357 lines
16 KiB
HTML
357 lines
16 KiB
HTML
<html>
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<head>
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<title>pcre2posix specification</title>
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</head>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcre2posix man page</h1>
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<p>
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Return to the <a href="index.html">PCRE2 index page</a>.
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</p>
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<p>
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This page is part of the PCRE2 HTML documentation. It was generated
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automatically from the original man page. If there is any nonsense in it,
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please consult the man page, in case the conversion went wrong.
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<br>
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<ul>
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<li><a name="TOC1" href="#SEC1">SYNOPSIS</a>
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<li><a name="TOC2" href="#SEC2">DESCRIPTION</a>
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<li><a name="TOC3" href="#SEC3">USING THE POSIX FUNCTIONS</a>
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<li><a name="TOC4" href="#SEC4">COMPILING A PATTERN</a>
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<li><a name="TOC5" href="#SEC5">MATCHING NEWLINE CHARACTERS</a>
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<li><a name="TOC6" href="#SEC6">MATCHING A PATTERN</a>
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<li><a name="TOC7" href="#SEC7">ERROR MESSAGES</a>
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<li><a name="TOC8" href="#SEC8">MEMORY USAGE</a>
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<li><a name="TOC9" href="#SEC9">AUTHOR</a>
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<li><a name="TOC10" href="#SEC10">REVISION</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">SYNOPSIS</a><br>
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<P>
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<b>#include <pcre2posix.h></b>
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</P>
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<P>
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<b>int pcre2_regcomp(regex_t *<i>preg</i>, const char *<i>pattern</i>,</b>
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<b> int <i>cflags</i>);</b>
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<br>
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<br>
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<b>int pcre2_regexec(const regex_t *<i>preg</i>, const char *<i>string</i>,</b>
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<b> size_t <i>nmatch</i>, regmatch_t <i>pmatch</i>[], int <i>eflags</i>);</b>
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<br>
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<br>
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<b>size_t pcre2_regerror(int <i>errcode</i>, const regex_t *<i>preg</i>,</b>
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<b> char *<i>errbuf</i>, size_t <i>errbuf_size</i>);</b>
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<br>
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<br>
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<b>void pcre2_regfree(regex_t *<i>preg</i>);</b>
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</P>
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<br><a name="SEC2" href="#TOC1">DESCRIPTION</a><br>
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<P>
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This set of functions provides a POSIX-style API for the PCRE2 regular
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expression 8-bit library. There are no POSIX-style wrappers for PCRE2's 16-bit
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and 32-bit libraries. See the
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<a href="pcre2api.html"><b>pcre2api</b></a>
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documentation for a description of PCRE2's native API, which contains much
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additional functionality.
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</P>
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<P>
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The functions described here are wrapper functions that ultimately call the
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PCRE2 native API. Their prototypes are defined in the <b>pcre2posix.h</b> header
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file, and they all have unique names starting with <b>pcre2_</b>. However, the
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<b>pcre2posix.h</b> header also contains macro definitions that convert the
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standard POSIX names such <b>regcomp()</b> into <b>pcre2_regcomp()</b> etc. This
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means that a program can use the usual POSIX names without running the risk of
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accidentally linking with POSIX functions from a different library.
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</P>
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<P>
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On Unix-like systems the PCRE2 POSIX library is called <b>libpcre2-posix</b>, so
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can be accessed by adding <b>-lpcre2-posix</b> to the command for linking an
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application. Because the POSIX functions call the native ones, it is also
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necessary to add <b>-lpcre2-8</b>.
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</P>
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<P>
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Although they were not defined as protypes in <b>pcre2posix.h</b>, releases
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10.33 to 10.36 of the library contained functions with the POSIX names
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<b>regcomp()</b> etc. These simply passed their arguments to the PCRE2
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functions. These functions were provided for backwards compatibility with
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earlier versions of PCRE2, which had only POSIX names. However, this has proved
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troublesome in situations where a program links with several libraries, some of
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which use PCRE2's POSIX interface while others use the real POSIX functions.
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For this reason, the POSIX names have been removed since release 10.37.
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</P>
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<P>
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Calling the header file <b>pcre2posix.h</b> avoids any conflict with other POSIX
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libraries. It can, of course, be renamed or aliased as <b>regex.h</b>, which is
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the "correct" name, if there is no clash. It provides two structure types,
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<i>regex_t</i> for compiled internal forms, and <i>regmatch_t</i> for returning
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captured substrings. It also defines some constants whose names start with
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"REG_"; these are used for setting options and identifying error codes.
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</P>
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<br><a name="SEC3" href="#TOC1">USING THE POSIX FUNCTIONS</a><br>
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<P>
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Those POSIX option bits that can reasonably be mapped to PCRE2 native options
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have been implemented. In addition, the option REG_EXTENDED is defined with the
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value zero. This has no effect, but since programs that are written to the
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POSIX interface often use it, this makes it easier to slot in PCRE2 as a
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replacement library. Other POSIX options are not even defined.
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</P>
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<P>
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There are also some options that are not defined by POSIX. These have been
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added at the request of users who want to make use of certain PCRE2-specific
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features via the POSIX calling interface or to add BSD or GNU functionality.
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</P>
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<P>
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When PCRE2 is called via these functions, it is only the API that is POSIX-like
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in style. The syntax and semantics of the regular expressions themselves are
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still those of Perl, subject to the setting of various PCRE2 options, as
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described below. "POSIX-like in style" means that the API approximates to the
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POSIX definition; it is not fully POSIX-compatible, and in multi-unit encoding
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domains it is probably even less compatible.
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</P>
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<P>
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The descriptions below use the actual names of the functions, but, as described
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above, the standard POSIX names (without the <b>pcre2_</b> prefix) may also be
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used.
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</P>
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<br><a name="SEC4" href="#TOC1">COMPILING A PATTERN</a><br>
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<P>
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The function <b>pcre2_regcomp()</b> is called to compile a pattern into an
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internal form. By default, the pattern is a C string terminated by a binary
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zero (but see REG_PEND below). The <i>preg</i> argument is a pointer to a
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<b>regex_t</b> structure that is used as a base for storing information about
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the compiled regular expression. (It is also used for input when REG_PEND is
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set.)
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</P>
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<P>
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The argument <i>cflags</i> is either zero, or contains one or more of the bits
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defined by the following macros:
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<pre>
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REG_DOTALL
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</pre>
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The PCRE2_DOTALL option is set when the regular expression is passed for
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compilation to the native function. Note that REG_DOTALL is not part of the
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POSIX standard.
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<pre>
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REG_ICASE
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</pre>
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The PCRE2_CASELESS option is set when the regular expression is passed for
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compilation to the native function.
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<pre>
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REG_NEWLINE
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</pre>
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The PCRE2_MULTILINE option is set when the regular expression is passed for
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compilation to the native function. Note that this does <i>not</i> mimic the
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defined POSIX behaviour for REG_NEWLINE (see the following section).
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<pre>
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REG_NOSPEC
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</pre>
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The PCRE2_LITERAL option is set when the regular expression is passed for
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compilation to the native function. This disables all meta characters in the
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pattern, causing it to be treated as a literal string. The only other options
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that are allowed with REG_NOSPEC are REG_ICASE, REG_NOSUB, REG_PEND, and
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REG_UTF. Note that REG_NOSPEC is not part of the POSIX standard.
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<pre>
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REG_NOSUB
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</pre>
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When a pattern that is compiled with this flag is passed to
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<b>pcre2_regexec()</b> for matching, the <i>nmatch</i> and <i>pmatch</i> arguments
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are ignored, and no captured strings are returned. Versions of the PCRE library
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prior to 10.22 used to set the PCRE2_NO_AUTO_CAPTURE compile option, but this
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no longer happens because it disables the use of backreferences.
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<pre>
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REG_PEND
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</pre>
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If this option is set, the <b>reg_endp</b> field in the <i>preg</i> structure
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(which has the type const char *) must be set to point to the character beyond
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the end of the pattern before calling <b>pcre2_regcomp()</b>. The pattern itself
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may now contain binary zeros, which are treated as data characters. Without
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REG_PEND, a binary zero terminates the pattern and the <b>re_endp</b> field is
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ignored. This is a GNU extension to the POSIX standard and should be used with
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caution in software intended to be portable to other systems.
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<pre>
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REG_UCP
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</pre>
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The PCRE2_UCP option is set when the regular expression is passed for
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compilation to the native function. This causes PCRE2 to use Unicode properties
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when matchine \d, \w, etc., instead of just recognizing ASCII values. Note
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that REG_UCP is not part of the POSIX standard.
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<pre>
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REG_UNGREEDY
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</pre>
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The PCRE2_UNGREEDY option is set when the regular expression is passed for
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compilation to the native function. Note that REG_UNGREEDY is not part of the
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POSIX standard.
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<pre>
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REG_UTF
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</pre>
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The PCRE2_UTF option is set when the regular expression is passed for
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compilation to the native function. This causes the pattern itself and all data
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strings used for matching it to be treated as UTF-8 strings. Note that REG_UTF
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is not part of the POSIX standard.
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</P>
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<P>
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In the absence of these flags, no options are passed to the native function.
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This means the the regex is compiled with PCRE2 default semantics. In
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particular, the way it handles newline characters in the subject string is the
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Perl way, not the POSIX way. Note that setting PCRE2_MULTILINE has only
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<i>some</i> of the effects specified for REG_NEWLINE. It does not affect the way
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newlines are matched by the dot metacharacter (they are not) or by a negative
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class such as [^a] (they are).
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</P>
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<P>
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The yield of <b>pcre2_regcomp()</b> is zero on success, and non-zero otherwise.
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The <i>preg</i> structure is filled in on success, and one other member of the
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structure (as well as <i>re_endp</i>) is public: <i>re_nsub</i> contains the
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number of capturing subpatterns in the regular expression. Various error codes
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are defined in the header file.
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</P>
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<P>
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NOTE: If the yield of <b>pcre2_regcomp()</b> is non-zero, you must not attempt
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to use the contents of the <i>preg</i> structure. If, for example, you pass it
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to <b>pcre2_regexec()</b>, the result is undefined and your program is likely to
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crash.
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</P>
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<br><a name="SEC5" href="#TOC1">MATCHING NEWLINE CHARACTERS</a><br>
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<P>
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This area is not simple, because POSIX and Perl take different views of things.
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It is not possible to get PCRE2 to obey POSIX semantics, but then PCRE2 was
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never intended to be a POSIX engine. The following table lists the different
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possibilities for matching newline characters in Perl and PCRE2:
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<pre>
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Default Change with
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. matches newline no PCRE2_DOTALL
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newline matches [^a] yes not changeable
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$ matches \n at end yes PCRE2_DOLLAR_ENDONLY
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$ matches \n in middle no PCRE2_MULTILINE
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^ matches \n in middle no PCRE2_MULTILINE
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</pre>
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This is the equivalent table for a POSIX-compatible pattern matcher:
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<pre>
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Default Change with
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. matches newline yes REG_NEWLINE
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newline matches [^a] yes REG_NEWLINE
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$ matches \n at end no REG_NEWLINE
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$ matches \n in middle no REG_NEWLINE
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^ matches \n in middle no REG_NEWLINE
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</pre>
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This behaviour is not what happens when PCRE2 is called via its POSIX
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API. By default, PCRE2's behaviour is the same as Perl's, except that there is
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no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both PCRE2 and Perl, there
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is no way to stop newline from matching [^a].
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</P>
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<P>
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Default POSIX newline handling can be obtained by setting PCRE2_DOTALL and
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PCRE2_DOLLAR_ENDONLY when calling <b>pcre2_compile()</b> directly, but there is
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no way to make PCRE2 behave exactly as for the REG_NEWLINE action. When using
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the POSIX API, passing REG_NEWLINE to PCRE2's <b>pcre2_regcomp()</b> function
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causes PCRE2_MULTILINE to be passed to <b>pcre2_compile()</b>, and REG_DOTALL
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passes PCRE2_DOTALL. There is no way to pass PCRE2_DOLLAR_ENDONLY.
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</P>
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<br><a name="SEC6" href="#TOC1">MATCHING A PATTERN</a><br>
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<P>
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The function <b>pcre2_regexec()</b> is called to match a compiled pattern
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<i>preg</i> against a given <i>string</i>, which is by default terminated by a
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zero byte (but see REG_STARTEND below), subject to the options in <i>eflags</i>.
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These can be:
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<pre>
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REG_NOTBOL
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</pre>
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The PCRE2_NOTBOL option is set when calling the underlying PCRE2 matching
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function.
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<pre>
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REG_NOTEMPTY
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</pre>
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The PCRE2_NOTEMPTY option is set when calling the underlying PCRE2 matching
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function. Note that REG_NOTEMPTY is not part of the POSIX standard. However,
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setting this option can give more POSIX-like behaviour in some situations.
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<pre>
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REG_NOTEOL
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</pre>
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The PCRE2_NOTEOL option is set when calling the underlying PCRE2 matching
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function.
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<pre>
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REG_STARTEND
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</pre>
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When this option is set, the subject string starts at <i>string</i> +
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<i>pmatch[0].rm_so</i> and ends at <i>string</i> + <i>pmatch[0].rm_eo</i>, which
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should point to the first character beyond the string. There may be binary
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zeros within the subject string, and indeed, using REG_STARTEND is the only
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way to pass a subject string that contains a binary zero.
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</P>
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<P>
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Whatever the value of <i>pmatch[0].rm_so</i>, the offsets of the matched string
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and any captured substrings are still given relative to the start of
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<i>string</i> itself. (Before PCRE2 release 10.30 these were given relative to
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<i>string</i> + <i>pmatch[0].rm_so</i>, but this differs from other
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implementations.)
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</P>
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<P>
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This is a BSD extension, compatible with but not specified by IEEE Standard
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1003.2 (POSIX.2), and should be used with caution in software intended to be
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portable to other systems. Note that a non-zero <i>rm_so</i> does not imply
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REG_NOTBOL; REG_STARTEND affects only the location and length of the string,
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not how it is matched. Setting REG_STARTEND and passing <i>pmatch</i> as NULL
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are mutually exclusive; the error REG_INVARG is returned.
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</P>
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<P>
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If the pattern was compiled with the REG_NOSUB flag, no data about any matched
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strings is returned. The <i>nmatch</i> and <i>pmatch</i> arguments of
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<b>pcre2_regexec()</b> are ignored (except possibly as input for REG_STARTEND).
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</P>
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<P>
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The value of <i>nmatch</i> may be zero, and the value <i>pmatch</i> may be NULL
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(unless REG_STARTEND is set); in both these cases no data about any matched
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strings is returned.
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</P>
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<P>
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Otherwise, the portion of the string that was matched, and also any captured
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substrings, are returned via the <i>pmatch</i> argument, which points to an
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array of <i>nmatch</i> structures of type <i>regmatch_t</i>, containing the
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members <i>rm_so</i> and <i>rm_eo</i>. These contain the byte offset to the first
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character of each substring and the offset to the first character after the end
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of each substring, respectively. The 0th element of the vector relates to the
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entire portion of <i>string</i> that was matched; subsequent elements relate to
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the capturing subpatterns of the regular expression. Unused entries in the
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array have both structure members set to -1.
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</P>
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<P>
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A successful match yields a zero return; various error codes are defined in the
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header file, of which REG_NOMATCH is the "expected" failure code.
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</P>
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<br><a name="SEC7" href="#TOC1">ERROR MESSAGES</a><br>
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<P>
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The <b>pcre2_regerror()</b> function maps a non-zero errorcode from either
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<b>pcre2_regcomp()</b> or <b>pcre2_regexec()</b> to a printable message. If
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<i>preg</i> is not NULL, the error should have arisen from the use of that
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structure. A message terminated by a binary zero is placed in <i>errbuf</i>. If
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the buffer is too short, only the first <i>errbuf_size</i> - 1 characters of the
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error message are used. The yield of the function is the size of buffer needed
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to hold the whole message, including the terminating zero. This value is
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greater than <i>errbuf_size</i> if the message was truncated.
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</P>
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<br><a name="SEC8" href="#TOC1">MEMORY USAGE</a><br>
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<P>
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Compiling a regular expression causes memory to be allocated and associated
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with the <i>preg</i> structure. The function <b>pcre2_regfree()</b> frees all
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such memory, after which <i>preg</i> may no longer be used as a compiled
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expression.
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</P>
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<br><a name="SEC9" href="#TOC1">AUTHOR</a><br>
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<P>
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Philip Hazel
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<br>
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University Computing Service
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<br>
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Cambridge, England.
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<br>
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</P>
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<br><a name="SEC10" href="#TOC1">REVISION</a><br>
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<P>
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Last updated: 26 April 2021
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<br>
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Copyright © 1997-2021 University of Cambridge.
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<br>
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<p>
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Return to the <a href="index.html">PCRE2 index page</a>.
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</p>
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