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<html>
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<head>
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<title>pcre 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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This HTML document has been generated automatically from the original man page.
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If there is any nonsense in it, please consult the man page, in case the
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conversion went wrong.<br>
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<ul>
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<li><a name="TOC1" href="#SEC1">DESCRIPTION</a>
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<li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
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<li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
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<li><a name="TOC4" href="#SEC4">UTF-8 SUPPORT</a>
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<li><a name="TOC5" href="#SEC5">AUTHOR</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">DESCRIPTION</a><br>
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<P>
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The PCRE library is a set of functions that implement regular expression
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pattern matching using the same syntax and semantics as Perl, with just a few
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differences. The current implementation of PCRE (release 4.x) corresponds
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approximately with Perl 5.8, including support for UTF-8 encoded strings.
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However, this support has to be explicitly enabled; it is not the default.
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</P>
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<P>
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PCRE is written in C and released as a C library. However, a number of people
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have written wrappers and interfaces of various kinds. A C++ class is included
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in these contributions, which can be found in the <i>Contrib</i> directory at
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the primary FTP site, which is:
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</P>
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<a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
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<P>
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Details of exactly which Perl regular expression features are and are not
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supported by PCRE are given in separate documents. See the
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<a href="pcrepattern.html"><b>pcrepattern</b></a>
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and
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<a href="pcrecompat.html"><b>pcrecompat</b></a>
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pages.
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</P>
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<P>
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Some features of PCRE can be included, excluded, or changed when the library is
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built. The
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<a href="pcre_config.html"><b>pcre_config()</b></a>
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function makes it possible for a client to discover which features are
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available. Documentation about building PCRE for various operating systems can
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be found in the <b>README</b> file in the source distribution.
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</P>
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<br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
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<P>
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The user documentation for PCRE has been split up into a number of different
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sections. In the "man" format, each of these is a separate "man page". In the
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HTML format, each is a separate page, linked from the index page. In the plain
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text format, all the sections are concatenated, for ease of searching. The
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sections are as follows:
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</P>
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<P>
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<pre>
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pcre this document
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pcreapi details of PCRE's native API
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pcrebuild options for building PCRE
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pcrecallout details of the callout feature
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pcrecompat discussion of Perl compatibility
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pcregrep description of the <b>pcregrep</b> command
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pcrepattern syntax and semantics of supported
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regular expressions
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pcreperform discussion of performance issues
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pcreposix the POSIX-compatible API
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pcresample discussion of the sample program
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pcretest the <b>pcretest</b> testing command
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</PRE>
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</P>
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<P>
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In addition, in the "man" and HTML formats, there is a short page for each
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library function, listing its arguments and results.
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</P>
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<br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
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<P>
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There are some size limitations in PCRE but it is hoped that they will never in
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practice be relevant.
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</P>
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<P>
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The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
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compiled with the default internal linkage size of 2. If you want to process
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regular expressions that are truly enormous, you can compile PCRE with an
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internal linkage size of 3 or 4 (see the <b>README</b> file in the source
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distribution and the
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<a href="pcrebuild.html"><b>pcrebuild</b></a>
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documentation for details). If these cases the limit is substantially larger.
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However, the speed of execution will be slower.
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</P>
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<P>
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All values in repeating quantifiers must be less than 65536.
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The maximum number of capturing subpatterns is 65535.
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</P>
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<P>
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There is no limit to the number of non-capturing subpatterns, but the maximum
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depth of nesting of all kinds of parenthesized subpattern, including capturing
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subpatterns, assertions, and other types of subpattern, is 200.
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</P>
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<P>
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The maximum length of a subject string is the largest positive number that an
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integer variable can hold. However, PCRE uses recursion to handle subpatterns
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and indefinite repetition. This means that the available stack space may limit
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the size of a subject string that can be processed by certain patterns.
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</P>
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<a name="utf8support"></a><br><a name="SEC4" href="#TOC1">UTF-8 SUPPORT</a><br>
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<P>
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Starting at release 3.3, PCRE has had some support for character strings
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encoded in the UTF-8 format. For release 4.0 this has been greatly extended to
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cover most common requirements.
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</P>
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<P>
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In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
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the code, and, in addition, you must call
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<a href="pcre_compile.html"><b>pcre_compile()</b></a>
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with the PCRE_UTF8 option flag. When you do this, both the pattern and any
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subject strings that are matched against it are treated as UTF-8 strings
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instead of just strings of bytes.
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</P>
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<P>
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If you compile PCRE with UTF-8 support, but do not use it at run time, the
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library will be a bit bigger, but the additional run time overhead is limited
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to testing the PCRE_UTF8 flag in several places, so should not be very large.
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</P>
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<P>
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The following comments apply when PCRE is running in UTF-8 mode:
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</P>
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<P>
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1. PCRE assumes that the strings it is given contain valid UTF-8 codes. It does
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not diagnose invalid UTF-8 strings. If you pass invalid UTF-8 strings to PCRE,
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the results are undefined.
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</P>
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<P>
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2. In a pattern, the escape sequence \x{...}, where the contents of the braces
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is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
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code number is the given hexadecimal number, for example: \x{1234}. If a
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non-hexadecimal digit appears between the braces, the item is not recognized.
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This escape sequence can be used either as a literal, or within a character
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class.
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</P>
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<P>
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3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8
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character if the value is greater than 127.
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</P>
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<P>
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4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
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bytes, for example: \x{100}{3}.
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</P>
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<P>
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5. The dot metacharacter matches one UTF-8 character instead of a single byte.
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</P>
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<P>
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6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
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but its use can lead to some strange effects.
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</P>
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<P>
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7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
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test characters of any code value, but the characters that PCRE recognizes as
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digits, spaces, or word characters remain the same set as before, all with
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values less than 256.
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</P>
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<P>
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8. Case-insensitive matching applies only to characters whose values are less
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than 256. PCRE does not support the notion of "case" for higher-valued
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characters.
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</P>
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<P>
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9. PCRE does not support the use of Unicode tables and properties or the Perl
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escapes \p, \P, and \X.
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</P>
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<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
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<P>
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Philip Hazel <ph10@cam.ac.uk>
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<br>
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University Computing Service,
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<br>
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Cambridge CB2 3QG, England.
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<br>
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Phone: +44 1223 334714
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</P>
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<P>
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Last updated: 04 February 2003
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<br>
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Copyright © 1997-2003 University of Cambridge.
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