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diff --git a/external-libs/pcre/doc/pcre.txt b/external-libs/pcre/doc/pcre.txt new file mode 100644 index 00000000..698baa52 --- /dev/null +++ b/external-libs/pcre/doc/pcre.txt @@ -0,0 +1,3169 @@ +This file contains a concatenation of the PCRE man pages, converted to plain +text format for ease of searching with a text editor, or for use on systems +that do not have a man page processor. The small individual files that give +synopses of each function in the library have not been included. There are +separate text files for the pcregrep and pcretest commands. +----------------------------------------------------------------------------- + +PCRE(3) PCRE(3) + + + +NAME + PCRE - Perl-compatible regular expressions + +DESCRIPTION + + 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. + + 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: + + ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre + + Details of exactly which Perl regular expression features are and are + not supported by PCRE are given in separate documents. See the pcrepat- + tern and pcrecompat pages. + + 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. + + +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: + + pcre this document + pcreapi details of PCRE's native API + pcrebuild options for building PCRE + pcrecallout details of the callout feature + pcrecompat discussion of Perl compatibility + pcregrep description of the pcregrep command + pcrepattern syntax and semantics of supported + regular expressions + pcreperform discussion of performance issues + pcreposix the POSIX-compatible API + pcresample discussion of the sample program + pcretest 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. + + +LIMITATIONS + + There are some size limitations in PCRE but it is hoped that they will + never in practice be relevant. + + The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE + is compiled with the default internal linkage size of 2. If you want to + 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 + of execution will be slower. + + All values in repeating quantifiers must be less than 65536. The maxi- + mum number of capturing subpatterns is 65535. + + There is no limit to the number of non-capturing subpatterns, but the + maximum depth of nesting of all kinds of parenthesized subpattern, + including capturing subpatterns, assertions, and other types of subpat- + tern, is 200. + + The maximum length of a subject string is the largest positive number + that an integer variable can hold. However, PCRE uses recursion to han- + dle subpatterns and indefinite repetition. This means that the avail- + able stack space may limit the size of a subject string that can be + processed by certain patterns. + + +UTF-8 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. + + 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 + not be very large. + + 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. + 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 + 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 + 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, + 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 + UTF-8 character if the value is greater than 127. + + 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. + + 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. + + 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. + + 9. PCRE does not support the use of Unicode tables and properties or + the Perl escapes \p, \P, and \X. + + +AUTHOR + + Philip Hazel <ph10@cam.ac.uk> + University Computing Service, + Cambridge CB2 3QG, England. + Phone: +44 1223 334714 + +Last updated: 20 August 2003 +Copyright (c) 1997-2003 University of Cambridge. +----------------------------------------------------------------------------- + +PCRE(3) PCRE(3) + + + +NAME + PCRE - Perl-compatible regular expressions + +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 + + ./configure --help + + The following sections describe certain options whose names begin with + --enable or --disable. These settings specify changes to the defaults + for the configure command. Because of the way that configure works, + --enable and --disable always come in pairs, so the complementary + option always exists as well, but as it specifies the default, it is + not described. + + +UTF-8 SUPPORT + + To build PCRE with support for UTF-8 character strings, add + + --enable-utf8 + + to the configure command. Of itself, this does not make PCRE treat + strings as UTF-8. As well as compiling PCRE with this option, you also + have have to set the PCRE_UTF8 option when you call the pcre_compile() + function. + + +CODE VALUE OF NEWLINE + + By default, PCRE treats character 10 (linefeed) as the newline charac- + ter. This is the normal newline character on Unix-like systems. You can + compile PCRE to use character 13 (carriage return) instead by adding + + --enable-newline-is-cr + + to the configure command. For completeness there is also a --enable- + newline-is-lf option, which explicitly specifies linefeed as the new- + line character. + + +BUILDING SHARED AND STATIC LIBRARIES + + The PCRE building process uses libtool to build both shared and static + Unix libraries by default. You can suppress one of these by adding one + of + + --disable-shared + --disable-static + + to the configure command, as required. + + +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 + 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. + The default threshold above which the stack is no longer used is 10; it + can be changed by adding a setting such as + + --with-posix-malloc-threshold=20 + + to the configure command. + + +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 + + --with-match-limit=500000 + + to the configure command. + + +HANDLING VERY LARGE PATTERNS + + Within a compiled pattern, offset values are used to point from one + part to another (for example, from an opening parenthesis to an alter- + nation metacharacter). By default 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 + is possible to compile PCRE to use three-byte or four-byte offsets by + adding a setting such as + + --with-link-size=3 + + to the configure command. The value given must be 2, 3, or 4. Using + longer offsets slows down the operation of PCRE because it has to load + additional bytes when handling them. + + If you build PCRE with an increased link size, test 2 (and test 5 if + you are using UTF-8) will fail. Part of the output of these tests is a + representation of the compiled pattern, and this changes with the link + size. + + +AVOIDING EXCESSIVE STACK USAGE + + PCRE implements backtracking while matching by making recursive calls + to an internal function called match(). In environments where the size + of the stack is limited, this can severely limit PCRE's operation. (The + Unix environment does not usually suffer from this problem.) An alter- + native approach that uses memory from the heap to remember data, + instead of using recursive function calls, has been implemented to work + round this problem. If you want to build a version of PCRE that works + this way, add + + --disable-stack-for-recursion + + to the configure command. With this configuration, PCRE will use the + pcre_stack_malloc and pcre_stack_free variables to call memory + management 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 + slowly when built in this way. + + +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 + + --enable-ebcdic + + to the configure command. + +Last updated: 09 December 2003 +Copyright (c) 1997-2003 University of Cambridge. +----------------------------------------------------------------------------- + +PCRE(3) PCRE(3) + + + +NAME + PCRE - Perl-compatible regular expressions + +SYNOPSIS OF PCRE API + + #include <pcre.h> + + pcre *pcre_compile(const char *pattern, int options, + const char **errptr, int *erroffset, + const unsigned char *tableptr); + + pcre_extra *pcre_study(const pcre *code, int options, + const char **errptr); + + int pcre_exec(const pcre *code, const pcre_extra *extra, + const char *subject, int length, int startoffset, + int options, int *ovector, int ovecsize); + + int pcre_copy_named_substring(const pcre *code, + const char *subject, int *ovector, + int stringcount, const char *stringname, + char *buffer, int buffersize); + + int pcre_copy_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, char *buffer, + int buffersize); + + int pcre_get_named_substring(const pcre *code, + const char *subject, int *ovector, + int stringcount, const char *stringname, + const char **stringptr); + + int pcre_get_stringnumber(const pcre *code, + const char *name); + + int pcre_get_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, + const char **stringptr); + + int pcre_get_substring_list(const char *subject, + int *ovector, int stringcount, const char ***listptr); + + void pcre_free_substring(const char *stringptr); + + void pcre_free_substring_list(const char **stringptr); + + const unsigned char *pcre_maketables(void); + + int pcre_fullinfo(const pcre *code, const pcre_extra *extra, + int what, void *where); + + int pcre_info(const pcre *code, int *optptr, int *firstcharptr); + + int pcre_config(int what, void *where); + + char *pcre_version(void); + + void *(*pcre_malloc)(size_t); + + void (*pcre_free)(void *); + + void *(*pcre_stack_malloc)(size_t); + + void (*pcre_stack_free)(void *); + + int (*pcre_callout)(pcre_callout_block *); + + +PCRE API + + 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. + + 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. + + 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_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_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 + 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- + tively. PCRE calls the memory management functions via these variables, + so a calling program can replace them if it wishes to intercept the + calls. This should be done before calling any PCRE functions. + + The global variables pcre_stack_malloc and pcre_stack_free are also + indirections to memory management functions. These special functions + are used only when PCRE is compiled to use the heap for remembering + data, instead of recursive function calls. 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 + 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 + pcrecallout documentation. + + +MULTITHREADING + + The PCRE functions can be used in multi-threading applications, with + the proviso that the memory management functions pointed to by + pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the + callout function pointed to by pcre_callout, are shared by all threads. + + The compiled form of a regular expression is not altered during match- + ing, so the same compiled pattern can safely be used by several threads + at once. + + +CHECKING BUILD-TIME OPTIONS + + int pcre_config(int what, void *where); + + The function pcre_config() makes it possible for a PCRE client to dis- + cover which optional features have been compiled into the PCRE library. + The pcrebuild documentation has more details about these optional fea- + tures. + + The first argument for pcre_config() is an integer, specifying which + information is required; the second argument is a pointer to a variable + into which the information is placed. The following information is + available: + + PCRE_CONFIG_UTF8 + + The output is an integer that is set to one if UTF-8 support is avail- + able; otherwise it is set to zero. + + 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 + operating system. + + PCRE_CONFIG_LINK_SIZE + + 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 + 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 + 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 + 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 + their state. This is the usual way that PCRE is compiled. The output is + zero if PCRE was compiled to use blocks of data on the heap instead of + recursive function calls. In this case, pcre_stack_malloc and + pcre_stack_free are called to manage memory blocks on the heap, thus + avoiding the use of the stack. + + +COMPILING A PATTERN + + pcre *pcre_compile(const char *pattern, int options, + const char **errptr, int *erroffset, + const unsigned char *tableptr); + + + 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 + it is no longer required. + + Although the compiled code of a PCRE regex is relocatable, that is, it + does not depend on memory location, the complete pcre data block is not + fully relocatable, because it contains a copy of the tableptr argument, + 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. + + If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, + if compilation of a pattern fails, pcre_compile() returns NULL, and + sets the variable pointed to by errptr to point to a textual error mes- + sage. The offset from the start of the pattern to the character where + the error was discovered is placed in the variable pointed to by + erroffset, which must not be NULL. If it is, an immediate error is + given. + + 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- + pile(): + + pcre *re; + const char *error; + int erroffset; + re = pcre_compile( + "^A.*Z", /* the pattern */ + 0, /* default options */ + &error, /* for error message */ + &erroffset, /* for error offset */ + NULL); /* use default character tables */ + + The following option bits are defined: + + 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 + achieved by appropriate constructs in the pattern itself, which is the + only way to do it in Perl. + + 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. + + PCRE_DOLLAR_ENDONLY + + If this bit is set, a dollar metacharacter in the pattern matches only + at the end of the subject string. Without this option, a dollar also + matches immediately before the final character if it is a newline (but + not before any other newlines). The PCRE_DOLLAR_ENDONLY option is + ignored if PCRE_MULTILINE is set. There is no equivalent to this option + in Perl, and no way to set it within a pattern. + + PCRE_DOTALL + + If this bit is set, a dot metacharater in the pattern matches all char- + acters, including newlines. Without it, newlines are excluded. This + option is equivalent to Perl's /s option, and it can be changed within + a pattern by a (?s) option setting. A negative class such as [^a] + always matches a newline character, independent of the setting of this + option. + + PCRE_EXTENDED + + If this bit is set, whitespace data characters in the pattern are + totally ignored except when escaped or inside a character class. + Whitespace does not include the VT character (code 11). In addition, + characters between an unescaped # outside a character class and the + next newline character, inclusive, are also ignored. This is equivalent + to Perl's /x option, and it can be changed within a pattern by a (?x) + option setting. + + This option makes it possible to include comments inside complicated + patterns. Note, however, that this applies only to data characters. + Whitespace characters may never appear within special character + sequences in a pattern, for example within the sequence (?( which + introduces a conditional subpattern. + + PCRE_EXTRA + + This option was invented in order to turn on additional functionality + of PCRE that is incompatible with Perl, but it is currently of very + little use. When set, any backslash in a pattern that is followed by a + letter that has no special meaning causes an error, thus reserving + these combinations for future expansion. By default, as in Perl, a + backslash followed by a letter with no special meaning is treated as a + literal. There are at present no other features controlled by this + option. It can also be set by a (?X) option setting within a pattern. + + PCRE_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. + + When PCRE_MULTILINE it is set, the "start of line" and "end of line" + constructs match immediately following or immediately before any new- + line in the subject string, respectively, as well as at the very start + and end. This is equivalent to Perl's /m option, and it can be changed + within a pattern by a (?m) option setting. If there are no "\n" charac- + ters in a subject string, or no occurrences of ^ or $ in a pattern, + setting PCRE_MULTILINE has no effect. + + PCRE_NO_AUTO_CAPTURE + + If this option is set, it disables the use of numbered capturing paren- + theses in the pattern. Any opening parenthesis that is not followed by + ? behaves as if it were followed by ?: but named parentheses can still + be used for capturing (and they acquire numbers in the usual way). + There is no equivalent of this option in Perl. + + PCRE_UNGREEDY + + This option inverts the "greediness" of the quantifiers so that they + are not greedy by default, but become greedy if followed by "?". It is + not compatible with Perl. It can also be set by a (?U) option setting + within the pattern. + + PCRE_UTF8 + + This option causes PCRE to regard both the pattern and the subject as + strings of UTF-8 characters instead of single-byte 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. + + PCRE_NO_UTF8_CHECK + + When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is + automatically checked. If an invalid UTF-8 sequence of bytes is found, + pcre_compile() returns an error. If you already know that your pattern + is valid, and you want to skip this check for performance 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(). + + + +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- + ing pcre_study(), to be sure that it has run successfully. + + This is a typical call to pcre_study(): + + pcre_extra *pe; + pe = pcre_study( + re, /* result of pcre_compile() */ + 0, /* no options exist */ + &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. + + +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"); + 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. + + +INFORMATION ABOUT A PATTERN + + int pcre_fullinfo(const pcre *code, const pcre_extra *extra, + int what, void *where); + + The pcre_fullinfo() function returns information about a compiled pat- + tern. It replaces the obsolete pcre_info() function, which is neverthe- + less retained for backwards compability (and is documented below). + + The first argument for pcre_fullinfo() is a pointer to the compiled + pattern. The second argument is the result of pcre_study(), or NULL if + the pattern was not studied. The third argument specifies which piece + of information is required, and the fourth argument is a pointer to a + variable to receive the data. The yield of the function is zero for + success, or one of the following negative numbers: + + PCRE_ERROR_NULL the argument code was NULL + the argument where was NULL + PCRE_ERROR_BADMAGIC the "magic number" was not found + PCRE_ERROR_BADOPTION the value of what was invalid + + Here is a typical call of pcre_fullinfo(), to obtain the length of the + compiled pattern: + + int rc; + unsigned long int length; + rc = pcre_fullinfo( + re, /* result of pcre_compile() */ + pe, /* result of pcre_study(), or NULL */ + PCRE_INFO_SIZE, /* what is required */ + &length); /* where to put the data */ + + The possible values for the third argument are defined in pcre.h, and + are as follows: + + PCRE_INFO_BACKREFMAX + + Return the number of the highest back reference in the pattern. The + fourth argument should point to an int variable. Zero is returned if + there are no back references. + + PCRE_INFO_CAPTURECOUNT + + Return the number of capturing subpatterns in the pattern. The fourth + argument should point to an int variable. + + PCRE_INFO_FIRSTBYTE + + Return information about the first byte of any matched string, for a + non-anchored pattern. (This option used to be called + 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 + (cat|cow|coyote), it is returned in the integer pointed to by where. + Otherwise, if either + + (a) the pattern was compiled with the PCRE_MULTILINE option, and every + branch starts with "^", or + + (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not + set (if it were set, the pattern would be anchored), + + -1 is returned, indicating that the pattern matches only at the start + of a subject string or after any newline within the string. Otherwise + -2 is returned. For anchored patterns, -2 is returned. + + PCRE_INFO_FIRSTTABLE + + If the pattern was studied, and this resulted in the construction of a + 256-bit table indicating a fixed set of bytes for the first byte in any + matching string, a pointer to the table is returned. Otherwise NULL is + returned. The fourth argument should point to an unsigned char * vari- + able. + + PCRE_INFO_LASTLITERAL + + Return the value of the rightmost literal byte that must exist in any + matched string, other than at its start, if such a byte has been + recorded. The fourth argument should point to an int variable. If there + is no such byte, -1 is returned. For anchored patterns, a last literal + byte is recorded only if it follows something of variable length. For + example, for the pattern /^a\d+z\d+/ the returned value is "z", but for + /^a\dz\d/ the returned value is -1. + + PCRE_INFO_NAMECOUNT + PCRE_INFO_NAMEENTRYSIZE + PCRE_INFO_NAMETABLE + + PCRE supports the use of named as well as numbered capturing parenthe- + ses. The names are just an additional way of identifying the parenthe- + ses, which still acquire 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. + + The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT + gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size + of each entry; both of these return an int value. The entry size + depends on the length of the longest name. PCRE_INFO_NAMETABLE returns + a pointer to the first entry of the table (a pointer to char). The + first two bytes of each entry are the number of the capturing parenthe- + sis, most significant byte first. The rest of the entry is the corre- + sponding name, zero terminated. The names are in alphabetical order. + For example, consider the following pattern (assume PCRE_EXTENDED is + set, so white space - including newlines - is ignored): + + (?P<date> (?P<year>(\d\d)?\d\d) - + (?P<month>\d\d) - (?P<day>\d\d) ) + + There are four named subpatterns, so the table has four entries, and + each entry in the table is eight bytes long. The table is as follows, + with non-printing bytes shows in hex, 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. + + PCRE_INFO_OPTIONS + + Return a copy of the options with which the pattern was compiled. The + fourth argument should point to an unsigned long int variable. These + option bits are those specified in the call to pcre_compile(), modified + by any top-level option settings within the pattern itself. + + A pattern is automatically anchored by PCRE if all of its top-level + alternatives begin with one of the following: + + ^ unless PCRE_MULTILINE is set + \A always + \G always + .* if PCRE_DOTALL is set and there are no back + references to the subpattern in which .* appears + + For such patterns, the PCRE_ANCHORED bit is set in the options returned + by pcre_fullinfo(). + + PCRE_INFO_SIZE + + Return the size of the compiled pattern, that is, the value that was + passed as the argument to pcre_malloc() when PCRE was getting memory in + which to place the compiled data. The fourth argument should point to a + size_t variable. + + 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 + 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. + + +OBSOLETE INFO FUNCTION + + int pcre_info(const pcre *code, int *optptr, int *firstcharptr); + + The pcre_info() function is now obsolete because its interface is too + restrictive to return all the available data about a compiled pattern. + New programs should use pcre_fullinfo() instead. The yield of + pcre_info() is the number of capturing subpatterns, or one of the fol- + lowing negative numbers: + + PCRE_ERROR_NULL the argument code was NULL + PCRE_ERROR_BADMAGIC the "magic number" was not found + + If the optptr argument is not NULL, a copy of the options with which + the pattern was compiled is placed in the integer it points to (see + PCRE_INFO_OPTIONS above). + + If the pattern is not anchored and the firstcharptr argument is not + NULL, it is used to pass back information about the first character of + any matched string (see PCRE_INFO_FIRSTBYTE above). + + +MATCHING A PATTERN + + int pcre_exec(const pcre *code, const pcre_extra *extra, + const char *subject, int length, int startoffset, + int options, int *ovector, int ovecsize); + + The function pcre_exec() is called to match a subject string against a + 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. + + Here is an example of a simple call to pcre_exec(): + + int rc; + int ovector[30]; + rc = pcre_exec( + re, /* result of pcre_compile() */ + NULL, /* we didn't study the pattern */ + "some string", /* the subject string */ + 11, /* the length of the subject string */ + 0, /* start at offset 0 in the subject */ + 0, /* default options */ + ovector, /* vector for substring information */ + 30); /* number of elements in the vector */ + + 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: + + unsigned long int flags; + void *study_data; + unsigned long int match_limit; + void *callout_data; + + The flags field is a bitmap that specifies which of the other fields + are set. The flag bits are: + + PCRE_EXTRA_STUDY_DATA + PCRE_EXTRA_MATCH_LIMIT + PCRE_EXTRA_CALLOUT_DATA + + 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 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 + exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT. + + 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. + + 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. + + 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. + + There are also three further options that can be set only 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. + + 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. + + PCRE_NOTEMPTY + + An empty string is not considered to be a valid match if this option is + set. If there are alternatives in the pattern, they are tried. If all + the alternatives match the empty string, the entire match fails. For + example, if the pattern + + a?b? + + is applied to a string not beginning with "a" or "b", it matches 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 + 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. + + 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 + with any kind of lookbehind. For example, consider the pattern + + \Biss\B + + which finds occurrences of "iss" in the middle of words. (\B matches + only if the current position in the subject is not a word boundary.) + When applied to the string "Mississipi" the first call to pcre_exec() + finds the first occurrence. If pcre_exec() is called again with just + the remainder of the subject, namely "issipi", it does not match, + because \B is always false at the start of the subject, which is deemed + to be a word boundary. However, if pcre_exec() is passed the entire + string again, but with startoffset set to 4, it finds the second + 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. + + 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, + 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 + after the end of a substring. The first pair, ovector[0] and ovec- + tor[1], identify the portion of the subject string matched by the + entire pattern. The next pair is used for the first capturing subpat- + tern, and so on. The value returned by pcre_exec() is the number of + pairs that have been set. If there are no capturing subpatterns, the + return value from a successful match is 1, indicating that just the + first pair of offsets has been set. + + Some convenience functions are provided for extracting the captured + substrings as separate strings. These are described in the following + section. + + It is possible for an capturing subpattern number n+1 to match some + part of the subject when subpattern n has not been used at all. For + example, if the string "abc" is matched against the pattern (a|(z))(bc) + subpatterns 1 and 3 are matched, but 2 is not. When this happens, both + 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. + Thus it is usually advisable to supply an ovector. + + Note that pcre_info() can be used to find out how many capturing sub- + patterns there are in a compiled pattern. The smallest size for ovector + that will allow for n captured substrings, in addition to the offsets + of the substring matched by the whole pattern, is (n+1)*3. + + If pcre_exec() fails, it returns a negative number. The following are + defined in the header file: + + PCRE_ERROR_NOMATCH (-1) + + The subject string did not match the pattern. + + PCRE_ERROR_NULL (-2) + + Either code or subject was passed as NULL, or ovector was NULL and + ovecsize was not zero. + + PCRE_ERROR_BADOPTION (-3) + + An unrecognized bit was set in the options argument. + + PCRE_ERROR_BADMAGIC (-4) + + PCRE stores a 4-byte "magic number" at the start of the compiled code, + to catch the case when it is passed a junk pointer. This is the error + it gives when the magic number isn't present. + + PCRE_ERROR_UNKNOWN_NODE (-5) + + While running the pattern match, an unknown item was encountered in the + compiled pattern. This error could be caused by a bug in PCRE or by + overwriting of the compiled pattern. + + PCRE_ERROR_NOMEMORY (-6) + + If a pattern contains back references, but the ovector that is passed + to pcre_exec() is not big enough to remember the referenced substrings, + PCRE gets a block of memory at the start of matching to use for this + purpose. If the call via pcre_malloc() fails, this error is given. The + memory is freed at the end of matching. + + PCRE_ERROR_NOSUBSTRING (-7) + + This error is used by the pcre_copy_substring(), pcre_get_substring(), + and pcre_get_substring_list() functions (see below). It is never + returned by pcre_exec(). + + PCRE_ERROR_MATCHLIMIT (-8) + + The recursion and backtracking limit, as specified by the match_limit + field in a pcre_extra structure (or defaulted) was reached. See the + description above. + + PCRE_ERROR_CALLOUT (-9) + + This error is never generated by pcre_exec() itself. It is provided for + use by callout functions that want to yield a distinctive error code. + See the pcrecallout documentation for details. + + PCRE_ERROR_BADUTF8 (-10) + + A string that contains an invalid UTF-8 byte sequence was passed as a + subject. + + PCRE_ERROR_BADUTF8_OFFSET (-11) + + The UTF-8 byte sequence that was passed as a subject was valid, but the + value of startoffset did not point to the beginning of a UTF-8 charac- + ter. + + +EXTRACTING CAPTURED SUBSTRINGS BY NUMBER + + int pcre_copy_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, char *buffer, + int buffersize); + + int pcre_get_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, + const char **stringptr); + + int pcre_get_substring_list(const char *subject, + int *ovector, int stringcount, const char ***listptr); + + Captured substrings can be accessed directly by using the offsets + returned by pcre_exec() in ovector. For convenience, the functions + pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub- + string_list() are provided for extracting captured substrings as new, + separate, zero-terminated strings. These functions identify substrings + by number. The next section describes functions for extracting named + substrings. A substring that contains a binary zero is correctly + extracted and has a further zero added on the end, but the result is + not, of course, a C string. + + The first three arguments are the same for all three of these func- + tions: subject is the subject string which 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 + the terminating zero, or one of + + PCRE_ERROR_NOMEMORY (-6) + + The buffer was too small for pcre_copy_substring(), or the attempt to + get memory failed for pcre_get_substring(). + + PCRE_ERROR_NOSUBSTRING (-7) + + There is no substring whose number is stringnumber. + + The pcre_get_substring_list() function extracts all available sub- + strings and builds a list of pointers to them. All this is done in a + single block of memory 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 + 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 + empty string. This can be distinguished from a genuine zero-length sub- + string by inspecting the appropriate offset in ovector, which is nega- + tive for unset substrings. + + The two convenience functions pcre_free_substring() and + pcre_free_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- + cial interface to another programming language which cannot use + pcre_free directly; it is for these cases that the functions are pro- + vided. + + +EXTRACTING CAPTURED SUBSTRINGS BY 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 + + ab(?<xxx>\d+)... + + 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. + + 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. + + First, instead of a substring number, a substring name is given. Sec- + ond, there is an extra argument, given at the start, which is a pointer + to the compiled pattern. This is needed in order to gain access to the + name-to-number translation table. + + These functions call pcre_get_stringnumber(), and if it succeeds, they + then call pcre_copy_substring() or pcre_get_substring(), as appropri- + ate. + +Last updated: 09 December 2003 +Copyright (c) 1997-2003 University of Cambridge. +----------------------------------------------------------------------------- + +PCRE(3) PCRE(3) + + + +NAME + PCRE - Perl-compatible regular expressions + +PCRE CALLOUTS + + int (*pcre_callout)(pcre_callout_block *); + + PCRE provides a feature called "callout", which is a means of temporar- + ily passing control to the caller of PCRE in the middle of pattern + matching. The caller of PCRE provides an external function by putting + its entry point in the global variable pcre_callout. By default, this + variable contains NULL, which disables all calling out. + + Within a regular expression, (?C) indicates the points at which the + external function is to be called. Different callout points can be + identified by putting a number less than 256 after the letter C. The + default value is zero. For example, this pattern has two callout + points: + + (?C1)abc(?C2)def + + 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: + + int version; + int callout_number; + int *offset_vector; + const char *subject; + int subject_length; + int start_match; + int current_position; + int capture_top; + int capture_last; + void *callout_data; + + 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. + + The callout_number field contains the number of the callout, as com- + piled into the pattern (that is, the number after ?C). + + 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 + 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. + + 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. + + The capture_last field contains the number of the most recently cap- + tured substring. + + 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. + + + +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. + + Negative values should normally be chosen from the set of + PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan- + dard "no match" failure. The error number PCRE_ERROR_CALLOUT is + reserved for use by callout functions; it will never be used by PCRE + itself. + +Last updated: 21 January 2003 +Copyright (c) 1997-2003 University of Cambridge. +----------------------------------------------------------------------------- + +PCRE(3) PCRE(3) + + + +NAME + PCRE - Perl-compatible regular expressions + +DIFFERENCES FROM PERL + + This document describes the differences in the ways that PCRE and Perl + handle regular expressions. The differences described here are with + respect to Perl 5.8. + + 1. PCRE does not have full UTF-8 support. Details of what it does have + are given in the section on UTF-8 support in the main pcre page. + + 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl + permits them, but they do not mean what you might think. For example, + (?!a){3} does not assert that the next three characters are not "a". It + just asserts that the next character is not "a" three times. + + 3. Capturing subpatterns that occur inside negative lookahead asser- + tions are counted, but their entries in the offsets vector are never + set. Perl sets its numerical variables from any such patterns that are + matched before the assertion fails to match something (thereby succeed- + ing), but only if the negative lookahead assertion contains just one + branch. + + 4. Though binary zero characters are supported in the subject string, + they are not allowed in a pattern string because it is passed as a nor- + mal C string, terminated by zero. The escape sequence "\0" can be used + in the pattern to represent a binary zero. + + 5. The following Perl escape sequences are not supported: \l, \u, \L, + \U, \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 + does not have variables). Note the following examples: + + Pattern PCRE matches Perl matches + + \Qabc$xyz\E abc$xyz abc followed by the + contents of $xyz + \Qabc\$xyz\E abc\$xyz abc\$xyz + \Qabc\E\$\Qxyz\E abc$xyz abc$xyz + + The \Q...\E sequence is recognized both inside and outside character + classes. + + 7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code}) + constructions. However, there is some experimental support for recur- + sive patterns using the non-Perl items (?R), (?number) and (?P>name). + Also, the PCRE "callout" feature allows an external function to be + called during pattern matching. + + 8. There are some differences that are concerned with the settings of + captured strings when part of a pattern is repeated. For example, + matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 + unset, but in PCRE it is set to "b". + + 9. PCRE provides some extensions to the Perl regular expression + facilities: + + (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 $ + 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- + 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. + + (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, + which PCRE cannot support.) + + (h) PCRE supports named capturing substrings, using the Python syntax. + + (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. +----------------------------------------------------------------------------- + +PCRE(3) PCRE(3) + + + +NAME + PCRE - Perl-compatible regular expressions + +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 + trivial example, the pattern + + The quick brown fox + + 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 + 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: + + \ general escape character with several uses + ^ assert start of string (or line, in multiline mode) + $ assert end of string (or line, in multiline mode) + . match any character except newline (by default) + [ start character class definition + | start of alternative branch + ( start subpattern + ) end subpattern + ? extends the meaning of ( + also 0 or 1 quantifier + also quantifier minimizer + * 0 or more quantifier + + 1 or more quantifier + also "possessive quantifier" + { start min/max quantifier + + Part of a pattern that is in square brackets is called a "character + class". In a character class the only meta-characters are: + + \ general escape character + ^ negate the class, but only if the first character + - indicates character range + [ POSIX character class (only if followed by POSIX + syntax) + ] terminates the character class + + The following sections describe the use of each of the meta-characters. + + +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 + 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 \\. + + 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- + 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- + tion. Note the following examples: + + Pattern PCRE matches Perl matches + + \Qabc$xyz\E abc$xyz abc followed by the + contents of $xyz + \Qabc\$xyz\E abc\$xyz abc\$xyz + \Qabc\E\$\Qxyz\E abc$xyz abc$xyz + + The \Q...\E sequence is recognized both inside and outside character + classes. + + 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 + sequences than the binary character it represents: + + \a alarm, that is, the BEL character (hex 07) + \cx "control-x", where x is any character + \e escape (hex 1B) + \f formfeed (hex 0C) + \n newline (hex 0A) + \r carriage return (hex 0D) + \t tab (hex 09) + \ddd character with octal code ddd, or backreference + \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; + 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 + 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. + + 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 + \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. + + 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- + its as a decimal number. If the number is less than 10, or if there + have been at least that many previous capturing left parentheses in the + expression, the entire sequence is taken as a back reference. A + description of how this works is given later, following the discussion + of parenthesized subpatterns. + + Inside a character class, or if the decimal number is greater than 9 + and there have not been that many capturing subpatterns, PCRE re-reads + up to three octal digits following the backslash, and generates a sin- + gle byte from the least significant 8 bits of the value. Any subsequent + digits stand for themselves. For example: + + \040 is another way of writing a space + \40 is the same, provided there are fewer than 40 + previous capturing subpatterns + \7 is always a back reference + \11 might be a back reference, or another way of + writing a tab + \011 is always a tab + \0113 is a tab followed by the character "3" + \113 might be a back reference, otherwise the + character with octal code 113 + \377 might be a back reference, otherwise + the byte consisting entirely of 1 bits + \81 is either a back reference, or a binary zero + followed by the two characters "8" and "1" + + Note that octal values of 100 or greater must not be introduced by a + leading zero, because no more than three octal digits are ever read. + + 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). + + The third use of backslash is for specifying generic character types: + + \d any decimal digit + \D any character that is not a decimal digit + \s any whitespace character + \S any character that is not a whitespace character + \w any "word" character + \W any "non-word" character + + Each pair of escape sequences partitions the complete set of characters + 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. + + 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. + + 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 + + \b matches at a word boundary + \B matches when not at a word boundary + \A matches at start of subject + \Z matches at end of subject or before newline at end + \z matches at end of subject + \G matches at first matching position in subject + + These assertions may not appear in character classes (but note that \b + has a different meaning, namely the backspace character, inside a char- + acter class). + + A word boundary is a position in the subject string where the current + character and the previous character do not both match \w or \W (i.e. + one matches \w and the other matches \W), or the start or end of the + string if the first or last character matches \w, respectively. + + 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- + ments, you can mimic Perl's /g option, and it is in this kind of imple- + mentation where \G can be useful. + + Note, however, that PCRE's interpretation of \G, as the start of the + current match, is subtly different from Perl's, which defines it as the + end of the previous match. In Perl, these can be different when the + previously matched string was empty. Because PCRE does just one match + at a time, it cannot reproduce this behaviour. + + If all the alternatives of a pattern begin with \G, the expression is + anchored to the starting match position, and the "anchored" flag is set + in the compiled regular expression. + + +CIRCUMFLEX AND DOLLAR + + 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 + has an entirely different meaning (see below). + + Circumflex need not be the first character of the pattern if a number + of alternatives are involved, but it should be the first thing in each + alternative in which it appears if the pattern is ever to match that + branch. If all possible alternatives start with a circumflex, that is, + if the pattern is constrained to match only at the start of the sub- + ject, it is said to be an "anchored" pattern. (There are also other + constructs that can cause a pattern to be anchored.) + + 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 + 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 + character class. + + The meaning of dollar can be changed so that it matches only at the + very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at + compile time. This does not affect the \Z assertion. + + The meanings of the circumflex and dollar characters are changed if the + PCRE_MULTILINE option is set. When this is the case, 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 + 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. + + +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. + + 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. + + +SQUARE BRACKETS + + 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 + 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 + must be in the set of characters defined by the class, unless the first + character in the class definition is a circumflex, in which case the + subject character must not be in the set defined by the class. If a + circumflex is actually required as a member of the class, ensure it is + not the first character, or escape it with a backslash. + + For example, the character class [aeiou] matches any lower case vowel, + while [^aeiou] matches any character that is not a lower case vowel. + Note that a circumflex is just a convenient notation for specifying the + characters 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. + + 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 + mechanism. + + When caseless matching is set, any letters in a class represent both + their upper case and lower case versions, so for example, a caseless + [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not + match "A", whereas a caseful version would. PCRE does not support the + concept of case for characters with values greater than 255. + + 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 + 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 + 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. + + +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, + + [01[:alpha:]%] + + matches "0", "1", any alphabetic character, or "%". The supported class + names are + + alnum letters and digits + alpha letters + ascii character codes 0 - 127 + blank space or tab only + cntrl control characters + digit decimal digits (same as \d) + graph printing characters, excluding space + lower lower case letters + print printing characters, including space + punct printing characters, excluding letters and digits + space white space (not quite the same as \s) + upper upper case letters + word "word" characters (same as \w) + xdigit hexadecimal digits + + The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), + and space (32). Notice that this list includes the VT character (code + 11). This makes "space" different to \s, which does not include VT (for + Perl compatibility). + + The name "word" is a Perl extension, and "blank" is a GNU extension + from Perl 5.8. Another Perl extension is negation, which is indicated + by a ^ character after the colon. For example, + + [12[:^digit:]] + + matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the + POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but + these are not supported, and an error is given if they are encountered. + + In UTF-8 mode, characters with values greater than 255 do not match any + of the POSIX character classes. + + +VERTICAL BAR + + Vertical bar characters are used to separate alternative patterns. For + example, the pattern + + gilbert|sullivan + + matches either "gilbert" or "sullivan". Any number of alternatives may + appear, and an empty alternative is permitted (matching the empty + string). The matching process tries each alternative in turn, from + left to right, and the first one that succeeds is used. If the alterna- + tives are within a subpattern (defined below), "succeeds" means match- + ing the rest of the main pattern as well as the alternative in the sub- + pattern. + + +INTERNAL OPTION SETTING + + The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and + PCRE_EXTENDED options can be changed from within the pattern by a + sequence of Perl option letters enclosed between "(?" and ")". The + option letters are + + i for PCRE_CASELESS + m for PCRE_MULTILINE + s for PCRE_DOTALL + x for PCRE_EXTENDED + + For example, (?im) sets caseless, multiline matching. It is also possi- + ble to unset these options by preceding the letter with a hyphen, and a + combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- + LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, + is also permitted. If a letter appears both before and after the + hyphen, the option is unset. + + When an option change occurs at top level (that is, not inside subpat- + tern parentheses), the change applies to the remainder of the pattern + that follows. If the change is placed right at the start of a pattern, + PCRE extracts it into the global options (and it will therefore show up + in data extracted by the pcre_fullinfo() function). + + An option change within a subpattern affects only that part of the cur- + rent pattern that follows it, so + + (a(?i)b)c + + matches abc and aBc and no other strings (assuming PCRE_CASELESS is not + used). By this means, options can be made to have different settings + in different parts of the pattern. Any changes made in one alternative + do carry on into subsequent branches within the same subpattern. For + example, + + (a(?i)b|c) + + matches "ab", "aB", "c", and "C", even though when matching "C" the + first branch is abandoned before the option setting. This is because + the effects of option settings happen at compile time. There would be + some very weird behaviour otherwise. + + The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed + 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. + + +SUBPATTERNS + + Subpatterns are delimited by parentheses (round brackets), which can be + nested. Marking part of a pattern as a subpattern does two things: + + 1. It localizes a set of alternatives. For example, the pattern + + cat(aract|erpillar|) + + matches one of the words "cat", "cataract", or "caterpillar". Without + 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 + 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 + subpatterns. + + For example, if the string "the red king" is matched against the pat- + tern + + the ((red|white) (king|queen)) + + the captured substrings are "red king", "red", and "king", and are num- + bered 1, 2, and 3, respectively. + + The fact that plain parentheses fulfil two functions is not always + helpful. There are often times when a grouping subpattern is required + without a capturing requirement. If an opening parenthesis is followed + by a question mark and a colon, the subpattern does not do any captur- + ing, and is not counted when computing the number of any subsequent + capturing subpatterns. For example, if the string "the white queen" is + matched against the pattern + + the ((?:red|white) (king|queen)) + + the captured substrings are "white queen" and "queen", and are numbered + 1 and 2. The maximum number of capturing subpatterns is 65535, and the + maximum depth of nesting of all subpatterns, both capturing and non- + capturing, is 200. + + As a convenient shorthand, if any option settings are required at the + start of a non-capturing subpattern, the option letters may appear + between the "?" and the ":". Thus the two patterns + + (?i:saturday|sunday) + (?:(?i)saturday|sunday) + + match exactly the same set of strings. Because alternative branches are + tried from left to right, and options are not reset until the end of + the subpattern is reached, an option setting in one branch does affect + subsequent branches, so the above patterns match "SUNDAY" as well as + "Saturday". + + +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- + 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. + + +REPETITION + + 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 + 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, + and the first must be less than or equal to the second. For example: + + z{2,4} + + matches "zz", "zzz", or "zzzz". A closing brace on its own is not a + special character. If the second number is omitted, but the comma is + present, there is no upper limit; if the second number and the comma + are both omitted, the quantifier specifies an exact number of required + matches. Thus + + [aeiou]{3,} + + matches at least 3 successive vowels, but may match many more, while + + \d{8} + + matches exactly 8 digits. An opening curly bracket that appears in a + position where a quantifier is not allowed, or one that does not match + the syntax of a quantifier, is taken as a literal character. For exam- + ple, {,6} is not a quantifier, but a literal string of four characters. + + In UTF-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. + + The quantifier {0} is permitted, causing the expression to behave as if + the previous item and the quantifier were not present. + + For convenience (and historical compatibility) the three most common + quantifiers have single-character abbreviations: + + * is equivalent to {0,} + + is equivalent to {1,} + ? is equivalent to {0,1} + + It is possible to construct infinite loops by following a subpattern + that can match no characters with a quantifier that has no upper limit, + for example: + + (a?)* + + Earlier versions of Perl and PCRE used to give an error at compile time + for such patterns. However, because there are cases where this can be + useful, such patterns are now accepted, but if any repetition of the + subpattern does in fact match no characters, the loop is forcibly bro- + ken. + + By default, the quantifiers are "greedy", that is, they match as much + as possible (up to the maximum number of permitted times), without + causing the rest of the pattern to fail. The classic example of where + this gives problems is in trying to match comments in C programs. These + appear between the sequences /* and */ and within the sequence, indi- + vidual * and / characters may appear. An attempt to match C comments by + applying the pattern + + /\*.*\*/ + + to the string + + /* first command */ not comment /* second comment */ + + fails, because it matches the entire string owing to the greediness of + the .* item. + + However, if a quantifier is followed by a question mark, it ceases to + be greedy, and instead matches the minimum number of times possible, so + the pattern + + /\*.*?\*/ + + does the right thing with the C comments. The meaning of the various + quantifiers is not otherwise changed, just the preferred number of + matches. Do not confuse this use of question mark with its use as a + quantifier in its own right. Because it has two uses, it can sometimes + appear doubled, as in + + \d??\d + + which matches one digit by preference, but can match two if that is the + only way the rest of the pattern matches. + + If the PCRE_UNGREEDY option is set (an option which is not available in + Perl), the quantifiers are not greedy by default, but individual ones + can be made greedy by following them with a question mark. In other + words, it inverts the default behaviour. + + When a parenthesized subpattern is quantified with a minimum repeat + count that is greater than 1 or with a limited maximum, more store is + required for the compiled pattern, in proportion to the size of the + minimum or maximum. + + If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv- + alent to Perl's /s) is set, thus allowing the . to match newlines, the + pattern is implicitly anchored, because whatever follows will be tried + against every character position in the subject string, so there is no + point in retrying the overall match at any position after the first. + PCRE normally treats such a pattern as though it were preceded by \A. + + In cases where it is known that the subject string contains no new- + lines, it is worth setting PCRE_DOTALL in order to obtain this opti- + mization, or alternatively using ^ to indicate anchoring explicitly. + + However, there is one situation where the optimization cannot be used. + When .* is inside capturing parentheses that are the subject of a + backreference elsewhere in the pattern, a match at the start may fail, + and a later one succeed. Consider, for example: + + (.*)abc\1 + + If the subject is "xyz123abc123" the match point is the fourth charac- + ter. For this reason, such a pattern is not implicitly anchored. + + When a capturing subpattern is repeated, the value captured is the sub- + string that matched the final iteration. For example, after + + (tweedle[dume]{3}\s*)+ + + has matched "tweedledum tweedledee" the value of the captured substring + is "tweedledee". However, if there are nested capturing subpatterns, + the corresponding captured values may have been set in previous itera- + tions. For example, after + + /(a|(b))+/ + + matches "aba" the value of the second captured substring is "b". + + +ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS + + With both maximizing and minimizing repetition, failure of what follows + normally causes the repeated item to be re-evaluated to see if a dif- + ferent number of repeats allows the rest of the pattern to match. Some- + times it is useful to prevent this, either to change the nature of the + match, or to cause it fail earlier than it otherwise might, when the + author of the pattern knows there is no point in carrying on. + + Consider, for example, the pattern \d+foo when applied to the subject + line + + 123456bar + + After matching all 6 digits and then failing to match "foo", the normal + action of the matcher is to try again with only 5 digits matching the + \d+ item, and then with 4, and so on, before ultimately failing. + "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides + the means for specifying that once a subpattern has matched, it is not + to be re-evaluated in this way. + + If we use atomic grouping for the previous example, the matcher would + give up immediately on failing to match "foo" the first time. The nota- + tion is a kind of special parenthesis, starting with (?> as in this + example: + + (?>\d+)foo + + This kind of parenthesis "locks up" the part of the pattern it con- + tains once it has matched, and a failure further into the pattern is + prevented from backtracking into it. Backtracking past it to previous + items, however, works as normal. + + An alternative description is that a subpattern of this type matches + the string of characters that an identical standalone pattern would + match, if anchored at the current point in the subject string. + + Atomic grouping subpatterns are not capturing subpatterns. Simple cases + such as the above example can be thought of as a maximizing repeat that + must swallow everything it can. So, while both \d+ and \d+? are pre- + pared to adjust the number of digits they match in order to make the + rest of the pattern match, (?>\d+) can only match an entire sequence of + digits. + + Atomic groups in general can of course contain arbitrarily complicated + subpatterns, and can be nested. However, when the subpattern for an + atomic group is just a single repeated item, as in the example above, a + simpler notation, called a "possessive quantifier" can be used. This + consists of an additional + character following a quantifier. Using + this notation, the previous example can be rewritten as + + \d++bar + + Possessive quantifiers are always greedy; the setting of the + PCRE_UNGREEDY option is ignored. They are a convenient notation for the + simpler forms of atomic group. However, there is no difference in the + meaning or processing of a possessive quantifier and the equivalent + atomic group. + + The possessive quantifier syntax is an extension to the Perl syntax. It + originates in Sun's Java package. + + When a pattern contains an unlimited repeat inside a subpattern that + can itself be repeated an unlimited number of times, the use of an + atomic group is the only way to avoid some failing matches taking a + very long time indeed. The pattern + + (\D+|<\d+>)*[!?] + + matches an unlimited number of substrings that either consist of non- + digits, or digits enclosed in <>, followed by either ! or ?. When it + matches, it runs quickly. However, if it is applied to + + aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa + + it takes a long time before reporting failure. This is because the + string can be divided between the 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 + + ((?>\D+)|<\d+>)*[!?] + + sequences of non-digits cannot be broken, and failure happens quickly. + + +BACK REFERENCES + + Outside a character class, a backslash followed by a digit greater than + 0 (and possibly further digits) is a back reference to a capturing sub- + pattern earlier (that is, to its left) in the pattern, provided there + have been that many previous capturing left parentheses. + + However, if the decimal number following the backslash is less than 10, + it is always taken as a back reference, and causes an error only if + there are not that many capturing left parentheses in the entire pat- + tern. In other words, the parentheses that are referenced need not be + to the left of the reference for numbers less than 10. 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 + the subpattern itself (see "Subpatterns as subroutines" below for a way + of doing that). So the pattern + + (sens|respons)e and \1ibility + + matches "sense and sensibility" and "response and responsibility", but + not "sense and responsibility". If caseful matching is in force at the + time of the back reference, the case of letters is relevant. For exam- + ple, + + ((?i)rah)\s+\1 + + matches "rah rah" and "RAH RAH", but not "RAH rah", even though the + original capturing subpattern is matched caselessly. + + 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 + 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. + 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. + + A back reference that occurs inside the parentheses to which it refers + fails when the subpattern is first used, so, for example, (a\1) never + matches. However, such references can be useful inside repeated sub- + patterns. For example, the pattern + + (a|b\1)+ + + matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- + ation of the subpattern, the back reference matches the character + string corresponding to the previous iteration. In order for this to + work, the pattern must be such that the first iteration does not need + to match the back reference. This can be done using alternation, as in + the example above, or by a quantifier with a minimum of zero. + + +ASSERTIONS + + An assertion is a test on the characters following or preceding the + current matching point that does not actually consume any characters. + The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are + described above. More complicated assertions are coded as subpatterns. + There are two kinds: those that look ahead of the current position in + the subject string, and those that look behind it. + + An assertion subpattern is matched in the normal way, except that it + does not cause the current matching position to be changed. Lookahead + assertions start with (?= for positive assertions and (?! for negative + assertions. For example, + + \w+(?=;) + + matches a word followed by a semicolon, but does not include the semi- + colon in the match, and + + foo(?!bar) + + matches any occurrence of "foo" that is not followed by "bar". Note + that the apparently similar pattern + + (?!foo)bar + + does not find an occurrence of "bar" that is preceded by something + other than "foo"; it finds any occurrence of "bar" whatsoever, because + the assertion (?!foo) is always true when the next three characters are + "bar". A lookbehind assertion is needed to achieve this 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 start with (?<= for positive assertions and (?<! + for negative assertions. For example, + + (?<!foo)bar + + does find an occurrence of "bar" that is not preceded by "foo". The + contents of a lookbehind assertion are restricted such that all the + strings it matches must have a fixed length. However, if there are sev- + eral alternatives, they do not all have to have the same fixed length. + Thus + + (?<=bullock|donkey) + + is permitted, but + + (?<!dogs?|cats?) + + causes an error at compile time. Branches that match different length + strings are permitted only at the top level of a lookbehind assertion. + This is an extension compared with Perl (at least for 5.8), which + requires all branches to match the same length of string. An assertion + such as + + (?<=ab(c|de)) + + is not permitted, because its single top-level branch can match two + different lengths, but it is acceptable if rewritten to use two top- + level branches: + + (?<=abc|abde) + + The implementation of lookbehind assertions is, for each alternative, + to temporarily move the current position back by the fixed width and + then try to match. If there are insufficient characters before the cur- + rent position, the match is deemed to fail. + + 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. + + 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 + proceeds from left to right, PCRE will look for each "a" in the subject + and then see if what follows matches the rest of the pattern. If the + pattern is specified as + + ^.*abcd$ + + the initial .* matches the entire string at first, but when this fails + (because there is no following "a"), it backtracks to match all but the + last character, then all but the last two characters, and so on. Once + again the search for "a" covers the entire string, from right to left, + so we are no better off. However, if the pattern is written as + + ^(?>.*)(?<=abcd) + + or, equivalently, + + ^.*+(?<=abcd) + + there can be no backtracking for the .* item; it can match only the + entire string. The subsequent lookbehind assertion does a single test + on the last four characters. If it fails, the match fails immediately. + For long strings, this approach makes a significant difference to the + processing time. + + Several assertions (of any sort) may occur in succession. For example, + + (?<=\d{3})(?<!999)foo + + matches "foo" preceded by three digits that are not "999". Notice that + each of the assertions is applied independently at the same point in + the subject string. First there is a check that the previous three + characters are all digits, and then there is a check that the same + three characters are not "999". This pattern does not match "foo" pre- + ceded by six characters, the first of which are digits and the last + three of which are not "999". For example, it doesn't match "123abc- + foo". A pattern to do that is + + (?<=\d{3}...)(?<!999)foo + + This time the first assertion looks at the preceding six characters, + checking that the first three are digits, and then the second assertion + checks that the preceding three characters are not "999". + + Assertions can be nested in any combination. For example, + + (?<=(?<!foo)bar)baz + + matches an occurrence of "baz" that is preceded by "bar" which in turn + is not preceded by "foo", while + + (?<=\d{3}(?!999)...)foo + + is another pattern which 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 + + (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) + + If the condition is satisfied, the yes-pattern is used; otherwise the + no-pattern (if present) is used. If there are more than two alterna- + tives in the subpattern, a compile-time error occurs. + + 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 + discussion: + + ( \( )? [^()]+ (?(1) \) ) + + The first part matches an optional opening parenthesis, and if that + character is present, sets it as the first captured substring. The sec- + ond part matches one or more characters that are not parentheses. The + third part is a conditional subpattern that tests whether 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, + 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 + 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 + white space, and with the two alternatives on the second line: + + (?(?=[^a-z]*[a-z]) + \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) + + The condition is a positive lookahead assertion that matches an + optional sequence of non-letters followed by a letter. In other words, + it tests for the presence of at least one letter in the subject. If a + letter is found, the subject is matched against the first alternative; + otherwise it is matched against the second. This pattern matches + strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are + letters and dd are digits. + + +COMMENTS + + 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 + at all. + + 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: + + $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 + individual subpattern recursion. + + 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 + expression. + + 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- + sized substring). Finally there is a closing parenthesis. + + 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 + 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. + 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 + 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 + + (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, + 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- + 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 + PCRE_ERROR_NOMEMORY error. + + Do not confuse the (?R) item with the condition (R), which tests for + recursion. Consider this pattern, which matches text in angle brack- + ets, allowing for arbitrary nesting. Only digits are allowed in nested + brackets (that is, when recursing), whereas any characters are permit- + ted at the outer level. + + < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > + + In this pattern, (?(R) is the start of a conditional subpattern, with + two different alternatives for the recursive and non-recursive cases. + The (?R) item is the actual recursive call. + + +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 + pointed out that the pattern + + (sens|respons)e and \1ibility + + matches "sense and sensibility" and "response and responsibility", but + not "sense and responsibility". If instead the pattern + + (sens|respons)e and (?1)ibility + + is used, it does match "sense and responsibility" as well as the other + two strings. 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. + 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 + all calling out. + + Within a regular expression, (?C) indicates the points at which the + external function is to be called. If you want to identify different + callout points, you can put a number less than 256 after the letter C. + The default value is zero. For example, this pattern has two callout + points: + + (?C1)abc(?C2)def + + 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. +----------------------------------------------------------------------------- + +PCRE(3) PCRE(3) + + + +NAME + PCRE - Perl-compatible regular expressions + +PCRE PERFORMANCE + + Certain items that may appear in regular expression patterns are more + efficient than others. It is more efficient to use a character class + 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. + + When a pattern begins with .* not in parentheses, or in parentheses + that are not the subject of a backreference, and the PCRE_DOTALL option + is set, the pattern is implicitly anchored by PCRE, since it can match + only at the start of a subject string. However, if PCRE_DOTALL is not + set, PCRE cannot make this optimization, because the . metacharacter + does not then match a newline, and if the subject string contains new- + lines, the pattern may match from the character immediately following + one of them instead of from the very start. For example, the pattern + + .*second + + matches the subject "first\nand second" (where \n stands for a newline + character), with the match starting at the seventh character. In order + to do this, PCRE has to retry the match starting after every newline in + the subject. + + If you are using such a pattern with subject strings that do not con- + tain newlines, the best performance is obtained by setting PCRE_DOTALL, + or starting the pattern with ^.* 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. + 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 + the pattern is such that the entire match is going to fail, PCRE has in + principle to try every possible variation, and this can take an + extremely long time. + + An optimization catches some of the more simple cases such as + + (a+)*b + + where a literal character follows. Before embarking on the standard + matching procedure, PCRE checks that there is a "b" later in the sub- + ject string, and if there is not, it fails the match immediately. How- + ever, when there is no following literal this optimization cannot be + used. You can see the difference by comparing the behaviour of + + (a+)*\d + + with the pattern above. The former gives a failure almost instantly + when applied to a whole line of "a" characters, whereas the latter + takes an appreciable time with strings longer than about 20 characters. + +Last updated: 03 February 2003 +Copyright (c) 1997-2003 University of Cambridge. +----------------------------------------------------------------------------- + +PCRE(3) PCRE(3) + + + +NAME + PCRE - Perl-compatible regular expressions. + +SYNOPSIS OF POSIX API + #include <pcreposix.h> + + int regcomp(regex_t *preg, const char *pattern, + int cflags); + + int regexec(regex_t *preg, const char *string, + size_t nmatch, regmatch_t pmatch[], int eflags); + + size_t regerror(int errcode, const regex_t *preg, + char *errbuf, size_t errbuf_size); + + void regfree(regex_t *preg); + + +DESCRIPTION + + This set of functions provides a POSIX-style API to the PCRE regular + expression package. See the pcreapi documentation for a description of + the 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 + functions call the native ones, it is also necessary to add -lpcre. + + I have implemented only those option bits that can be reasonably mapped + to PCRE native options. In addition, the options REG_EXTENDED and + REG_NOSUB are defined with the value zero. They have no effect, but + since programs that are written to the POSIX interface often use them, + this makes it easier to slot in PCRE as a replacement library. Other + POSIX options are not even defined. + + When PCRE is called via these functions, it is only the API that is + POSIX-like in style. The syntax and semantics of the regular expres- + sions themselves are still those of Perl, subject to the setting of + various PCRE options, as described below. "POSIX-like in style" means + that the API approximates to the POSIX definition; it is not fully + POSIX-compatible, and in multi-byte encoding domains it is probably + even less compatible. + + The header for these functions is supplied as pcreposix.h to avoid any + potential clash with other POSIX libraries. It can, of course, be + renamed or aliased as regex.h, which is the "correct" name. It provides + two structure types, regex_t for compiled internal forms, and reg- + match_t for returning captured substrings. It also defines some con- + stants whose names start with "REG_"; these are used for setting + options and identifying error codes. + + +COMPILING A PATTERN + + The function regcomp() is called to compile a pattern into an internal + form. The pattern is a C string terminated by a binary zero, and is + passed in the argument pattern. The preg argument is a pointer to a + regex_t structure which 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 + defined by the following macros: + + REG_ICASE + + The PCRE_CASELESS option is set when the expression is passed for com- + pilation to the native function. + + REG_NEWLINE + + The PCRE_MULTILINE option is set when the expression is passed for com- + pilation to the native function. Note that this does not mimic the + defined POSIX behaviour for REG_NEWLINE (see the following section). + + In the absence of these flags, no options are passed to the native + function. This means the the regex is compiled with PCRE default + semantics. In particular, the way it handles newline characters in the + subject string is the Perl way, not the POSIX way. Note that setting + PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE. + It does not affect the way newlines are matched by . (they aren't) or + by a negative class such as [^a] (they are). + + The yield of regcomp() is zero on success, and non-zero otherwise. The + preg structure is filled in on success, and one member of the structure + is public: re_nsub contains the number of capturing subpatterns in the + regular expression. Various error codes are defined in the header file. + + +MATCHING NEWLINE CHARACTERS + + This area is not simple, because POSIX and Perl take different views of + things. It is not possible to get PCRE to obey POSIX semantics, but + then PCRE was never intended to be a POSIX engine. The following table + lists the different possibilities for matching newline characters in + PCRE: + + Default Change with + + . matches newline no PCRE_DOTALL + newline matches [^a] yes not changeable + $ matches \n at end yes PCRE_DOLLARENDONLY + $ matches \n in middle no PCRE_MULTILINE + ^ matches \n in middle no PCRE_MULTILINE + + This is the equivalent table for POSIX: + + Default Change with + + . matches newline yes REG_NEWLINE + newline matches [^a] yes REG_NEWLINE + $ matches \n at end no REG_NEWLINE + $ matches \n in middle no REG_NEWLINE + ^ matches \n in middle no REG_NEWLINE + + PCRE's behaviour is the same as Perl's, except that there is no equiva- + lent for PCRE_DOLLARENDONLY 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 + behave exactly as for the REG_NEWLINE action. + + +MATCHING A PATTERN + + The function regexec() is called to match a pre-compiled pattern preg + against a given string, which is terminated by a zero byte, subject to + the options in eflags. These can be: + + REG_NOTBOL + + The PCRE_NOTBOL option is set when calling the underlying PCRE matching + function. + + REG_NOTEOL + + The PCRE_NOTEOL option is set when calling the underlying PCRE matching + function. + + The portion of the string that was matched, and also any captured sub- + strings, are returned via the pmatch argument, which points to an array + of nmatch structures of type regmatch_t, containing the members rm_so + and rm_eo. These contain the offset to the first character of each sub- + string and the offset to the first character after the end of each sub- + string, respectively. The 0th element of the vector relates to the + entire portion of string that was matched; subsequent elements relate + to the capturing subpatterns of the regular expression. Unused entries + in the array have both structure members set to -1. + + A successful match yields a zero return; various error codes are + defined in the header file, of which REG_NOMATCH is the "expected" + failure code. + + +ERROR MESSAGES + + The regerror() function maps a non-zero errorcode from either regcomp() + or regexec() to a printable message. If preg is not NULL, the error + should have arisen from the use of that structure. A message terminated + by a binary zero is placed in errbuf. The length of the message, + including the zero, is limited to errbuf_size. The yield of the func- + tion is the size of buffer needed to hold the whole message. + + +STORAGE + + Compiling a regular expression causes memory to be allocated and asso- + ciated with the preg structure. The function regfree() frees all such + memory, after which preg may no longer be used as a compiled expres- + sion. + + +AUTHOR + + Philip Hazel <ph10@cam.ac.uk> + University Computing Service, + Cambridge CB2 3QG, England. + +Last updated: 03 February 2003 +Copyright (c) 1997-2003 University of Cambridge. +----------------------------------------------------------------------------- + +PCRE(3) PCRE(3) + + + +NAME + PCRE - Perl-compatible regular expressions + +PCRE SAMPLE PROGRAM + + A simple, complete demonstration program, to get you started with using + PCRE, is supplied in the file pcredemo.c in the PCRE distribution. + + The program compiles the regular expression that is its first argument, + and matches it against the subject string in its second argument. No + PCRE options are set, and default character tables are used. If match- + ing succeeds, the program outputs the portion of the subject that + matched, together with the contents of any captured substrings. + + If the -g option is given on the command line, the program then goes on + to check for further matches of the same regular expression in the same + subject string. The logic is a little bit tricky because of the possi- + bility of matching an empty string. Comments in the code explain what + is going on. + + On a Unix 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 \ + -L/usr/local/lib -lpcre + + Then you can run simple tests like this: + + ./pcredemo 'cat|dog' 'the cat sat on the mat' + ./pcredemo -g 'cat|dog' 'the dog sat on the cat' + + Note that there is a much more comprehensive test program, called + pcretest, which supports many more facilities for testing regular + expressions and 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: + + ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or + directory + + This is caused by the way shared library support works on those sys- + tems. You need to add + + -R/usr/local/lib + + to the compile command to get round this problem. + +Last updated: 28 January 2003 +Copyright (c) 1997-2003 University of Cambridge. +----------------------------------------------------------------------------- + |