at the start of the - string, while the "end of line" metacharacter ($) matches only at the - end of the string, or before a terminating newline (unless PCRE_DOL- - LAR_ENDONLY is set). This is the same as Perl. + line of characters (even if it actually contains newlines). The "start + of line" metacharacter (^) matches only at the start of the string, + while the "end of line" metacharacter ($) matches only at the end of + the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY + is set). This 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 any new- @@ -678,10 +749,10 @@ COMPILING A PATTERN This option causes PCRE to regard both the pattern and the subject as strings of UTF-8 characters instead of single-byte character strings. - However, it is available only if PCRE has been built to include UTF-8 - 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 section - on UTF-8 support in the main pcre page. + However, it is available only when PCRE is built to include UTF-8 sup- + port. If not, the use of this option provokes an error. Details of how + this option changes the behaviour of PCRE are given in the section on + UTF-8 support in the main pcre page. PCRE_NO_UTF8_CHECK @@ -691,9 +762,9 @@ COMPILING A PATTERN is valid, and you want to skip this check for performance reasons, 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. Note that there is a similar option for sup- - pressing the checking of subject strings passed to pcre_exec(). - + your program to crash. Note that this option can also be passed to + pcre_exec(), to suppress the UTF-8 validity checking of subject + strings. STUDYING A PATTERN @@ -701,29 +772,31 @@ STUDYING A PATTERN pcre_extra *pcre_study(const pcre *code, int options, const char **errptr); - When a 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 match- - ing. The function pcre_study() takes a pointer to a compiled pattern as - its first argument. If studing the pattern produces additional informa- - tion 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 a pcre_study() can be passed directly to - pcre_exec(). However, the pcre_extra block also contains other fields - that can be set by the caller before the block is passed; these are - described below. If studying the pattern does not produce any addi- - tional information, pcre_study() returns NULL. In that circumstance, if - the calling program wants to pass some of the other fields to - pcre_exec(), it must set up its own pcre_extra block. - - The second argument contains option bits. At present, no options are - defined for pcre_study(), and this argument should always be zero. - - 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 points to a textual error mes- - sage. You should therefore test the error pointer for NULL after call- + 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(). However, a pcre_extra block also contains 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 additional information, + pcre_study() returns NULL. In that circumstance, if the calling program + wants to pass any of the other fields to pcre_exec(), it must set up + its own pcre_extra block. + + The second argument of pcre_study() contains option bits. At present, + no options are defined, and this argument should always be zero. + + 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 points to a textual error mes- + sage. You should therefore test the error pointer for NULL after call- ing pcre_study(), to be sure that it has run successfully. This is a typical call to pcre_study(): @@ -735,40 +808,53 @@ STUDYING A PATTERN &error); /* set to NULL or points to a message */ At present, studying a pattern is useful only for non-anchored patterns - that do not have a single fixed starting character. A bitmap of possi- - ble starting characters is created. + that do not have a single fixed starting character. A bitmap of possi- + ble starting bytes is created. LOCALE SUPPORT - PCRE handles caseless matching, and determines whether characters are - letters, digits, or whatever, by reference to a set of tables. When - running in UTF-8 mode, this applies only to characters with codes less - than 256. The library contains a default set of tables that is created - in the default C locale when PCRE is compiled. This is used when the - final argument of pcre_compile() is NULL, and is sufficient for many - applications. - - An alternative set of tables can, however, be supplied. Such tables are - built by calling the pcre_maketables() function, which has no argu- - ments, 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 char- - acters with codes greater than 128 are treated as letters), the follow- - ing code could be used: - - setlocale(LC_CTYPE, "fr"); + PCRE handles caseless matching, and determines whether characters are + letters, digits, or whatever, by reference to a set of tables, indexed + by character value. (When running in UTF-8 mode, this applies only to + characters with codes less than 128. Higher-valued codes never match + escapes such as \w or \d, but can be tested with \p if PCRE is built + with Unicode character property support.) + + An internal set of tables is created in the default C locale when PCRE + is built. This is used when the final argument of pcre_compile() is + NULL, and is sufficient for many applications. An alternative set of + tables can, however, be supplied. These may be created in a different + locale from the default. As more and more applications change to using + Unicode, 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() or pcre_exec() 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 letters), the following code could be used: + + setlocale(LC_CTYPE, "fr_FR"); tables = pcre_maketables(); re = pcre_compile(..., tables); - The tables are built in memory that is obtained via pcre_malloc. The - pointer that is passed to pcre_compile is saved with the compiled pat- - tern, and the same tables are used via this pointer by pcre_study() and - pcre_exec(). Thus, for any single pattern, compilation, studying and - matching all happen in the same locale, but different patterns can be - compiled in different locales. It is the caller's responsibility to - ensure that the memory containing the tables remains available for as - long as it is needed. + 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 normally also by pcre_exec(). Thus, by default, for any single pat- + tern, compilation, studying and matching all happen in the same locale, + but different patterns can be compiled in different locales. + + It is possible to pass a table pointer or NULL (indicating the use of + the internal tables) to pcre_exec(). Although not intended for this + purpose, this facility could be used to match a pattern in a different + locale from the one in which it was compiled. Passing table pointers at + run time is discussed below in the section on matching a pattern. INFORMATION ABOUT A PATTERN @@ -792,8 +878,10 @@ INFORMATION ABOUT A PATTERN PCRE_ERROR_BADMAGIC the "magic number" was not found PCRE_ERROR_BADOPTION the value of what was invalid - Here is a typical call of pcre_fullinfo(), to obtain the length of the - compiled pattern: + The "magic number" is placed at the start of each compiled pattern as + an simple check against passing an arbitrary memory pointer. Here is a + typical call of pcre_fullinfo(), to obtain the length of the compiled + pattern: int rc; unsigned long int length; @@ -817,6 +905,14 @@ INFORMATION ABOUT A PATTERN Return the number of capturing subpatterns in the pattern. The fourth argument should point to an int variable. + PCRE_INFO_DEFAULTTABLES + + 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 Return information about the first byte of any matched string, for a @@ -824,7 +920,7 @@ INFORMATION ABOUT A PATTERN PCRE_INFO_FIRSTCHAR; the old name is still recognized for backwards compatibility.) - If there is a fixed first byte, e.g. from a pattern such as + If there is a fixed first byte, for example, from a pattern such as (cat|cow|coyote), it is returned in the integer pointed to by where. Otherwise, if either @@ -862,11 +958,13 @@ INFORMATION ABOUT A PATTERN 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 a number. A caller that wants to extract data - from a named subpattern must convert the name to a number in order to - access the correct pointers in the output vector (described with - pcre_exec() below). In order to do this, it must first use these three - values to obtain the name-to-number mapping table for the pattern. + ses, which still acquire numbers. A convenience function called + pcre_get_named_substring() is provided for extracting an individual + captured substring 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 @@ -884,15 +982,17 @@ INFORMATION ABOUT A PATTERN 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 hex, and undefined bytes shown as ??: + 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, remember that - the length of each entry may be different for each compiled pattern. + When writing code to extract data from named subpatterns using the + name-to-number map, remember that the length of each entry is likely to + be different for each compiled pattern. PCRE_INFO_OPTIONS @@ -922,8 +1022,8 @@ INFORMATION ABOUT A PATTERN PCRE_INFO_STUDYSIZE - Returns the size of the data block pointed to by the study_data field - in a pcre_extra block. That is, it is the value that was passed to + Return the size of the data block pointed to by the study_data field in + a pcre_extra block. That is, it is the value that was passed to pcre_malloc() when PCRE was getting memory into which to place the data created by pcre_study(). The fourth argument should point to a size_t variable. @@ -958,9 +1058,15 @@ MATCHING A PATTERN int options, int *ovector, int ovecsize); The function pcre_exec() is called to match a subject string against a - pre-compiled pattern, which is passed in the code argument. If the pat- - tern has been studied, the result of the study should be passed in the - extra argument. + compiled pattern, which is passed in the code argument. If the pattern + has been studied, the result of the study should be passed in the extra + argument. + + 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(): @@ -973,18 +1079,23 @@ MATCHING A PATTERN 11, /* the length of the subject string */ 0, /* start at offset 0 in the subject */ 0, /* default options */ - ovector, /* vector for substring information */ - 30); /* number of elements in the vector */ + ovector, /* vector of integers for substring information */ + 30); /* number of elements in the vector (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 fields in the block are as follows: + 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 fields in a pcre_extra block are as fol- + lows: unsigned long int flags; void *study_data; unsigned long int match_limit; void *callout_data; + const unsigned char *tables; The flags field is a bitmap that specifies which of the other fields are set. The flag bits are: @@ -992,153 +1103,190 @@ MATCHING A PATTERN PCRE_EXTRA_STUDY_DATA PCRE_EXTRA_MATCH_LIMIT PCRE_EXTRA_CALLOUT_DATA + PCRE_EXTRA_TABLES Other flag bits should be set to zero. The study_data field is set in the pcre_extra block that is returned by pcre_study(), together with - the appropriate flag bit. You should not set this yourself, but you can - add to the block by setting the other fields. + the appropriate flag bit. You should not set this yourself, but you may + add to the block by setting the 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 the use of nested unlimited - repeats. Internally, PCRE uses a function called match() which it calls - repeatedly (sometimes recursively). The limit is imposed on the number - of times this function is called during a match, which has the effect - of limiting the amount of recursion and backtracking that can take - place. For patterns that are not anchored, the count starts from zero - for each position in the subject string. - - The default limit for the library can be set when PCRE is built; the - default default is 10 million, which handles all but the most extreme - cases. You can reduce the default by suppling pcre_exec() with a - pcre_extra block in which match_limit is set to a smaller value, and - PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is + 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 the use of nested unlimited + repeats. + + Internally, PCRE uses a function called match() which it calls repeat- + edly (sometimes recursively). The limit is imposed on the number of + times this function is called during a match, which has the effect of + limiting the amount of recursion and backtracking that can take place. + For patterns that are not anchored, the count starts from zero for each + position in the subject string. + + The default limit for the library can be set when PCRE is built; the + default default is 10 million, which handles all but the most extreme + cases. You can reduce the default by suppling pcre_exec() with a + pcre_extra block in which match_limit is set to a smaller value, and + PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT. - The pcre_callout field is used in conjunction with the "callout" fea- + The pcre_callout field is used in conjunction with the "callout" fea- ture, which is described in the pcrecallout documentation. - The PCRE_ANCHORED option can be passed in the options argument, whose - unused bits must be zero. This limits pcre_exec() to matching at the - first matching position. However, 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. + The tables field is used to pass a character tables pointer to + pcre_exec(); this overrides the value that is stored with the compiled + pattern. A non-NULL value is stored with the compiled pattern only if + custom tables were supplied to pcre_compile() via its tableptr argu- + ment. If NULL is passed to pcre_exec() using this mechanism, it forces + PCRE's internal tables to be used. This facility is helpful when re- + using patterns that have been saved after compiling with an external + set of tables, because the external tables might be at a different + address when pcre_exec() is called. See the pcreprecompile documenta- + tion for a discussion of saving compiled patterns for later use. - When PCRE_UTF8 was set at compile time, the validity of the subject as - a UTF-8 string is automatically checked, and the value of startoffset - is also checked to ensure that it points to the start of a UTF-8 char- - acter. If an invalid UTF-8 sequence of bytes is found, pcre_exec() - returns the error PCRE_ERROR_BADUTF8. If startoffset contains an - invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned. + Option bits for pcre_exec() - 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 UTF-8 character. When PCRE_NO_UTF8_CHECK is - set, the effect of passing an invalid UTF-8 string as a subject, or a - value of startoffset that does not point to the start of a UTF-8 char- - acter, is undefined. Your program may crash. + The unused bits of the options argument for pcre_exec() must be zero. + The only bits that may be set are PCRE_ANCHORED, PCRE_NOTBOL, + PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and PCRE_PARTIAL. + + PCRE_ANCHORED - There are also three further options that can be set only at matching - time: + 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_NOTBOL - The first character of the 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 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 + behaviour of the circumflex metacharacter. It does not affect \A. PCRE_NOTEOL - The end of the string is not the end of a line, so the dollar metachar- - acter should not match it nor (except in multiline mode) a newline - immediately before it. Setting this without PCRE_MULTILINE (at compile - time) causes dollar never to match. + 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 + 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 the - empty string at the start of the subject. With PCRE_NOTEMPTY set, this + is applied to a string not beginning with "a" or "b", it matches the + 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". Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe- - cial case of a pattern match of the empty string within its split() - function, and when using the /g modifier. It is possible to emulate + cial 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 offset with PCRE_NOTEMPTY set, and then if that fails - by advancing the starting offset (see below) and trying an ordinary - match again. + again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then + if that fails by advancing the starting offset (see below) and trying + an ordinary match again. There is some code that demonstrates how to do + this in the pcredemo.c sample program. + + 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 value of startoffset is also checked to ensure that it + points to the start of a UTF-8 character. If an invalid UTF-8 sequence + of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If + startoffset contains an invalid value, 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 UTF-8 character. When PCRE_NO_UTF8_CHECK is + set, the effect of passing an invalid UTF-8 string as a subject, or a + value of startoffset that does not point to the start of a UTF-8 char- + acter, is undefined. Your program may crash. + + PCRE_PARTIAL + + This option turns on the partial matching feature. If the subject + string fails to match the pattern, but at some point during the match- + ing process the end of the subject was reached (that is, the subject + partially matches the pattern and the failure to match occurred only + because there were not enough subject characters), pcre_exec() returns + PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is + used, there are restrictions on what may appear in the pattern. These + are discussed 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 byte offset in startoffset. Unlike the - pattern string, the subject may contain binary zero bytes. 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. - - If the pattern was compiled with the PCRE_UTF8 option, the subject must - be a sequence of bytes that is a valid UTF-8 string, and the starting - offset must point to the beginning of a UTF-8 character. If an invalid - UTF-8 string or offset is passed, an error (either PCRE_ERROR_BADUTF8 - or PCRE_ERROR_BADUTF8_OFFSET) is returned, unless the option - PCRE_NO_UTF8_CHECK is set, in which case PCRE's behaviour is not - defined. - - 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 + length in length, and a starting byte offset in startoffset. In UTF-8 + mode, the byte offset must point to the start of a UTF-8 character. + Unlike the pattern string, the subject may contain binary zero bytes. + 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. + + 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, + 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 - occurrence of "iss" because it is able to look behind the starting - point to discover that it is preceded by a letter. - - If a non-zero starting offset is passed when the pattern is anchored, - one attempt to match at the given offset is tried. This can only suc- - ceed if the pattern does not require the match to be at the start of - the subject. + 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. + + 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 + 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 integer - offsets whose address is passed in ovector. The number of elements in - the vector is passed in ovecsize. The first two-thirds of the vector is - used to pass back captured substrings, each substring using a pair of - integers. The remaining third of the vector is used as workspace by - pcre_exec() while matching capturing subpatterns, and is not available - for passing back information. The length passed in ovecsize should - always be a multiple of three. If it is not, it is rounded down. - - When a match has been successful, information about captured substrings - is returned in pairs of integers, starting at the beginning of ovector, + Captured substrings are returned to the caller via a vector of integer + offsets whose address is passed in ovector. The number of elements in + the vector 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 length 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 a pair is set to the offset of the first character in a sub- string, and the second is set to the offset of the first character @@ -1161,15 +1309,15 @@ MATCHING A PATTERN offset values corresponding to the unused subpattern are set to -1. If a capturing subpattern is matched repeatedly, it is the last portion - of the string that it matched that gets returned. - - If the vector is too small to hold all the captured substrings, it is - used as far as possible (up to two-thirds of its length), and the func- - tion returns a value of zero. In particular, if the substring offsets - are not of interest, pcre_exec() may be called with ovector passed as - NULL and ovecsize as zero. However, if the pattern contains back refer- - ences and the ovector isn't big enough to remember the related sub- - strings, PCRE has to get additional memory for use during matching. + 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. In particular, if the substring off- + sets are not of interest, pcre_exec() may be called with ovector passed + as NULL and ovecsize as zero. However, if the pattern 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. Note that pcre_info() can be used to find out how many capturing sub- @@ -1177,6 +1325,8 @@ MATCHING A PATTERN that will allow for n captured substrings, in addition to the offsets of the substring matched by the whole pattern, is (n+1)*3. + Return values from pcre_exec() + If pcre_exec() fails, it returns a negative number. The following are defined in the header file: @@ -1196,8 +1346,10 @@ MATCHING A PATTERN 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. This is the error - it gives when the magic number isn't present. + 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_NODE (-5) @@ -1211,7 +1363,7 @@ MATCHING A PATTERN 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 freed at the end of matching. + memory is automatically freed at the end of matching. PCRE_ERROR_NOSUBSTRING (-7) @@ -1242,6 +1394,26 @@ MATCHING A PATTERN value of startoffset did not point to the beginning of a UTF-8 charac- ter. + 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_BAD_PARTIAL (-13) + + The PCRE_PARTIAL option was used with a compiled pattern containing + items that are not supported for partial matching. See the pcrepartial + documentation for details of 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. + EXTRACTING CAPTURED SUBSTRINGS BY NUMBER @@ -1256,117 +1428,121 @@ EXTRACTING CAPTURED SUBSTRINGS BY NUMBER 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 + 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 + 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. - The first three arguments are the same for all three of these func- - tions: subject is the subject string which has just been successfully + 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 size of 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, while - 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 + 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 PCRE_ERROR_NOMEMORY (-6) - The buffer was too small for pcre_copy_substring(), or the attempt to + 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 which 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 + 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 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 + 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- + 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_substring_list() can be used to free the memory returned by a - previous call of pcre_get_substring() or pcre_get_substring_list(), - respectively. 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- + 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 which cannot use - pcre_free directly; it is for these cases that the functions are pro- - vided. + pcre_free directly; it is for these cases that the functions are + provided. 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_stringnumber(const pcre *code, - const char *name); - 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. This can be done by calling pcre_get_stringnumber(). The first - argument is the compiled pattern, and the second is the name. For exam- - ple, for this pattern + To extract a substring by name, you first have to find associated num- + ber. For example, for this pattern + + (a+)b(?<xxx>\d+)... - ab(?<xxx>\d+)... + the number of the subpattern called "xxx" is 2. You can find the number + from the name by calling pcre_get_stringnumber(). The first argument is + the compiled 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. - the number of the subpattern called "xxx" is 1. Given the number, you - can then 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. + 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 functions that - extract by number, and so are not re-described here. There are just two - differences. + 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- + 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 + 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- + These functions call pcre_get_stringnumber(), and if it succeeds, they + then call pcre_copy_substring() or pcre_get_substring(), as appropri- ate. -Last updated: 09 December 2003 -Copyright (c) 1997-2003 University of Cambridge. +Last updated: 09 September 2004 +Copyright (c) 1997-2004 University of Cambridge. ----------------------------------------------------------------------------- PCRE(3) PCRE(3) @@ -1392,11 +1568,47 @@ PCRE CALLOUTS default value is zero. For example, this pattern has two callout points: - (?C1)abc(?C2)def + (?C1)eabc(?C2)def - During matching, when PCRE reaches a callout point (and pcre_callout is - set), the external function is called. Its only argument is a pointer - to a pcre_callout block. This contains the following variables: + If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is + called, 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. Automatic callouts can be used for tracking the + progress of pattern matching. The pcretest command has an option that + sets automatic callouts; when it is used, the output indicates how the + pattern is 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 + matches patterns, callouts sometimes do not happen. 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. + + +THE CALLOUT INTERFACE + + During matching, when PCRE reaches a callout point, the external func- + tion defined by pcre_callout is called (if it is set). The only argu- + ment is a pointer to a pcre_callout block. This structure contains the + following fields: int version; int callout_number; @@ -1408,55 +1620,72 @@ PCRE CALLOUTS int capture_top; int capture_last; void *callout_data; + int pattern_position; + int next_item_length; The version field is an integer containing the version number of the - block format. The current version is zero. The version number may - change in future if additional fields are added, but the intention is - never to remove any of the existing fields. + block format. The initial version was 0; the current version is 1. 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). + 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 pcre_exec(). 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. - The subject and subject_length fields contain copies the values that + The subject and subject_length fields contain copies of the values that were passed to pcre_exec(). The start_match field contains the offset within the subject at which the current match attempt started. If the pattern is not anchored, the - callout function may be called several times for different starting - points. + 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. The capture_top field contains one more than the number of the highest - numbered captured substring so far. If no substrings have been - captured, the value of capture_top is one. + numbered captured substring so far. If no substrings have been cap- + tured, the value of capture_top is one. The capture_last field contains the number of the most recently cap- - tured substring. + tured substring. If no substrings have been captured, its value is -1. - The callout_data field contains a value that is passed to pcre_exec() - by the caller specifically so that it can be passed back in callouts. - It is passed in the pcre_callout field of the pcre_extra data struc- - ture. If no such data was passed, the value of callout_data in a - pcre_callout block is NULL. There is a description of the pcre_extra + The callout_data field contains a value that is passed to pcre_exec() + by the caller specifically so that it can be passed back in callouts. + It is passed in the pcre_callout field of the pcre_extra data struc- + ture. If no such data was passed, the value of callout_data in a + pcre_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 pcre_call- + out 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 pcre_call- + out structure. It contains the length of the next item to be matched in + the pattern string. When the callout immediately precedes an alterna- + tion 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. RETURN VALUES - The callout function returns an integer. If the value is zero, matching - proceeds as normal. If the value is greater than zero, matching fails - at the current point, but backtracking to test other possibilities goes - ahead, just as if a lookahead assertion had failed. If the value is - less than zero, the match is abandoned, and pcre_exec() returns the - value. + 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 backtracking to test + other matching possibilities goes ahead, just as if a lookahead asser- + tion had failed. If the value is less than zero, the match is aban- + doned, and pcre_exec() 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- @@ -1464,8 +1693,8 @@ RETURN VALUES reserved for use by callout functions; it will never be used by PCRE itself. -Last updated: 21 January 2003 -Copyright (c) 1997-2003 University of Cambridge. +Last updated: 09 September 2004 +Copyright (c) 1997-2004 University of Cambridge. ----------------------------------------------------------------------------- PCRE(3) PCRE(3) @@ -1475,7 +1704,7 @@ PCRE(3) PCRE(3) NAME PCRE - Perl-compatible regular expressions -DIFFERENCES FROM PERL +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 @@ -1498,18 +1727,23 @@ DIFFERENCES FROM PERL 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. + 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, \P, \p, \N, and \X. 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. - - 6. 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 + \U, and \N. In fact these are implemented by Perl's general string-han- + dling and are not part of its pattern matching engine. If any of these + are encountered by PCRE, an error is generated. + + 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. + + 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 @@ -1519,58 +1753,63 @@ DIFFERENCES FROM PERL \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 + The \Q...\E sequence is recognized both inside and outside character classes. - 7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code}) - constructions. However, there is some experimental support for recur- - sive patterns using the non-Perl items (?R), (?number) and (?P>name). - Also, the PCRE "callout" feature allows an external function to be - called during pattern matching. + 8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code}) + constructions. However, there is support for recursive patterns using + the non-Perl items (?R), (?number), and (?P>name). Also, the PCRE + "callout" feature allows an external function to be called during pat- + tern matching. See the pcrecallout documentation for details. - 8. 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 $2 + 9. There are some differences that are concerned with the settings of + captured strings when part of a pattern is repeated. For example, + matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b". - 9. PCRE provides some extensions to the Perl regular expression - facilities: + 10. PCRE provides some extensions to the Perl regular expression facil- + ities: - (a) Although lookbehind assertions must match fixed length strings, + (a) Although lookbehind assertions 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 $ + (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. - (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti- + (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 to force a pattern to be tried only at - the first matching position in the subject string. + (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, and PCRE_NO_AUTO_CAP- + (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP- TURE options for pcre_exec() have no Perl equivalents. - (g) The (?R), (?number), and (?P>name) constructs allows for recursive - pattern matching (Perl can do this using the (?p{code}) construct, + (g) The (?R), (?number), and (?P>name) constructs allows for recursive + pattern matching (Perl can do this using the (?p{code}) construct, which PCRE cannot support.) - (h) PCRE supports named capturing substrings, using the Python syntax. + (h) PCRE supports named capturing substrings, using the Python syntax. - (i) PCRE supports the possessive quantifier "++" syntax, taken from + (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java package. (j) The (R) condition, for testing recursion, is a PCRE extension. (k) The callout facility is PCRE-specific. -Last updated: 09 December 2003 -Copyright (c) 1997-2003 University of Cambridge. + (l) The partial matching facility is PCRE-specific. + + (m) Patterns compiled by PCRE can be saved and re-used at a later time, + even on different hosts that have the other endianness. + +Last updated: 09 September 2004 +Copyright (c) 1997-2004 University of Cambridge. ----------------------------------------------------------------------------- PCRE(3) PCRE(3) @@ -1584,35 +1823,36 @@ PCRE REGULAR EXPRESSION DETAILS The syntax and semantics of the regular expressions supported by PCRE are described below. Regular expressions are also described in the Perl - documentation and in a number of other books, some of which have copi- - ous examples. Jeffrey Friedl's "Mastering Regular Expressions", pub- - lished by O'Reilly, covers them in great detail. The description here - is intended as reference documentation. - - The basic operation of PCRE is on strings of bytes. However, there is - also support for UTF-8 character strings. To use this support you must - build PCRE to include UTF-8 support, and then call pcre_compile() with - the PCRE_UTF8 option. How this affects the pattern matching is men- - tioned in several places below. There is also a summary of UTF-8 fea- - tures in the section on UTF-8 support in the main pcre page. - - 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 + documentation and 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 descrip- + tion of PCRE's regular expressions is intended as reference material. + + The original operation of PCRE was on strings of one-byte characters. + However, there is now also support for UTF-8 character strings. To use + this, you must build PCRE to include UTF-8 support, and then call + pcre_compile() with the PCRE_UTF8 option. How this affects pattern + matching is mentioned in several places below. There is also a summary + of UTF-8 features in the section on UTF-8 support in the main pcre + page. + + 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. The + matches a portion of a subject string that is identical to itself. The power of regular expressions comes from the ability to include alterna- - tives and repetitions in the pattern. These are encoded in the pattern - by the use of meta-characters, which do not stand for themselves but + tives 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 meta-characters: those that are recog- - nized anywhere in the pattern except within square brackets, and those - that are recognized in square brackets. Outside square brackets, the - meta-characters are as follows: + 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 in 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) @@ -1630,8 +1870,8 @@ PCRE REGULAR EXPRESSION DETAILS 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 meta-characters are: + 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 @@ -1640,33 +1880,33 @@ PCRE REGULAR EXPRESSION DETAILS syntax) ] terminates the character class - The following sections describe the use of each of the meta-characters. + 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 non-alphameric character, it takes away any special meaning that - character may have. This use of backslash as an escape character + a non-alphanumeric character, 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 meta-character, so it is - always safe to precede a non-alphameric with backslash to specify that - it stands for itself. In particular, if you want to match a backslash, - you write \\. + 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 \\. - If a pattern is compiled with the PCRE_EXTENDED option, whitespace in - the pattern (other than in a character class) and characters between a + If a pattern is compiled with the PCRE_EXTENDED option, whitespace in + the pattern (other than in a character class) and characters between a # outside a character class and the next newline character are ignored. - An escaping backslash can be used to include a whitespace or # charac- + An escaping backslash can be used to include a whitespace or # charac- ter 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- + 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 @@ -1676,14 +1916,16 @@ BACKSLASH \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 + The \Q...\E sequence is recognized both inside and outside character classes. + 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 usually easier to use one of the following escape + 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 usually easier to use one of the following escape sequences than the binary character it represents: \a alarm, that is, the BEL character (hex 07) @@ -1697,31 +1939,32 @@ BACKSLASH \xhh character with hex code hh \x{hhh..} character with hex code hhh... (UTF-8 mode only) - The precise effect of \cx 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 \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; + The precise effect of \cx 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 \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex 7B. - After \x, from zero to two hexadecimal digits are read (letters can be - in upper or lower case). In UTF-8 mode, any number of hexadecimal dig- - its may appear between \x{ and }, but the value of the character code - must be less than 2**31 (that is, the maximum hexadecimal value is - 7FFFFFFF). If characters other than hexadecimal digits appear between - \x{ and }, or if there is no terminating }, this form of escape is not + After \x, from zero to two hexadecimal digits are read (letters can be + in upper or lower case). In UTF-8 mode, any number of hexadecimal dig- + its may appear between \x{ and }, but the value of the character code + must be less than 2**31 (that is, the maximum hexadecimal value is + 7FFFFFFF). If characters other than hexadecimal digits appear between + \x{ and }, or if there is no terminating }, this form of escape is not recognized. Instead, the initial \x will be interpreted as a basic hex- - adecimal escape, with no following digits, giving a byte whose value is - zero. + adecimal escape, with no following digits, giving a character whose + value is zero. Characters whose value is less than 256 can be defined by either of the - two syntaxes for \x when PCRE is in UTF-8 mode. There is no difference - in the way they are handled. For example, \xdc is exactly the same as + two syntaxes for \x when PCRE is in UTF-8 mode. There is no difference + in the way they are handled. For example, \xdc is exactly the same as \x{dc}. - After \0 up to two further octal digits are read. In both cases, if - there are fewer than two digits, just those that are present are used. - Thus the sequence \0\x\07 specifies two binary zeros followed by a BEL - character (code value 7). Make sure you supply two digits after the - initial zero if the character that follows is itself an octal digit. + After \0 up to two further octal digits are read. In both cases, if + there are fewer than two digits, just those that are present are used. + Thus the sequence \0\x\07 specifies two binary zeros followed by a BEL + character (code value 7). Make sure you supply two digits after the + initial zero if the pattern character that follows is itself an octal + digit. The handling of a backslash followed by a digit other than 0 is compli- cated. Outside a character class, PCRE reads it and any following dig- @@ -1758,10 +2001,14 @@ BACKSLASH All the sequences that define a single byte value or a single UTF-8 character (in UTF-8 mode) can be used both inside and outside character classes. In addition, inside a character class, the sequence \b is - interpreted as the backspace character (hex 08). Outside a character - class it has a different meaning (see below). + interpreted as the backspace character (hex 08), and the sequence \X is + interpreted as the character "X". Outside a character class, these + sequences have different meanings (see below). + + Generic character types - The third use of backslash is for specifying generic character types: + The third use of backslash is for specifying generic character types. + The following are always recognized: \d any decimal digit \D any character that is not a decimal digit @@ -1774,31 +2021,126 @@ BACKSLASH into two disjoint sets. Any given character matches one, and only one, of each pair. - In UTF-8 mode, characters with values greater than 255 never match \d, - \s, or \w, and always match \D, \S, and \W. + These character type sequences can appear both inside and outside char- + acter 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, since there is no character to match. For compatibility with Perl, \s does not match the VT character (code 11). This makes it different from the the POSIX "space" class. The \s characters are HT (9), LF (10), FF (12), CR (13), and space (32). - A "word" character is any letter or digit or the underscore character, - that is, any character which can be part of a Perl "word". The defini- - tion of letters and digits is controlled by PCRE's character tables, - and may vary if locale- specific matching is taking place (see "Locale - support" in the pcreapi page). For example, in the "fr" (French) - locale, some character codes greater than 128 are used for accented - letters, and these are matched by \w. - - These character type sequences can appear both inside and outside char- - acter 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, since there is no character to match. + A "word" character is an underscore or any character less than 256 that + is a letter or digit. 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 the "fr_FR" (French) locale, some character + codes greater than 128 are used for accented letters, and these are + matched by \w. + + In UTF-8 mode, characters with values greater than 128 never match \d, + \s, or \w, and always match \D, \S, and \W. This is true even when Uni- + code character property support is available. + + Unicode character properties + + When PCRE is built with Unicode character property support, three addi- + tional escape sequences to match generic character types are available + when UTF-8 mode is selected. They are: + + \p{xx} a character with the xx property + \P{xx} a character without the xx property + \X an extended Unicode sequence + + The property names represented by xx above are limited to the Unicode + general category properties. Each character has exactly one such prop- + erty, specified by a two-letter abbreviation. For compatibility with + Perl, negation 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 + 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 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 + + Extended properties such as "Greek" or "InMusicalSymbols" are not sup- + ported by PCRE. + + Specifying caseless matching does not affect these escape sequences. + For example, \p{Lu} always matches only upper case letters. + + The \X escape matches any number of Unicode characters that form an + extended Unicode sequence. \X is equivalent to + + (?>\PM\pM*) + + That is, it matches a character without the "mark" property, followed + by zero or more characters with the "mark" property, and treats the + sequence as an atomic group (see below). Characters with the "mark" + property are typically accents that affect the preceding character. + + Matching characters by Unicode property is not fast, because PCRE has + to search a structure that contains data for over fifteen thousand + characters. That is why the traditional escape sequences such as \d and + \w do not use Unicode properties in PCRE. + + Simple assertions The fourth 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 + The backslashed assertions are: \b matches at a word boundary \B matches when not at a word boundary @@ -1817,31 +2159,32 @@ BACKSLASH string if the first or last character matches \w, respectively. The \A, \Z, and \z assertions differ from the traditional circumflex - and dollar (described below) 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. - - They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the - startoffset argument of pcre_exec() is non-zero, indicating that match- - ing 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 that is the last character of the string as well as at - the end of the string, 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- + 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 that is the last character of the + string as well as at the end of the string, 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 + 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 + 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 + 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. @@ -1849,98 +2192,99 @@ BACKSLASH CIRCUMFLEX AND DOLLAR Outside a character class, in the default matching mode, the circumflex - character is an assertion which 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 + 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 + 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.) - A dollar character is an assertion which is true only if the current - matching point is at the end of the subject string, or immediately + A 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 character that is the last character in the string (by - default). 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. Dollar has no special meaning in a + default). 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. Dollar 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 + 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, they match immedi- - ately after and immediately before an internal newline character, - respectively, in addition to matching at the start and end of the sub- - ject string. For example, the pattern /^abc$/ matches the subject - string "def\nabc" in multiline mode, but not otherwise. Consequently, - patterns that are anchored in single line mode because all branches - start with ^ are not anchored in multiline mode, and a match for cir- - cumflex is possible when the startoffset argument of pcre_exec() is - non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE - is set. - - Note that the sequences \A, \Z, and \z can be used to match the start - and end of the subject in both modes, and if all branches of a pattern - start with \A it is always anchored, whether PCRE_MULTILINE is set or + ately after and immediately before an internal newline character, + respectively, in addition to matching at the start and end of the sub- + ject string. For example, the pattern /^abc$/ matches the subject + string "def\nabc" (where \n represents a newline character) in multi- + line 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_DOL- + LAR_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 PCRE_MULTILINE is set or not. FULL STOP (PERIOD, DOT) Outside a character class, a dot in the pattern matches any one charac- - ter in the subject, including a non-printing character, but not (by - default) newline. In UTF-8 mode, a dot matches any UTF-8 character, - which might be more than one byte long, except (by default) for new- - line. If the PCRE_DOTALL option is set, dots match newlines as well. - The handling of dot is entirely independent of the handling of circum- - flex and dollar, the only relationship being that they both involve - newline characters. Dot has no special meaning in a character class. + ter in the subject, including a non-printing character, but not (by + default) newline. In UTF-8 mode, a dot matches any UTF-8 character, + which might be more than one byte long, except (by default) newline. If + the PCRE_DOTALL option is set, dots match newlines as well. The han- + dling of dot is entirely independent of the handling of circumflex and + dollar, the only relationship being that they both involve newline + characters. Dot has no special meaning in a character class. MATCHING A SINGLE BYTE Outside a character class, the escape sequence \C matches any one byte, - both in and out of UTF-8 mode. Unlike a dot, it always matches a new- - line. The feature is provided in Perl in order to match individual - bytes in UTF-8 mode. Because it breaks up UTF-8 characters into indi- - vidual bytes, what remains in the string may be a malformed UTF-8 - string. For this reason it is best avoided. + both in and out of UTF-8 mode. Unlike a dot, it can match a newline. + The feature is provided in Perl in order to match individual bytes in + UTF-8 mode. Because it breaks up UTF-8 characters into individual + bytes, what remains in the string may be a malformed UTF-8 string. For + this reason, the \C escape sequence is best avoided. - PCRE does not allow \C to appear in lookbehind assertions (see below), - because in UTF-8 mode it makes it impossible to calculate the length of - the lookbehind. + PCRE does not allow \C to appear in lookbehind assertions (described + below), because in UTF-8 mode this would make it impossible to calcu- + late the length of the lookbehind. -SQUARE BRACKETS +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. 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 + 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 UTF-8 - mode, the character may occupy more than one byte. A matched character + A character class matches a single character in the subject. In UTF-8 + mode, the character may occupy more than one byte. 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 + 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. + 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 which are in the class by enumerating those that are not. It - is not an assertion: it still consumes a character from the subject - string, and fails if the current pointer is at the end of the string. + 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 mode, characters with values greater than 255 can be included in a class as a literal string of bytes, or by using the \x{ escaping @@ -1949,58 +2293,64 @@ SQUARE BRACKETS 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. PCRE does not support the - concept of case for characters with values greater than 255. + match "A", whereas a caseful version would. When running in UTF-8 mode, + PCRE supports the concept of case for characters with values greater + than 128 only when it is compiled with Unicode property support. - The newline character is never treated in any special way in character - classes, whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE + The newline character is never treated in any special way in character + classes, whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class such as [^a] will always match a newline. - 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 + 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. 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 single class containing a range followed by two separate - characters. The octal or hexadecimal representation of "]" can also be - used to end a range. - - Ranges operate in the collating sequence of character values. They can - also be used for characters specified numerically, for example - [\000-\037]. In UTF-8 mode, ranges can include characters whose values + 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. + + Ranges operate in the collating sequence of character values. They can + also be used for characters specified numerically, for example + [\000-\037]. In UTF-8 mode, ranges can include characters whose values are greater than 255, for example [\x{100}-\x{2ff}]. 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 if character tables for the - "fr" locale are in use, [\xc8-\xcb] matches accented E characters in - both cases. - - The character types \d, \D, \s, \S, \w, and \W may also appear in a - character class, and add the characters that they match to the class. - For example, [\dABCDEF] matches any hexadecimal digit. 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. - - All non-alphameric characters other than \, -, ^ (at the start) and the - terminating ] are non-special in character classes, but it does no harm - if they are escaped. + to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if + character tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches + accented E characters in both cases. In UTF-8 mode, PCRE supports the + concept of case for characters with values greater than 128 only when + it is compiled with Unicode property support. + + The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear + in a character class, and add the characters that they match to the + class. For example, [\dABCDEF] matches any hexadecimal digit. A circum- + flex 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. + + 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 - see the + next section), 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, which uses - names enclosed by [: and :] within the enclosing square brackets. PCRE - also supports this notation. For example, + 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:]%] @@ -2037,7 +2387,7 @@ POSIX CHARACTER CLASSES 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. - In UTF-8 mode, characters with values greater than 255 do not match any + In UTF-8 mode, characters with values greater than 128 do not match any of the POSIX character classes. @@ -2104,13 +2454,14 @@ INTERNAL OPTION SETTING in the same way as the Perl-compatible options by using the characters U and X respectively. The (?X) flag setting is special in that it must always occur earlier in the pattern than any of the additional features - it turns on, even when it is at top level. It is best put at the start. + it turns on, even when it is at top level. It is best to put it at the + start. SUBPATTERNS Subpatterns are delimited by parentheses (round brackets), which can be - nested. Marking part of a pattern as a subpattern does two things: + nested. Turning part of a pattern into a subpattern does two things: 1. It localizes a set of alternatives. For example, the pattern @@ -2120,11 +2471,11 @@ SUBPATTERNS the parentheses, it would match "cataract", "erpillar" or the empty string. - 2. It sets up the subpattern as a capturing subpattern (as defined - above). When the whole pattern matches, that portion of the subject + 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 pcre_exec(). Opening parentheses are counted from - left to right (starting from 1) to obtain the numbers of the capturing + 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 pat- @@ -2169,41 +2520,43 @@ 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 the difficulty, PCRE supports the naming of sub- + change. To help with this difficulty, PCRE supports the naming of sub- patterns, something that Perl does not provide. The Python syntax (?P<name>...) is used. Names consist of alphanumeric characters and underscores, and must be unique within a pattern. Named capturing parentheses are still allocated numbers as well as names. The PCRE API provides function calls for extracting the name-to- - number translation table from a compiled pattern. For further details - see the pcreapi documentation. + number translation table from a compiled pattern. There is also a con- + venience function for extracting a captured substring by name. For fur- + ther details see the pcreapi documentation. REPETITION - Repetition is specified by quantifiers, which can follow any of the + Repetition is specified by quantifiers, which can follow any of the following items: a literal data character the . metacharacter the \C escape sequence - escapes such as \d that match single characters + the \X escape sequence (in UTF-8 mode with Unicode properties) + an escape such as \d that matches a single character a character class a back reference (see next section) a parenthesized subpattern (unless it is an assertion) - 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, + 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 "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,} @@ -2212,14 +2565,17 @@ REPETITION \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- + 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-8 mode, quantifiers apply to UTF-8 characters rather than to + In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char- - acters, each of which is represented by a two-byte sequence. + acters, each of which is represented by a two-byte sequence. Similarly, + when Unicode property support is available, \X{3} matches three Unicode + extended sequences, each of which may be several bytes 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. @@ -2247,15 +2603,15 @@ REPETITION 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 the sequences /* and */ and within the sequence, indi- - vidual * and / characters may appear. An attempt to match C comments by - applying the pattern + 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 command */ not comment /* second comment */ + /* first comment */ not comment /* second comment */ fails, because it matches the entire string owing to the greediness of the .* item. @@ -2283,7 +2639,7 @@ REPETITION 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 store is + 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. @@ -2374,7 +2730,7 @@ ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS consists of an additional + character following a quantifier. Using this notation, the previous example can be rewritten as - \d++bar + \d++foo Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY option is ignored. They are a convenient notation for the @@ -2399,68 +2755,70 @@ ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa it takes a long time before reporting failure. This is because the - string can be divided between the two repeats in a large number of - ways, and all have to be tried. (The example used [!?] rather than a - single character at the end, because both PCRE and Perl have an opti- - mization that allows for fast failure when a single character is used. - They remember the last single character that is required for a match, - and fail early if it is not present in the string.) If the pattern is - changed to + 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. + 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 + 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. See the section - entitled "Backslash" above for further details of the handling of dig- - its following a backslash. - - A back reference matches whatever actually matched the capturing sub- - pattern in the current subject string, rather than anything matching + 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. See the subsec- + tion entitled "Non-printing characters" above for further details of + the handling of digits following a backslash. + + 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- + 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 + matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original capturing subpattern is matched caselessly. - Back references to named subpatterns use the Python syntax (?P=name). + Back references to named subpatterns use the Python syntax (?P=name). We could rewrite the above example as follows: (?<p1>(?i)rah)\s+(?P=p1) - 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 + 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. For example, the pattern (a|(bc))\2 - always fails if it starts to match "a" rather than "bc". Because there - may be many capturing parentheses in a pattern, all digits following - the backslash are taken as part of a potential back reference number. + always fails if it starts to match "a" rather than "bc". Because there + may be many capturing parentheses in a pattern, all digits following + the backslash are taken as part of a potential back reference 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 whitespace. Otherwise an empty comment can be used. + used to terminate the back reference. If the PCRE_EXTENDED option is + set, this can be whitespace. Otherwise an empty comment (see "Com- + ments" below) can be used. 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 @@ -2482,14 +2840,26 @@ 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. + 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, and may not be + repeated, because it makes no sense to assert the same thing several + times. If any kind of assertion contains capturing subpatterns within + it, these are counted for the purposes of numbering the capturing sub- + patterns in the whole pattern. However, substring capturing is carried + out only for positive assertions, because it does not make sense for + negative assertions. - An assertion subpattern is matched in the normal way, except that it - does not cause the current matching position to be changed. Lookahead - assertions start with (?= for positive assertions and (?! for negative - assertions. For example, + Lookahead assertions + + Lookahead assertions start with (?= for positive assertions and (?! for + negative assertions. For example, \w+(?=;) @@ -2506,13 +2876,15 @@ ASSERTIONS 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 this effect. + "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. + Lookbehind assertions + Lookbehind assertions start with (?<= for positive assertions and (?<! for negative assertions. For example, @@ -2551,55 +2923,58 @@ ASSERTIONS PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode) to appear in lookbehind assertions, because it makes it impossi- - ble to calculate the length of the lookbehind. + ble to calculate the length of the lookbehind. The \X escape, which can + match different numbers of bytes, is also not permitted. - Atomic groups can be used in conjunction with lookbehind assertions to + Atomic groups can be used in conjunction with lookbehind assertions to specify efficient matching at the end of the subject string. Consider a simple pattern such as abcd$ - when applied to a long string that does not match. Because matching + 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 + 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 + 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, + 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) - or, equivalently, + or, equivalently, using the possessive quantifier syntax, ^.*+(?<=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 + 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 + 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- + 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, + 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". @@ -2607,245 +2982,449 @@ ASSERTIONS (?<=(?<!foo)bar)baz - matches an occurrence of "baz" that is preceded by "bar" which in turn + 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 which matches "foo" preceded by three digits and any + is another pattern that matches "foo" preceded by three digits and any three characters that are not "999". - Assertion subpatterns are not capturing subpatterns, and may not be - repeated, because it makes no sense to assert the same thing several - times. If any kind of assertion contains capturing subpatterns within - it, these are counted for the purposes of numbering the capturing sub- - patterns in the whole pattern. However, substring capturing is carried - out only for positive assertions, because it does not make sense for - negative assertions. - 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 previous capturing - subpattern matched or not. The two possible forms of conditional sub- - pattern are + 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 previous capturing subpat- + tern matched or not. 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- + 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. There are three kinds of condition. If the text between the parentheses - consists of a sequence of digits, the condition is satisfied if the - capturing subpattern of that number has previously matched. The number - must be greater than zero. Consider the following pattern, which con- - tains non-significant white space to make it more readable (assume the - PCRE_EXTENDED option) and to divide it into three parts for ease of + consists of a sequence of digits, the condition is satisfied if the + capturing subpattern of that number has previously matched. The number + must be greater than zero. Consider the following pattern, which con- + tains 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 + 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 + ond part matches one or more characters that are not parentheses. The third part is a conditional subpattern that tests whether the first set of parentheses matched or not. If they did, that is, if subject started with an opening parenthesis, the condition is true, and so the yes-pat- - tern is executed and a closing parenthesis is required. Otherwise, - since no-pattern is not present, the subpattern matches nothing. In - other words, this pattern matches a sequence of non-parentheses, + tern is executed and a closing parenthesis is required. Otherwise, + 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 the condition is the string (R), it is satisfied if a recursive call - to the pattern or subpattern has been made. At "top level", the condi- - tion is false. This is a PCRE extension. Recursive patterns are + to the pattern or subpattern has been made. At "top level", the condi- + tion is false. This is a PCRE extension. Recursive patterns are described in the next section. - If the condition is not a sequence of digits or (R), it must be an - assertion. This may be a positive or negative lookahead or lookbehind - assertion. Consider this pattern, again containing non-significant + If the condition is not a sequence of digits or (R), 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 + 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 - The sequence (?# marks the start of a comment which continues up to the - next closing parenthesis. Nested parentheses are not permitted. 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. The + characters that make up a comment play no part in the pattern matching at all. - If the PCRE_EXTENDED option is set, an unescaped # character outside a + If the PCRE_EXTENDED option is set, an unescaped # character outside a character class introduces a comment that continues up to the next new- line character in the pattern. 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. Perl has provided an experimental facility that allows regular - expressions to recurse (amongst other things). It does this by interpo- - lating Perl code in the expression at run time, and the code can refer - to the expression itself. A Perl pattern to solve the parentheses prob- - lem can be created like this: + 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. Perl provides a facility that allows regular expressions 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 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 - some special syntax for recursion of the entire pattern, and also for + refers recursively to the pattern in which it appears. Obviously, PCRE + cannot support the interpolation of Perl code. Instead, it supports + some special syntax for recursion of the entire pattern, and also for individual subpattern recursion. - The special item that consists of (? followed by a number greater than + The special item that consists of (? followed by a number greater than zero and a closing parenthesis is a recursive call of the subpattern of - the given number, provided that it occurs inside that subpattern. (If - not, it is a "subroutine" call, which is described in the next sec- - tion.) The special item (?R) is a recursive call of the entire regular + the given number, provided that it occurs inside that subpattern. (If + not, it is a "subroutine" call, which is described in the next sec- + tion.) The special item (?R) is a recursive call of the entire regular expression. - For example, this PCRE pattern solves the nested parentheses problem - (assume the PCRE_EXTENDED option is set so that white space is + For example, 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- + 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. - If this were part of a larger pattern, you would not want to recurse + 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, keep- - ing track of parenthesis numbers can be tricky. It may be more conve- - nient to use named parentheses instead. For this, PCRE uses (?P>name), - which is an extension to the Python syntax that PCRE uses for named + We have put the pattern into parentheses, and caused the recursion to + refer to them instead of the whole pattern. In a larger pattern, keep- + ing track of parenthesis numbers can be tricky. It may be more conve- + nient to use named parentheses instead. For this, PCRE uses (?P>name), + which is an extension to the Python syntax that PCRE uses for named parentheses (Perl does not provide named parentheses). We could rewrite the above example as follows: (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) ) - This particular example pattern contains nested unlimited repeats, and - so the use of atomic grouping for matching strings of non-parentheses - is important when applying the pattern to strings that do not match. + This particular example pattern contains nested unlimited repeats, and + so the use of atomic grouping for matching strings of non-parentheses + is important when applying the pattern to strings that do not match. For example, when this pattern is applied to (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() - it yields "no match" quickly. However, if atomic grouping 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 + it yields "no match" quickly. However, if atomic grouping 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 set for any capturing subpatterns are those from the outermost level of the recursion at which the subpattern - value is set. If you want to obtain intermediate values, a callout - function can be used (see below and the pcrecallout documentation). If - the pattern above is matched against + value is set. If you want to obtain intermediate values, a callout + function can be used (see the next section and the pcrecallout documen- + tation). If the pattern above is matched against (ab(cd)ef) - the value for the capturing parentheses is "ef", which is the last - value taken on at the top level. If additional parentheses are added, + the value for the capturing parentheses is "ef", which is the last + value taken on at the top level. If additional parentheses are added, giving \( ( ( (?>[^()]+) | (?R) )* ) \) ^ ^ ^ ^ - the string they capture is "ab(cd)ef", the contents of the top level - parentheses. If there are more than 15 capturing parentheses in a pat- + the string they capture is "ab(cd)ef", the contents of the top level + parentheses. If there are more than 15 capturing parentheses in a pat- tern, PCRE has to obtain extra memory to store data during a recursion, - which it does by using pcre_malloc, freeing it via pcre_free after- - wards. If no memory can be obtained, the match fails with the + which it does by using pcre_malloc, freeing it via pcre_free after- + wards. 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- + 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. + 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. SUBPATTERNS AS SUBROUTINES If the syntax for a recursive subpattern reference (either by number or - by name) is used outside the parentheses to which it refers, it oper- - ates like a subroutine in a programming language. An earlier example + by name) is used outside the parentheses to which it refers, it oper- + ates like a subroutine in a programming language. An earlier example pointed out that the pattern (sens|respons)e and \1ibility - matches "sense and sensibility" and "response and responsibility", but + 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. Such references must, however, follow the subpattern to + is used, it does match "sense and responsibility" as well as the other + two strings. Such references must, however, follow the subpattern to which they refer. CALLOUTS Perl has a feature whereby using the sequence (?{...}) causes arbitrary - Perl code to be obeyed in the middle of matching a regular expression. + 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. By default, this variable contains NULL, which disables + an external function by putting its entry point in the global variable + pcre_callout. 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 + 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 pcre_compile(), callouts are + automatically installed before each item in the pattern. They are all + numbered 255. + During matching, when PCRE reaches a callout point (and pcre_callout is - set), the external function is called. It is provided with the number - of the callout, and, optionally, one item of data originally supplied - by the caller of pcre_exec(). The callout function may cause matching - to backtrack, or to fail altogether. A complete description of the - interface to the callout function is given in the pcrecallout documen- - tation. - -Last updated: 03 February 2003 -Copyright (c) 1997-2003 University of Cambridge. + set), the external function 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 pcre_exec(). The callout + function may cause matching to proceed, to backtrack, or to fail alto- + gether. A complete description of the interface to the callout function + is given in the pcrecallout documentation. + +Last updated: 09 September 2004 +Copyright (c) 1997-2004 University of Cambridge. +----------------------------------------------------------------------------- + +PCRE(3) PCRE(3) + + + +NAME + PCRE - Perl-compatible regular expressions + +PARTIAL MATCHING IN PCRE + + In normal use of PCRE, if the subject string that is passed to + pcre_exec() 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, possibly beeping and not + reflecting the character that has been typed. This immediate feedback + is likely to be a better user interface than a check that is delayed + until the entire string has been entered. + + PCRE supports the concept of partial matching by means of the PCRE_PAR- + TIAL option, which can be set when calling pcre_exec(). When this is + done, the return code PCRE_ERROR_NOMATCH is converted into + PCRE_ERROR_PARTIAL if at any time during the matching process the + entire subject string matched part of the pattern. No captured data is + set when this occurs. + + Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers + the last literal byte in a pattern, and abandons matching immediately + if such a byte is not present in the subject string. This optimization + cannot be used for a subject string that might match only partially. + + +RESTRICTED PATTERNS FOR PCRE_PARTIAL + + Because of the way certain internal optimizations are implemented in + PCRE, the PCRE_PARTIAL option cannot be used with all patterns. + Repeated single characters such as + + a{2,4} + + and repeated single metasequences such as + + \d+ + + are not permitted if the maximum number of occurrences is greater than + one. Optional items such as \d? (where the maximum is one) are permit- + ted. Quantifiers with any values are permitted after parentheses, so + the invalid examples above can be coded thus: + + (a){2,4} + (\d)+ + + These constructions run more slowly, but for the kinds of application + that are envisaged for this facility, this is not felt to be a major + restriction. + + If PCRE_PARTIAL is set for a pattern that does not conform to the + restrictions, pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL + (-13). + + +EXAMPLE OF PARTIAL MATCHING USING PCRETEST + + If the escape sequence \P is present in a pcretest data line, the + PCRE_PARTIAL flag 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> 25jun04P + 0: 25jun04 + 1: jun + data> 25dec3P + Partial match + data> 3juP + Partial match + data> 3jujP + No match + data> jP + 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. + +Last updated: 08 September 2004 +Copyright (c) 1997-2004 University of Cambridge. +----------------------------------------------------------------------------- + +PCRE(3) PCRE(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. + + If you save compiled patterns to a file, you can copy them to a differ- + ent host and run them there. This works even if the new host has the + opposite endianness to the one on which the patterns were compiled. + There may be a small performance penalty, but it should be insignifi- + cant. + + +SAVING A COMPILED PATTERN + The value returned by pcre_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_fullinfo() with an argu- + ment of PCRE_INFO_SIZE. You can then save the data in any appropriate + manner. Here is sample code 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 study + data in a similar way to the compiled pattern itself. When studying + generates additional information, pcre_study() returns a pointer to a + pcre_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_extra block itself). The length of the study data can be obtained + by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE. + Remember to check that pcre_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, you pass its pointer to pcre_exec() in the usual way. + This should work even on another host, and even if that host has the + opposite endianness to the one where the pattern was compiled. + + However, if you passed a pointer to custom character tables when the + pattern was compiled (the tableptr argument of pcre_compile()), you + must now pass a similar pointer to pcre_exec(), because the value saved + with the compiled pattern will obviously be nonsense. A field in a + pcre_extra() block is used to pass this data, as described in the sec- + tion on matching a pattern in the pcreapi documentation. + + If you did not provide custom character tables when the pattern was + compiled, the pointer in the compiled pattern is NULL, which causes + pcre_exec() 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_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_extra block to pcre_exec() in the usual way. + + +COMPATIBILITY WITH DIFFERENT PCRE RELEASES + + The layout of the control block that is at the start of the data that + makes up a compiled pattern was changed for release 5.0. If you have + any saved patterns that were compiled with previous releases (not a + facility that was previously advertised), you will have to recompile + them for release 5.0. However, from now on, it should be possible to + make changes in a compabible manner. + +Last updated: 10 September 2004 +Copyright (c) 1997-2004 University of Cambridge. ----------------------------------------------------------------------------- PCRE(3) PCRE(3) @@ -2862,63 +3441,73 @@ PCRE PERFORMANCE like [aeiou] than a set of alternatives such as (a|e|i|o|u). In gen- eral, the simplest construction that provides the required behaviour is usually the most efficient. Jeffrey Friedl's book contains a lot of - discussion about optimizing regular expressions for efficient perfor- - mance. + useful general discussion about optimizing regular expressions for + efficient performance. This document contains a few observations about + PCRE. - When a pattern begins with .* not in parentheses, or in parentheses + Using Unicode character properties (the \p, \P, and \X escapes) is + slow, because PCRE has to scan a structure that contains data for over + fifteen thousand characters 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. + + 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 + 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 + 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- + 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 ^.* to indicate explicit anchoring. That - saves PCRE from having to scan along the subject looking for a newline + or starting the pattern with ^.* to indicate explicit anchoring. 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. + 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 33 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, the + - repeats can match different numbers of times.) When the remainder of + This can match "aaaa" in 33 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, 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 + principle to try every possible variation, and this can take an extremely long time. 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 + where a literal character follows. Before embarking on the standard + matching procedure, PCRE checks that there is a "b" later in the + subject string, and if there is not, it fails the match immediately. + However, 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 + 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. -Last updated: 03 February 2003 -Copyright (c) 1997-2003 University of Cambridge. + In many cases, the solution to this kind of performance issue is to use + an atomic group or a possessive quantifier. + +Last updated: 09 September 2004 +Copyright (c) 1997-2004 University of Cambridge. ----------------------------------------------------------------------------- PCRE(3) PCRE(3) @@ -2929,6 +3518,7 @@ NAME PCRE - Perl-compatible regular expressions. SYNOPSIS OF POSIX API + #include <pcreposix.h> int regcomp(regex_t *preg, const char *pattern, @@ -2947,13 +3537,13 @@ DESCRIPTION This set of functions provides a POSIX-style API to the PCRE regular expression package. See the pcreapi documentation for a description of - the native API, which contains additional functionality. + PCRE's native API, which contains additional functionality. 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 which uses them. Because the POSIX + 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 option bits that can be reasonably mapped @@ -2985,7 +3575,7 @@ 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 which is used as a base for storing information about + regex_t structure that is used as a base for storing information about the compiled expression. The argument cflags is either zero, or contains one or more of the bits @@ -3036,24 +3626,24 @@ MATCHING NEWLINE CHARACTERS 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 + . 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_DOLLARENDONLY in Perl. In both PCRE and Perl, there is no - way to stop newline from matching [^a]. + 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_DOLLARENDONLY, but there is no way to make PCRE + 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 pre-compiled pattern preg + The function regexec() is called to match a compiled pattern preg against a given string, which is terminated by a zero byte, subject to the options in eflags. These can be: @@ -3092,7 +3682,7 @@ ERROR MESSAGES tion is the size of buffer needed to hold the whole message. -STORAGE +MEMORY USAGE Compiling a regular expression causes memory to be allocated and asso- ciated with the preg structure. The function regfree() frees all such @@ -3106,8 +3696,8 @@ AUTHOR University Computing Service, Cambridge CB2 3QG, England. -Last updated: 03 February 2003 -Copyright (c) 1997-2003 University of Cambridge. +Last updated: 07 September 2004 +Copyright (c) 1997-2004 University of Cambridge. ----------------------------------------------------------------------------- PCRE(3) PCRE(3) @@ -3134,24 +3724,34 @@ PCRE SAMPLE PROGRAM bility of matching an empty string. Comments in the code explain what is going on. - On a Unix system that has PCRE installed in /usr/local, you can compile - the demonstration program using a command like this: + If PCRE is installed in the standard include and library directories + for your system, you should be able to compile the demonstration pro- + gram 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 pcredemo.c -I/usr/local/include \ + gcc -o pcredemo -I/usr/local/include pcredemo.c \ -L/usr/local/lib -lpcre - Then you can run simple tests like this: + Once you have compiled 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 + Note that there is a much more comprehensive test program, called + pcretest, which supports many more facilities for testing regular expressions and the PCRE library. The pcredemo program is provided as a simple coding example. - On some operating systems (e.g. Solaris) you may get an error like this - when you try to run pcredemo: + On some operating systems (e.g. Solaris), when PCRE is not installed in + the standard library directory, you may get an error like this when you + try to run pcredemo: ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or directory @@ -3161,9 +3761,9 @@ PCRE SAMPLE PROGRAM -R/usr/local/lib - to the compile command to get round this problem. + (for example) to the compile command to get round this problem. -Last updated: 28 January 2003 -Copyright (c) 1997-2003 University of Cambridge. +Last updated: 09 September 2004 +Copyright (c) 1997-2004 University of Cambridge. ----------------------------------------------------------------------------- |