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Sat Feb 24 21:39:42 2007 UTC
+revision 75 by nigel,
Sat Feb 24 21:40:37 2007 UTC
 Line 1
  .TH PCRE 3
  .SH NAME
- pcre - Perl-compatible regular expressions.
+ PCRE - Perl-compatible regular expressions
- .SH SYNOPSIS
+ .SH INTRODUCTION
- .B #include <pcre.h>
+ .rs
- .PP
+ .sp
- .SM
- .br
- .B pcre *pcre_compile(const char *\fIpattern\fR, int \fIoptions\fR,
- .ti +5n
- .B const char **\fIerrptr\fR, int *\fIerroffset\fR,
- .ti +5n
- .B const unsigned char *\fItableptr\fR);
- .PP
- .br
- .B pcre_extra *pcre_study(const pcre *\fIcode\fR, int \fIoptions\fR,
- .ti +5n
- .B const char **\fIerrptr\fR);
- .PP
- .br
- .B int pcre_exec(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
- .ti +5n
- .B "const char *\fIsubject\fR," int \fIlength\fR, int \fIstartoffset\fR,
- .ti +5n
- .B int \fIoptions\fR, int *\fIovector\fR, int \fIovecsize\fR);
- .PP
- .br
- .B int pcre_copy_substring(const char *\fIsubject\fR, int *\fIovector\fR,
- .ti +5n
- .B int \fIstringcount\fR, int \fIstringnumber\fR, char *\fIbuffer\fR,
- .ti +5n
- .B int \fIbuffersize\fR);
- .PP
- .br
- .B int pcre_get_substring(const char *\fIsubject\fR, int *\fIovector\fR,
- .ti +5n
- .B int \fIstringcount\fR, int \fIstringnumber\fR,
- .ti +5n
- .B const char **\fIstringptr\fR);
- .PP
- .br
- .B int pcre_get_substring_list(const char *\fIsubject\fR,
- .ti +5n
- .B int *\fIovector\fR, int \fIstringcount\fR, "const char ***\fIlistptr\fR);"
- .PP
- .br
- .B void pcre_free_substring(const char *\fIstringptr\fR);
- .PP
- .br
- .B void pcre_free_substring_list(const char **\fIstringptr\fR);
- .PP
- .br
- .B const unsigned char *pcre_maketables(void);
- .PP
- .br
- .B int pcre_fullinfo(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
- .ti +5n
- .B int \fIwhat\fR, void *\fIwhere\fR);
- .PP
- .br
- .B int pcre_info(const pcre *\fIcode\fR, int *\fIoptptr\fR, int
- .B *\fIfirstcharptr\fR);
- .PP
- .br
- .B char *pcre_version(void);
- .PP
- .br
- .B void *(*pcre_malloc)(size_t);
- .PP
- .br
- .B void (*pcre_free)(void *);
- .SH DESCRIPTION
  The PCRE library is a set of functions that implement regular expression
- pattern matching using the same syntax and semantics as Perl 5, with just a few
+ pattern matching using the same syntax and semantics as Perl, with just a few
- differences (see below). The current implementation corresponds to Perl 5.005,
+ differences. The current implementation of PCRE (release 5.x) corresponds
- with some additional features from later versions. This includes some
+ approximately with Perl 5.8, including support for UTF-8 encoded strings and
- experimental, incomplete support for UTF-8 encoded strings. Details of exactly
+ Unicode general category properties. However, this support has to be explicitly
- what is and what is not supported are given below.
+ enabled; it is not the default.
+ .P
- PCRE has its own native API, which is described in this document. There is also
+ PCRE is written in C and released as a C library. A number of people have
- a set of wrapper functions that correspond to the POSIX regular expression API.
+ written wrappers and interfaces of various kinds. A C++ class is included in
- These are described in the \fBpcreposix\fR documentation.
+ these contributions, which can be found in the \fIContrib\fR directory at the
+ primary FTP site, which is:
- The native API function prototypes are defined in the header file \fBpcre.h\fR,
+ .sp
- and on Unix systems the library itself is called \fBlibpcre.a\fR, so can be
+ .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
- accessed by adding \fB-lpcre\fR to the command for linking an application which
+ .\" </a>
- calls it. The header file defines the macros PCRE_MAJOR and PCRE_MINOR to
+ ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
- contain the major and minor release numbers for the library. Applications can
+ .P
- use these to include support for different releases.
+ Details of exactly which Perl regular expression features are and are not
+ supported by PCRE are given in separate documents. See the
- The functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and \fBpcre_exec()\fR
+ .\" HREF
- are used for compiling and matching regular expressions. A sample program that
+ \fBpcrepattern\fR
- demonstrates the simplest way of using them is given in the file
+ .\"
- \fIpcredemo.c\fR. The last section of this man page describes how to run it.
+ and
+ .\" HREF
- The functions \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
+ \fBpcrecompat\fR
- \fBpcre_get_substring_list()\fR are convenience functions for extracting
+ .\"
- captured substrings from a matched subject string; \fBpcre_free_substring()\fR
+ pages.
- and \fBpcre_free_substring_list()\fR are also provided, to free the memory used
+ .P
- for extracted strings.
+ Some features of PCRE can be included, excluded, or changed when the library is
+ built. The
- The function \fBpcre_maketables()\fR is used (optionally) to build a set of
+ .\" HREF
- character tables in the current locale for passing to \fBpcre_compile()\fR.
+ \fBpcre_config()\fR
+ .\"
- The function \fBpcre_fullinfo()\fR is used to find out information about a
+ function makes it possible for a client to discover which features are
- compiled pattern; \fBpcre_info()\fR is an obsolete version which returns only
+ available. The features themselves are described in the
- some of the available information, but is retained for backwards compatibility.
+ .\" HREF
- The function \fBpcre_version()\fR returns a pointer to a string containing the
+ \fBpcrebuild\fP
- version of PCRE and its date of release.
+ .\"
+ page. Documentation about building PCRE for various operating systems can be
- The global variables \fBpcre_malloc\fR and \fBpcre_free\fR initially contain
+ found in the \fBREADME\fP file in the source distribution.
- the entry points of the standard \fBmalloc()\fR and \fBfree()\fR functions
+ .
- respectively. PCRE calls the memory management functions via these variables,
+ .
- so a calling program can replace them if it wishes to intercept the calls. This
+ .SH "USER DOCUMENTATION"
- should be done before calling any PCRE functions.
+ .rs
+ .sp
+ The user documentation for PCRE comprises a number of different sections. In
- .SH MULTI-THREADING
+ the "man" format, each of these is a separate "man page". In the HTML format,
- The PCRE functions can be used in multi-threading applications, with the
+ each is a separate page, linked from the index page. In the plain text format,
- proviso that the memory management functions pointed to by \fBpcre_malloc\fR
+ all the sections are concatenated, for ease of searching. The sections are as
- and \fBpcre_free\fR are shared by all threads.
+ follows:
+ .sp
- The compiled form of a regular expression is not altered during matching, so
+   pcre              this document
- the same compiled pattern can safely be used by several threads at once.
+   pcreapi           details of PCRE's native API
+   pcrebuild         options for building PCRE
+   pcrecallout       details of the callout feature
- .SH COMPILING A PATTERN
+   pcrecompat        discussion of Perl compatibility
- The function \fBpcre_compile()\fR is called to compile a pattern into an
+   pcregrep          description of the \fBpcregrep\fP command
- internal form. The pattern is a C string terminated by a binary zero, and
+   pcrepartial       details of the partial matching facility
- is passed in the argument \fIpattern\fR. A pointer to a single block of memory
+ .\" JOIN
- that is obtained via \fBpcre_malloc\fR is returned. This contains the compiled
+   pcrepattern       syntax and semantics of supported
- code and related data. The \fBpcre\fR type is defined for the returned block;
+                       regular expressions
- this is a typedef for a structure whose contents are not externally defined. It
+   pcreperform       discussion of performance issues
- is up to the caller to free the memory when it is no longer required.
+   pcreposix         the POSIX-compatible API
+   pcreprecompile    details of saving and re-using precompiled patterns
- Although the compiled code of a PCRE regex is relocatable, that is, it does not
+   pcresample        discussion of the sample program
- depend on memory location, the complete \fBpcre\fR data block is not
+   pcretest          description of the \fBpcretest\fP testing command
- fully relocatable, because it contains a copy of the \fItableptr\fR argument,
+ .sp
- which is an address (see below).
+ In addition, in the "man" and HTML formats, there is a short page for each
+ library function, listing its arguments and results.
- The size of a compiled pattern is roughly proportional to the length of the
+ .
- pattern string, except that each character class (other than those containing
+ .
- just a single character, negated or not) requires 33 bytes, and repeat
- quantifiers with a minimum greater than one or a bounded maximum cause the
- relevant portions of the compiled pattern to be replicated.
- The \fIoptions\fR argument contains independent bits that affect the
- compilation. 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
- below). For these options, the contents of the \fIoptions\fR 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 \fIerrptr\fR is NULL, \fBpcre_compile()\fR returns NULL immediately.
- Otherwise, if compilation of a pattern fails, \fBpcre_compile()\fR returns
- NULL, and sets the variable pointed to by \fIerrptr\fR to point to a textual
- error message. The offset from the start of the pattern to the character where
- the error was discovered is placed in the variable pointed to by
- \fIerroffset\fR, which must not be NULL. If it is, an immediate error is given.
- If the final argument, \fItableptr\fR, is NULL, PCRE uses a default set of
- character tables which are built when it is compiled, using the default C
- locale. Otherwise, \fItableptr\fR must be the result of a call to
- \fBpcre_maketables()\fR. See the section on locale support below.
- This code fragment shows a typical straightforward call to \fBpcre_compile()\fR:
-   pcre *re;
-   const char *error;
-   int erroffset;
-   re = pcre_compile(
-     "^A.*Z",          /* the pattern */
-,                /* default options */
-     &error,           /* for error message */
-     &erroffset,       /* for error offset */
-     NULL);            /* use default character tables */
- The following option bits are defined in the header file:
-   PCRE_ANCHORED
- If this bit is set, the pattern is forced to be "anchored", that is, it is
- constrained to match only at the start of 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.
-   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.
-   PCRE_DOTALL
- If this bit is set, a dot metacharater in the pattern matches all characters,
- including newlines. Without it, newlines are excluded. This option is
- equivalent to Perl's /s option. 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, and 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 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_DOLLAR_ENDONLY is set). This is the same as
- Perl.
- When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs
- match immediately following or immediately before any newline in the subject
- string, respectively, as well as at the very start and end. This is equivalent
- to Perl's /m option. If there are no "\\n" characters in a subject string, or
- no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no
- effect.
-   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 just byte 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. Support for UTF-8 is new, experimental, and incomplete.
- Details of exactly what it entails are given below.
- .SH STUDYING A PATTERN
- 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 matching. The
- function \fBpcre_study()\fR takes a pointer to a compiled pattern as its first
- argument, and returns a pointer to a \fBpcre_extra\fR block (another typedef
- for a structure with hidden contents) containing additional information about
- the pattern; this can be passed to \fBpcre_exec()\fR. If no additional
- information is available, NULL is returned.
- The second argument contains option bits. At present, no options are defined
- for \fBpcre_study()\fR, and this argument should always be zero.
- The third argument for \fBpcre_study()\fR is a pointer to 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 message.
- This is a typical call to \fBpcre_study\fR():
-   pcre_extra *pe;
-   pe = pcre_study(
-     re,             /* result of pcre_compile() */
-,              /* 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 possible starting
- characters is created.
- .SH LOCALE SUPPORT
- PCRE handles caseless matching, and determines whether characters are letters,
- digits, or whatever, by reference to a set of tables. The library contains a
- default set of tables which is created in the default C locale when PCRE is
- compiled. This is used when the final argument of \fBpcre_compile()\fR is NULL,
- and is sufficient for many applications.
- An alternative set of tables can, however, be supplied. Such tables are built
- by calling the \fBpcre_maketables()\fR function, which has no arguments, in the
- relevant locale. The result can then be passed to \fBpcre_compile()\fR as often
- as necessary. For example, to build and use tables that are appropriate for the
- French locale (where accented characters with codes greater than 128 are
- treated as letters), the following 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 \fBpcre_malloc\fR. The
- pointer that is passed to \fBpcre_compile\fR is saved with the compiled
- pattern, and the same tables are used via this pointer by \fBpcre_study()\fR
- and \fBpcre_exec()\fR. 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.
- .SH INFORMATION ABOUT A PATTERN
- The \fBpcre_fullinfo()\fR function returns information about a compiled
- pattern. It replaces the obsolete \fBpcre_info()\fR function, which is
- nevertheless retained for backwards compability (and is documented below).
- The first argument for \fBpcre_fullinfo()\fR is a pointer to the compiled
- pattern. The second argument is the result of \fBpcre_study()\fR, or NULL if
- the pattern was not studied. The third argument specifies which piece of
- information is required, while 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 \fIcode\fR was NULL
-                         the argument \fIwhere\fR was NULL
-   PCRE_ERROR_BADMAGIC   the "magic number" was not found
-   PCRE_ERROR_BADOPTION  the value of \fIwhat\fR was invalid
- Here is a typical call of \fBpcre_fullinfo()\fR, 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 \fBpcre.h\fR, and are
- as follows:
-   PCRE_INFO_OPTIONS
- Return a copy of the options with which the pattern was compiled. The fourth
- argument should point to an \fBunsigned long int\fR variable. These option bits
- are those specified in the call to \fBpcre_compile()\fR, modified by any
- top-level option settings within the pattern itself, and with the PCRE_ANCHORED
- bit forcibly set if the form of the pattern implies that it can match only at
- the start of a subject string.
-   PCRE_INFO_SIZE
- Return the size of the compiled pattern, that is, the value that was passed as
- the argument to \fBpcre_malloc()\fR when PCRE was getting memory in which to
- place the compiled data. The fourth argument should point to a \fBsize_t\fR
- variable.
-   PCRE_INFO_CAPTURECOUNT
- Return the number of capturing subpatterns in the pattern. The fourth argument
- should point to an \fbint\fR variable.
-   PCRE_INFO_BACKREFMAX
- Return the number of the highest back reference in the pattern. The fourth
- argument should point to an \fBint\fR variable. Zero is returned if there are
- no back references.
-   PCRE_INFO_FIRSTCHAR
- Return information about the first character of any matched string, for a
- non-anchored pattern. If there is a fixed first character, e.g. from a pattern
- such as (cat|cow|coyote), it is returned in the integer pointed to by
- \fIwhere\fR. 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 "\\n" 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 characters for the first character in any
- matching string, a pointer to the table is returned. Otherwise NULL is
- returned. The fourth argument should point to an \fBunsigned char *\fR
- variable.
-   PCRE_INFO_LASTLITERAL
- For a non-anchored pattern, return the value of the rightmost literal character
- which must exist in any matched string, other than at its start. The fourth
- argument should point to an \fBint\fR variable. If there is no such character,
- or if the pattern is anchored, -1 is returned. For example, for the pattern
- /a\\d+z\\d+/ the returned value is 'z'.
- The \fBpcre_info()\fR function is now obsolete because its interface is too
- restrictive to return all the available data about a compiled pattern. New
- programs should use \fBpcre_fullinfo()\fR instead. The yield of
- \fBpcre_info()\fR is the number of capturing subpatterns, or one of the
- following negative numbers:
-   PCRE_ERROR_NULL       the argument \fIcode\fR was NULL
-   PCRE_ERROR_BADMAGIC   the "magic number" was not found
- If the \fIoptptr\fR 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 \fIfirstcharptr\fR argument is not NULL,
- it is used to pass back information about the first character of any matched
- string (see PCRE_INFO_FIRSTCHAR above).
- .SH MATCHING A PATTERN
- The function \fBpcre_exec()\fR is called to match a subject string against a
- pre-compiled pattern, which is passed in the \fIcode\fR argument. If the
- pattern has been studied, the result of the study should be passed in the
- \fIextra\fR argument. Otherwise this must be NULL.
- Here is an example of a simple call to \fBpcre_exec()\fR:
-   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 */
-,             /* the length of the subject string */
-,              /* start at offset 0 in the subject */
-,              /* default options */
-     ovector,        /* vector for substring information */
-);            /* number of elements in the vector */
- The PCRE_ANCHORED option can be passed in the \fIoptions\fR argument, whose
- unused bits must be zero. 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.
- 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 metacharacter
- 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 occurrences of "a" or "b".
- Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a special case
- of a pattern match of the empty string within its \fBsplit()\fR 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 as a pointer in \fIsubject\fR, a length in
- \fIlength\fR, and a starting offset in \fIstartoffset\fR. Unlike the pattern
- string, the subject may contain binary zero characters. When the starting
- offset is zero, the search for a match starts at the beginning of the subject,
- and this is by far the most common case.
- A non-zero starting offset is useful when searching for another match in the
- same subject by calling \fBpcre_exec()\fR again after a previous success.
- Setting \fIstartoffset\fR 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 \fBpcre_exec()\fR finds the first
- occurrence. If \fBpcre_exec()\fR 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
- \fBpcre_exec()\fR is passed the entire string again, but with \fIstartoffset\fR
- 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 succeed 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 substring. 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 \fIovector\fR. The number of elements in the vector
- is passed in \fIovecsize\fR. 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 \fBpcre_exec()\fR while
- matching capturing subpatterns, and is not available for passing back
- information. The length passed in \fIovecsize\fR 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 \fIovector\fR, 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 substring, and the second
- is set to the offset of the first character after the end of a substring. The
- first pair, \fIovector[0]\fR and \fIovector[1]\fR, identify the portion of the
- subject string matched by the entire pattern. The next pair is used for the
- first capturing subpattern, and so on. The value returned by \fBpcre_exec()\fR
- 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 \fIn+1\fR to match some
- part of the subject when subpattern \fIn\fR 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 function returns a
- value of zero. In particular, if the substring offsets are not of interest,
- \fBpcre_exec()\fR may be called with \fIovector\fR passed as NULL and
- \fIovecsize\fR as zero. However, if the pattern contains back references and
- the \fIovector\fR isn't big enough to remember the related substrings, PCRE has
- to get additional memory for use during matching. Thus it is usually advisable
- to supply an \fIovector\fR.
- Note that \fBpcre_info()\fR can be used to find out how many capturing
- subpatterns there are in a compiled pattern. The smallest size for
- \fIovector\fR that will allow for \fIn\fR captured substrings in addition to
- the offsets of the substring matched by the whole pattern is (\fIn\fR+1)*3.
- If \fBpcre_exec()\fR 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 \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was
- NULL and \fIovecsize\fR was not zero.
-   PCRE_ERROR_BADOPTION      (-3)
- An unrecognized bit was set in the \fIoptions\fR 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 \fIovector\fR that is passed to
- \fBpcre_exec()\fR 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 \fBpcre_malloc()\fR fails, this error is given. The memory is freed at
- the end of matching.
- .SH EXTRACTING CAPTURED SUBSTRINGS
- Captured substrings can be accessed directly by using the offsets returned by
- \fBpcre_exec()\fR in \fIovector\fR. For convenience, the functions
- \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
- \fBpcre_get_substring_list()\fR are provided for extracting captured substrings
- as new, separate, zero-terminated strings. A substring that contains a binary
- zero is correctly extracted and has a further zero added on the end, but the
- result does not, of course, function as a C string.
- The first three arguments are the same for all three functions: \fIsubject\fR
- is the subject string which has just been successfully matched, \fIovector\fR
- is a pointer to the vector of integer offsets that was passed to
- \fBpcre_exec()\fR, and \fIstringcount\fR 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 \fBpcre_exec\fR if it
- is greater than zero. If \fBpcre_exec()\fR returned zero, indicating that it
- ran out of space in \fIovector\fR, the value passed as \fIstringcount\fR should
- be the size of the vector divided by three.
- The functions \fBpcre_copy_substring()\fR and \fBpcre_get_substring()\fR
- extract a single substring, whose number is given as \fIstringnumber\fR. A
- value of zero extracts the substring that matched the entire pattern, while
- higher values extract the captured substrings. For \fBpcre_copy_substring()\fR,
- the string is placed in \fIbuffer\fR, whose length is given by
- \fIbuffersize\fR, while for \fBpcre_get_substring()\fR a new block of memory is
- obtained via \fBpcre_malloc\fR, and its address is returned via
- \fIstringptr\fR. 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 \fBpcre_copy_substring()\fR, or the attempt to get
- memory failed for \fBpcre_get_substring()\fR.
-   PCRE_ERROR_NOSUBSTRING    (-7)
- There is no substring whose number is \fIstringnumber\fR.
- The \fBpcre_get_substring_list()\fR function extracts all available substrings
- and builds a list of pointers to them. All this is done in a single block of
- memory which is obtained via \fBpcre_malloc\fR. The address of the memory block
- is returned via \fIlistptr\fR, 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 \fIn+1\fR matches some part of the
- subject, but subpattern \fIn\fR has not been used at all, they return an empty
- string. This can be distinguished from a genuine zero-length substring by
- inspecting the appropriate offset in \fIovector\fR, which is negative for unset
- substrings.
- The two convenience functions \fBpcre_free_substring()\fR and
- \fBpcre_free_substring_list()\fR can be used to free the memory returned by
- a previous call of \fBpcre_get_substring()\fR or
- \fBpcre_get_substring_list()\fR, respectively. They do nothing more than call
- the function pointed to by \fBpcre_free\fR, which of course could be called
- directly from a C program. However, PCRE is used in some situations where it is
- linked via a special interface to another programming language which cannot use
- \fBpcre_free\fR directly; it is for these cases that the functions are
- provided.
  .SH LIMITATIONS
+ .rs
+ .sp
  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.
+ .P
+ 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 \fBREADME\fP file in the source
+ distribution and the
+ .\" HREF
+ \fBpcrebuild\fP
+ .\"
+ documentation for details). In these cases the limit is substantially larger.
+ However, the speed of execution will be slower.
+ .P
  All values in repeating quantifiers must be less than 65536.
- There maximum number of capturing subpatterns is 65535.
+ The maximum number of capturing subpatterns is 65535.
+ .P
  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 subpattern, is 200.
+ .P
  The maximum length of a subject string is the largest positive number that an
  integer variable can hold. However, PCRE uses recursion to handle subpatterns
  and indefinite repetition. This means that the available stack space may limit
  the size of a subject string that can be processed by certain patterns.
+ .sp
+ .\" HTML <a name="utf8support"></a>
- .SH DIFFERENCES FROM PERL
+ .
- The differences described here are with respect to Perl 5.005.
+ .
+ .SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
-. By default, a whitespace character is any character that the C library
+ .rs
- function \fBisspace()\fR recognizes, though it is possible to compile PCRE with
+ .sp
- alternative character type tables. Normally \fBisspace()\fR matches space,
+ From release 3.3, PCRE has had some support for character strings encoded in
- formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5
+ the UTF-8 format. For release 4.0 this was greatly extended to cover most
- no longer includes vertical tab in its set of whitespace characters. The \\v
+ common requirements, and in release 5.0 additional support for Unicode general
- escape that was in the Perl documentation for a long time was never in fact
+ category properties was added.
- recognized. However, the character itself was treated as whitespace at least
+ .P
- up to 5.002. In 5.004 and 5.005 it does not match \\s.
+ 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 does not allow repeat quantifiers on lookahead assertions. Perl permits
+ .\" HREF
- them, but they do not mean what you might think. For example, (?!a){3} does
+ \fBpcre_compile()\fP
- not assert that the next three characters are not "a". It just asserts that the
+ .\"
- next character is not "a" three times.
+ 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
-. Capturing subpatterns that occur inside negative lookahead assertions are
+ instead of just strings of bytes.
- counted, but their entries in the offsets vector are never set. Perl sets its
+ .P
- numerical variables from any such patterns that are matched before the
- assertion fails to match something (thereby succeeding), but only if the
- negative lookahead assertion contains just one branch.
-. Though binary zero characters are supported in the subject string, they are
- not allowed in a pattern string because it is passed as a normal C string,
- terminated by zero. The escape sequence "\\0" can be used in the pattern to
- represent a binary zero.
-. The following Perl escape sequences are not supported: \\l, \\u, \\L, \\U,
- \\E, \\Q. In fact these are implemented by Perl's general string-handling and
- are not part of its pattern matching engine.
-. The Perl \\G assertion is not supported as it is not relevant to single
- pattern matches.
-. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
- constructions. However, there is some experimental support for recursive
- patterns using the non-Perl item (?R).
-. There are at the time of writing some oddities in Perl 5.005_02 concerned
- with the settings of captured strings when part of a pattern is repeated. For
- example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
- "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if
- the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) are set.
- In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the
- future Perl changes to a consistent state that is different, PCRE may change to
- follow.
-. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern
- /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not.
- However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.
-. 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 5.005 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 special
- meaning is faulted.
- (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
- inverted, that is, by default they are not greedy, but if followed by a
- question mark they are.
- (e) PCRE_ANCHORED can be used to force a pattern to be tried only at the start
- of the subject.
- (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for
- \fBpcre_exec()\fR have no Perl equivalents.
- (g) The (?R) construct allows for recursive pattern matching (Perl 5.6 can do
- this using the (?p{code}) construct, which PCRE cannot of course support.)
- .SH 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 copious
- examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
- O'Reilly (ISBN 1-56592-257), 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 the beginnings of
- some support for UTF-8 character strings. To use this support you must
- configure PCRE to include it, and then call \fBpcre_compile()\fR with the
- PCRE_UTF8 option. How this affects the pattern matching is described in the
- final section of this document.
- 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 alternatives and
- repetitions in the pattern. These are encoded in the pattern by the use of
- \fImeta-characters\fR, 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 recognized
- 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 subject (or line, in multiline mode)
-   $      assert end of subject (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
-   {      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
-   ]      terminates the character class
- The following sections describe the use of each of the meta-characters.
- .SH 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 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 "\\" 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 "#" character as part of the
- pattern.
- A second use of backslash provides a way of encoding non-printing characters
- 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)
-   \\xhh   character with hex code hh
-   \\ddd   character with octal code ddd, or backreference
- 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
-B.
- After "\\x", up to two hexadecimal digits are read (letters can be in upper or
- lower case).
- 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.
- 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 complicated.
- Outside a character class, PCRE reads it and any following digits 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 \fIback reference\fR. 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 single 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   is the character with octal code 113 (since there
-             can be no more than 99 back references)
-   \\377   is a 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 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.
- 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 definition of letters and
- digits is controlled by PCRE's character tables, and may vary if locale-
- specific matching is taking place (see "Locale support" above). 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 character
- classes. They each match one character of the appropriate type. If the current
- matching point is at the end of the subject string, all of them fail, since
- there is no character to match.
- The fourth use of backslash is for certain simple assertions. An assertion
- 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     word boundary
-   \\B     not a word boundary
-   \\A     start of subject (independent of multiline mode)
-   \\Z     end of subject or newline at end (independent of multiline mode)
-   \\z     end of subject (independent of multiline mode)
- These assertions may not appear in character classes (but note that "\\b" has a
- different meaning, namely the backspace character, inside a character 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. They are not affected by the
- PCRE_NOTBOL or PCRE_NOTEOL options. If the \fIstartoffset\fR argument of
- \fBpcre_exec()\fR is non-zero, \\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.
- .SH 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 \fIstartoffset\fR argument of
- \fBpcre_exec()\fR is non-zero, circumflex can never match. 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 subject, 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 or matching
- 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 immediately
- after and immediately before an internal "\\n" character, respectively, in
- addition to matching at the start and end of the subject 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 circumflex is possible when the \fIstartoffset\fR argument of
- \fBpcre_exec()\fR 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.
- .SH FULL STOP (PERIOD, DOT)
- Outside a character class, a dot in the pattern matches any one character in
- the subject, including a non-printing character, but not (by default) newline.
- If the PCRE_DOTALL option is set, dots match newlines as well. The handling of
- dot is entirely independent of the handling of circumflex and dollar, the only
- relationship being that they both involve newline characters. Dot has no
- special meaning in a character class.
- .SH 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 special. 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; the character must
- be in the set of characters defined by the class, unless the first character in
- the class 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.
- 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.
- 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 characters 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 character 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 interpreted 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 ASCII collating sequence. They can also be used for
- characters specified numerically, for example [\\000-\\037]. 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.
- .SH POSIX CHARACTER CLASSES
- Perl 5.6 (not yet released at the time of writing) is going to support the
- POSIX notation for character classes, which uses names enclosed by [: and :]
- within the enclosing square brackets. PCRE 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
-   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 (same as \\s)
-   upper    upper case letters
-   word     "word" characters (same as \\w)
-   xdigit   hexadecimal digits
- The names "ascii" and "word" are Perl extensions. 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.
- .SH VERTICAL BAR
- Vertical bar characters are used to separate alternative patterns. For example,
- the pattern
-   gilbert|sullivan
- matches either "gilbert" or "sullivan". Any number of alternatives may appear,
- and an empty alternative is permitted (matching the empty string).
- The matching process tries each alternative in turn, from left to right,
- and the first one that succeeds is used. If the alternatives are within a
- subpattern (defined below), "succeeds" means matching the rest of the main
- pattern as well as the alternative in the subpattern.
- .SH INTERNAL OPTION SETTING
- The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED
- 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 possible to
- unset these options by preceding the letter with a hyphen, and a combined
- setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
- PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
- permitted. If a letter appears both before and after the hyphen, the option is
- unset.
- The scope of these option changes depends on where in the pattern the setting
- occurs. For settings that are outside any subpattern (defined below), the
- effect is the same as if the options were set or unset at the start of
- matching. The following patterns all behave in exactly the same way:
-   (?i)abc
-   a(?i)bc
-   ab(?i)c
-   abc(?i)
- which in turn is the same as compiling the pattern abc with PCRE_CASELESS set.
- In other words, such "top level" settings apply to the whole pattern (unless
- there are other changes inside subpatterns). If there is more than one setting
- of the same option at top level, the rightmost setting is used.
- If an option change occurs inside a subpattern, the effect is different. This
- is a change of behaviour in Perl 5.005. An option change inside a subpattern
- affects only that part of the subpattern that follows it, so
-   (a(?i)b)c
- matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
- By this means, options can be made to have different settings in different
- parts of the pattern. Any changes made in one alternative do carry on
- into subsequent branches within the same subpattern. For example,
-   (a(?i)b|c)
- matches "ab", "aB", "c", and "C", even though when matching "C" the first
- branch is abandoned before the option setting. This is because the effects of
- option settings happen at compile time. There would be some very weird
- behaviour otherwise.
- 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.
- .SH SUBPATTERNS
- Subpatterns are delimited by parentheses (round brackets), which can be nested.
- Marking part of a pattern as a subpattern does two things:
-. 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.
-. 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 \fIovector\fR argument of
- \fBpcre_exec()\fR. 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 pattern
-   the ((red|white) (king|queen))
- the captured substrings are "red king", "red", and "king", and are numbered 1,
-, 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 "?:", the
- subpattern does not do any capturing, 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
-. The maximum number of captured substrings is 99, and the maximum number 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".
- .SH REPETITION
- Repetition is specified by quantifiers, which can follow any of the following
- items:
-   a single character, possibly escaped
-   the . metacharacter
-   a character class
-   a back reference (see next section)
-   a parenthesized subpattern (unless it is an assertion - see below)
- The general repetition quantifier specifies a minimum and maximum number 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 example, {,6} is not a
- quantifier, but a literal string of four characters.
- 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 broken.
- 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, individual * 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 (equivalent
- 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 treats such a pattern as
- though it were preceded by \\A. In cases where it is known that the subject
- string contains no newlines, it is worth setting PCRE_DOTALL when the pattern
- begins with .* in order to obtain this optimization, or alternatively using ^
- to indicate anchoring explicitly.
- When a capturing subpattern is repeated, the value captured is the substring
- 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 iterations. For
- example, after
-   /(a|(b))+/
- matches "aba" the value of the second captured substring is "b".
- .SH 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 subpattern earlier
- (i.e. 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 pattern. 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 digits following a backslash.
- A back reference matches whatever actually matched the capturing subpattern in
- the current subject string, rather than anything matching the subpattern
- itself. 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 example,
-   ((?i)rah)\\s+\\1
- matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
- capturing subpattern is matched caselessly.
- 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
- up to 99 back references, 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 subpatterns. For
- example, the pattern
-   (a|b\\1)+
- matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration 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.
- .SH 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, \\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 semicolon 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.
- 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 several 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 5.005, 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 current position, the
- match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
- can be particularly useful for matching at the ends of strings; an example is
- given at the end of the section on once-only subpatterns.
- 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 \fInot\fR match "foo" preceded by six characters, the first
- of which are digits and the last three of which are not "999". For example, it
- doesn't match "123abcfoo". 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 subpatterns in the whole pattern.
- However, substring capturing is carried out only for positive assertions,
- because it does not make sense for negative assertions.
- Assertions count towards the maximum of 200 parenthesized subpatterns.
- .SH ONCE-ONLY SUBPATTERNS
- With both maximizing and minimizing repetition, failure of what follows
- normally causes the repeated item to be re-evaluated to see if a different
- number of repeats allows the rest of the pattern to match. Sometimes it is
- useful to prevent this, either to change the nature of the match, or to cause
- it fail earlier than it otherwise might, when the author of the pattern knows
- there is no point in carrying on.
- Consider, for example, the pattern \\d+foo when applied to the subject line
-bar
- 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. Once-only
- subpatterns provide the means for specifying that once a portion of the pattern
- has matched, it is not to be re-evaluated in this way, so the matcher would
- give up immediately on failing to match "foo" the first time. The notation is
- another kind of special parenthesis, starting with (?> as in this example:
-   (?>\\d+)bar
- This kind of parenthesis "locks up" the  part of the pattern it contains 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.
- Once-only 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 prepared 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.
- This construction can of course contain arbitrarily complicated subpatterns,
- and it can be nested.
- Once-only subpatterns 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 which does not match. Because matching proceeds
- from left to right, PCRE will look for each "a" in the subject and then see if
- what follows matches the rest of the pattern. If the pattern is specified as
-   ^.*abcd$
- the initial .* matches the entire string at first, but when this fails (because
- there is no following "a"), it backtracks to match all but the last character,
- then all but the last two characters, and so on. Once again the search for "a"
- covers the entire string, from right to left, so we are no better off. However,
- if the pattern is written as
-   ^(?>.*)(?<=abcd)
- there can be no backtracking for the .* item; it can match only the entire
- string. The subsequent lookbehind assertion does a single test on the last four
- characters. If it fails, the match fails immediately. For long strings, this
- approach makes a significant difference to the processing time.
- When a pattern contains an unlimited repeat inside a subpattern that can itself
- be repeated an unlimited number of times, the use of a once-only subpattern 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 optimization 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.
- .SH CONDITIONAL SUBPATTERNS
- It is possible to cause the matching process to obey a subpattern
- conditionally 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 subpattern are
-   (?(condition)yes-pattern)
-   (?(condition)yes-pattern|no-pattern)
- If the condition is satisfied, the yes-pattern is used; otherwise the
- no-pattern (if present) is used. If there are more than two alternatives in the
- subpattern, a compile-time error occurs.
- There are two 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 contains non-significant white space to
- make it more readable (assume the PCRE_EXTENDED option) and to divide it into
- three parts for ease of discussion:
-   ( \\( )?    [^()]+    (?(1) \\) )
- The first part matches an optional opening parenthesis, and if that
- character is present, sets it as the first captured substring. The second 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-pattern 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 not a sequence of digits, 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.
- .SH 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 newline
- character in the pattern.
- .SH 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 5.6 has provided an
- experimental facility that allows regular expressions to recurse (amongst other
- things). It does this by interpolating Perl code in the expression at run time,
- and the code can refer to the expression itself. A Perl pattern to solve the
- parentheses problem can be created like this:
-   $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;
- The (?p{...}) item interpolates Perl code at run time, and in this case refers
- recursively to the pattern in which it appears. Obviously, PCRE cannot support
- the interpolation of Perl code. Instead, the special item (?R) is provided for
- the specific case of recursion. This PCRE pattern solves the 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 (i.e. a correctly parenthesized substring). Finally
- there is a closing parenthesis.
- This particular example pattern contains nested unlimited repeats, and so the
- use of a once-only subpattern for matching strings of non-parentheses is
- important when applying the pattern to strings that do not match. For example,
- when it is applied to
-   (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
- it yields "no match" quickly. However, if a once-only subpattern 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.
- The values set for any capturing subpatterns are those from the outermost level
- of the recursion at which the subpattern value is set. 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 pattern, PCRE
- has to obtain extra memory to store data during a recursion, which it does by
- using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no
- memory can be obtained, it saves data for the first 15 capturing parentheses
- only, as there is no way to give an out-of-memory error from within a
- recursion.
- .SH PERFORMANCE
- Certain items that may appear in 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 general, 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
- performance.
- When a pattern begins with .* 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 newlines, 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 first captured substring being "and". 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 contain
- 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 subject string, and if
- there is not, it fails the match immediately. However, when there is no
- following literal this optimization cannot be used. You can see the difference
- by comparing the behaviour of
-   (a+)*\\d
- with the pattern above. The former gives a failure almost instantly when
- applied to a whole line of "a" characters, whereas the latter takes an
- appreciable time with strings longer than about 20 characters.
- .SH UTF-8 SUPPORT
- Starting at release 3.3, PCRE has some support for character strings encoded
- in the UTF-8 format. This is incomplete, and is regarded as experimental. In
- order to use it, you must configure PCRE to include UTF-8 support in the code,
- and, in addition, you must call \fBpcre_compile()\fR 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, but only in the cases that are mentioned below.
  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.
+ .P
- PCRE assumes that the strings it is given contain valid UTF-8 codes. It does
+ If PCRE is built with Unicode character property support (which implies UTF-8
- not diagnose invalid UTF-8 strings. If you pass invalid UTF-8 strings to PCRE,
+ support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
- the results are undefined.
+ The available properties that can be tested are limited to the general
+ category properties such as Lu for an upper case letter or Nd for a decimal
- Running with PCRE_UTF8 set causes these changes in the way PCRE works:
+ number. A full list is given in the
+ .\" HREF
-. In a pattern, the escape sequence \\x{...}, where the contents of the braces
+ \fBpcrepattern\fP
+ .\"
+ documentation. The PCRE library is increased in size by about 90K when Unicode
+ property support is included.
+ .P
+ The following comments apply when PCRE is running in UTF-8 mode:
+ .P
+. 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.
+ .P
+. In a pattern, the escape sequence \ex{...}, 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 example: \\x{1234}. This
+ code number is the given hexadecimal number, for example: \ex{1234}. If a
- inserts from one to six literal bytes into the pattern, using the UTF-8
+ non-hexadecimal digit appears between the braces, the item is not recognized.
- encoding. If a non-hexadecimal digit appears between the braces, the item is
+ This escape sequence can be used either as a literal, or within a character
- not recognized.
+ class.
+ .P
-. The original hexadecimal escape sequence, \\xhh, generates a two-byte UTF-8
+. The original hexadecimal escape sequence, \exhh, matches a two-byte UTF-8
- character if its value is greater than 127.
+ character if the value is greater than 127.
+ .P
-. Repeat quantifiers are NOT correctly handled if they follow a multibyte
+. Repeat quantifiers apply to complete UTF-8 characters, not to individual
- character. For example, \\x{100}* and \\xc3+ do not work. If you want to
+ bytes, for example: \ex{100}{3}.
- repeat such characters, you must enclose them in non-capturing parentheses,
+ .P
- for example (?:\\x{100}), at present.
+. The dot metacharacter matches one UTF-8 character instead of a single byte.
+ .P
-. The dot metacharacter matches one UTF-8 character instead of a single byte.
+. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
+ but its use can lead to some strange effects.
-. Unlike literal UTF-8 characters, the dot metacharacter followed by a
+ .P
- repeat quantifier does operate correctly on UTF-8 characters instead of
+. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
- single bytes.
+ test characters of any code value, but the characters that PCRE recognizes as
+ digits, spaces, or word characters remain the same set as before, all with
-. Although the \\x{...} escape is permitted in a character class, characters
+ values less than 256. This remains true even when PCRE includes Unicode
- whose values are greater than 255 cannot be included in a class.
+ property support, because to do otherwise would slow down PCRE in many common
+ cases. If you really want to test for a wider sense of, say, "digit", you
-. A class is matched against a UTF-8 character instead of just a single byte,
+ must use Unicode property tests such as \ep{Nd}.
- but it can match only characters whose values are less than 256. Characters
+ .P
- with greater values always fail to match a class.
+. Similarly, characters that match the POSIX named character classes are all
+ low-valued characters.
-. Repeated classes work correctly on multiple characters.
+ .P
+. Case-insensitive matching applies only to characters whose values are less
-. Classes containing just a single character whose value is greater than 127
+ than 128, unless PCRE is built with Unicode property support. Even when Unicode
- (but less than 256), for example, [\\x80] or [^\\x{93}], do not work because
+ property support is available, PCRE still uses its own character tables when
- these are optimized into single byte matches. In the first case, of course,
+ checking the case of low-valued characters, so as not to degrade performance.
- the class brackets are just redundant.
+ The Unicode property information is used only for characters with higher
+ values.
-. Lookbehind assertions move backwards in the subject by a fixed number of
+ .
- characters instead of a fixed number of bytes. Simple cases have been tested
- to work correctly, but there may be hidden gotchas herein.
-. The character types such as \\d and \\w do not work correctly with UTF-8
- characters. They continue to test a single byte.
-. Anything not explicitly mentioned here continues to work in bytes rather
- than in characters.
- The following UTF-8 features of Perl 5.6 are not implemented:
-. The escape sequence \\C to match a single byte.
-. The use of Unicode tables and properties and escapes \\p, \\P, and \\X.
- .SH SAMPLE PROGRAM
- The code below is a simple, complete demonstration program, to get you started
- with using PCRE. This code is also supplied in the file \fIpcredemo.c\fR 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 options are
- set, and default character tables are used. If matching succeeds, the program
- outputs the portion of the subject that matched, together with the contents of
- any captured substrings.
- On a Unix system that has PCRE installed in \fI/usr/local\fR, 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'
- Note that there is a much more comprehensive test program, called
- \fBpcretest\fR, which supports many more facilities for testing regular
- expressions. The \fBpcredemo\fR 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 \fBpcredemo\fR:
-   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 systems. You
- need to add
-   -R/usr/local/lib
- to the compile command to get round this problem. Here's the code:
-   #include <stdio.h>
-   #include <string.h>
-   #include <pcre.h>
-   #define OVECCOUNT 30    /* should be a multiple of 3 */
-   int main(int argc, char **argv)
-   {
-   pcre *re;
-   const char *error;
-   int erroffset;
-   int ovector[OVECCOUNT];
-   int rc, i;
-   if (argc != 3)
-     {
-     printf("Two arguments required: a regex and a "
-       "subject string\\n");
-     return 1;
-     }
-   /* Compile the regular expression in the first argument */
-   re = pcre_compile(
-     argv[1],     /* the pattern */
-,           /* default options */
-     &error,      /* for error message */
-     &erroffset,  /* for error offset */
-     NULL);       /* use default character tables */
-   /* Compilation failed: print the error message and exit */
-   if (re == NULL)
-     {
-     printf("PCRE compilation failed at offset %d: %s\\n",
-       erroffset, error);
-     return 1;
-     }
-   /* Compilation succeeded: match the subject in the second
-      argument */
-   rc = pcre_exec(
-     re,          /* the compiled pattern */
-     NULL,        /* we didn't study the pattern */
-     argv[2],     /* the subject string */
-     (int)strlen(argv[2]), /* the length of the subject */
-,           /* start at offset 0 in the subject */
-,           /* default options */
-     ovector,     /* vector for substring information */
-     OVECCOUNT);  /* number of elements in the vector */
-   /* Matching failed: handle error cases */
-   if (rc < 0)
-     {
-     switch(rc)
-       {
-       case PCRE_ERROR_NOMATCH: printf("No match\\n"); break;
-       /*
-       Handle other special cases if you like
-       */
-       default: printf("Matching error %d\\n", rc); break;
-       }
-     return 1;
-     }
-   /* Match succeded */
-   printf("Match succeeded\\n");
-   /* The output vector wasn't big enough */
-   if (rc == 0)
-     {
-     rc = OVECCOUNT/3;
-     printf("ovector only has room for %d captured "
-       substrings\\n", rc - 1);
-     }
-   /* Show substrings stored in the output vector */
-   for (i = 0; i < rc; i++)
-     {
-     char *substring_start = argv[2] + ovector[2*i];
-     int substring_length = ovector[2*i+1] - ovector[2*i];
-     printf("%2d: %.*s\\n", i, substring_length,
-       substring_start);
-     }
-   return 0;
-   }
  .SH AUTHOR
+ .rs
+ .sp
  Philip Hazel <ph10@cam.ac.uk>
  .br
  University Computing Service,
  .br
- New Museums Site,
- .br
  Cambridge CB2 3QG, England.
  .br
  Phone: +44 1223 334714
+ .sp
- Last updated: 15 August 2001
+ .in 0
+ Last updated: 09 September 2004
  .br
- Copyright (c) 1997-2001 University of Cambridge.
+ Copyright (c) 1997-2004 University of Cambridge.

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