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Documentation and tidies preparatory to 8.37 release.
1 <html>
2 <head>
3 <title>pcre specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcre man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
14 <br>
15 <ul>
16 <li><a name="TOC1" href="#SEC1">PLEASE TAKE NOTE</a>
17 <li><a name="TOC2" href="#SEC2">INTRODUCTION</a>
18 <li><a name="TOC3" href="#SEC3">SECURITY CONSIDERATIONS</a>
19 <li><a name="TOC4" href="#SEC4">USER DOCUMENTATION</a>
20 <li><a name="TOC5" href="#SEC5">AUTHOR</a>
21 <li><a name="TOC6" href="#SEC6">REVISION</a>
22 </ul>
23 <br><a name="SEC1" href="#TOC1">PLEASE TAKE NOTE</a><br>
24 <P>
25 This document relates to PCRE releases that use the original API,
26 with library names libpcre, libpcre16, and libpcre32. January 2015 saw the
27 first release of a new API, known as PCRE2, with release numbers starting at
28 10.00 and library names libpcre2-8, libpcre2-16, and libpcre2-32. The old
29 libraries (now called PCRE1) are still being maintained for bug fixes, but
30 there will be no new development. New projects are advised to use the new PCRE2
31 libraries.
32 </P>
33 <br><a name="SEC2" href="#TOC1">INTRODUCTION</a><br>
34 <P>
35 The PCRE library is a set of functions that implement regular expression
36 pattern matching using the same syntax and semantics as Perl, with just a few
37 differences. Some features that appeared in Python and PCRE before they
38 appeared in Perl are also available using the Python syntax, there is some
39 support for one or two .NET and Oniguruma syntax items, and there is an option
40 for requesting some minor changes that give better JavaScript compatibility.
41 </P>
42 <P>
43 Starting with release 8.30, it is possible to compile two separate PCRE
44 libraries: the original, which supports 8-bit character strings (including
45 UTF-8 strings), and a second library that supports 16-bit character strings
46 (including UTF-16 strings). The build process allows either one or both to be
47 built. The majority of the work to make this possible was done by Zoltan
48 Herczeg.
49 </P>
50 <P>
51 Starting with release 8.32 it is possible to compile a third separate PCRE
52 library that supports 32-bit character strings (including UTF-32 strings). The
53 build process allows any combination of the 8-, 16- and 32-bit libraries. The
54 work to make this possible was done by Christian Persch.
55 </P>
56 <P>
57 The three libraries contain identical sets of functions, except that the names
58 in the 16-bit library start with <b>pcre16_</b> instead of <b>pcre_</b>, and the
59 names in the 32-bit library start with <b>pcre32_</b> instead of <b>pcre_</b>. To
60 avoid over-complication and reduce the documentation maintenance load, most of
61 the documentation describes the 8-bit library, with the differences for the
62 16-bit and 32-bit libraries described separately in the
63 <a href="pcre16.html"><b>pcre16</b></a>
64 and
65 <a href="pcre32.html"><b>pcre32</b></a>
66 pages. References to functions or structures of the form <i>pcre[16|32]_xxx</i>
67 should be read as meaning "<i>pcre_xxx</i> when using the 8-bit library,
68 <i>pcre16_xxx</i> when using the 16-bit library, or <i>pcre32_xxx</i> when using
69 the 32-bit library".
70 </P>
71 <P>
72 The current implementation of PCRE corresponds approximately with Perl 5.12,
73 including support for UTF-8/16/32 encoded strings and Unicode general category
74 properties. However, UTF-8/16/32 and Unicode support has to be explicitly
75 enabled; it is not the default. The Unicode tables correspond to Unicode
76 release 6.3.0.
77 </P>
78 <P>
79 In addition to the Perl-compatible matching function, PCRE contains an
80 alternative function that matches the same compiled patterns in a different
81 way. In certain circumstances, the alternative function has some advantages.
82 For a discussion of the two matching algorithms, see the
83 <a href="pcrematching.html"><b>pcrematching</b></a>
84 page.
85 </P>
86 <P>
87 PCRE is written in C and released as a C library. A number of people have
88 written wrappers and interfaces of various kinds. In particular, Google Inc.
89 have provided a comprehensive C++ wrapper for the 8-bit library. This is now
90 included as part of the PCRE distribution. The
91 <a href="pcrecpp.html"><b>pcrecpp</b></a>
92 page has details of this interface. Other people's contributions can be found
93 in the <i>Contrib</i> directory at the primary FTP site, which is:
94 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
95 </P>
96 <P>
97 Details of exactly which Perl regular expression features are and are not
98 supported by PCRE are given in separate documents. See the
99 <a href="pcrepattern.html"><b>pcrepattern</b></a>
100 and
101 <a href="pcrecompat.html"><b>pcrecompat</b></a>
102 pages. There is a syntax summary in the
103 <a href="pcresyntax.html"><b>pcresyntax</b></a>
104 page.
105 </P>
106 <P>
107 Some features of PCRE can be included, excluded, or changed when the library is
108 built. The
109 <a href="pcre_config.html"><b>pcre_config()</b></a>
110 function makes it possible for a client to discover which features are
111 available. The features themselves are described in the
112 <a href="pcrebuild.html"><b>pcrebuild</b></a>
113 page. Documentation about building PCRE for various operating systems can be
114 found in the
115 <a href="README.txt"><b>README</b></a>
116 and
117 <a href="NON-AUTOTOOLS-BUILD.txt"><b>NON-AUTOTOOLS_BUILD</b></a>
118 files in the source distribution.
119 </P>
120 <P>
121 The libraries contains a number of undocumented internal functions and data
122 tables that are used by more than one of the exported external functions, but
123 which are not intended for use by external callers. Their names all begin with
124 "_pcre_" or "_pcre16_" or "_pcre32_", which hopefully will not provoke any name
125 clashes. In some environments, it is possible to control which external symbols
126 are exported when a shared library is built, and in these cases the
127 undocumented symbols are not exported.
128 </P>
129 <br><a name="SEC3" href="#TOC1">SECURITY CONSIDERATIONS</a><br>
130 <P>
131 If you are using PCRE in a non-UTF application that permits users to supply
132 arbitrary patterns for compilation, you should be aware of a feature that
133 allows users to turn on UTF support from within a pattern, provided that PCRE
134 was built with UTF support. For example, an 8-bit pattern that begins with
135 "(*UTF8)" or "(*UTF)" turns on UTF-8 mode, which interprets patterns and
136 subjects as strings of UTF-8 characters instead of individual 8-bit characters.
137 This causes both the pattern and any data against which it is matched to be
138 checked for UTF-8 validity. If the data string is very long, such a check might
139 use sufficiently many resources as to cause your application to lose
140 performance.
141 </P>
142 <P>
143 One way of guarding against this possibility is to use the
144 <b>pcre_fullinfo()</b> function to check the compiled pattern's options for UTF.
145 Alternatively, from release 8.33, you can set the PCRE_NEVER_UTF option at
146 compile time. This causes an compile time error if a pattern contains a
147 UTF-setting sequence.
148 </P>
149 <P>
150 If your application is one that supports UTF, be aware that validity checking
151 can take time. If the same data string is to be matched many times, you can use
152 the PCRE_NO_UTF[8|16|32]_CHECK option for the second and subsequent matches to
153 save redundant checks.
154 </P>
155 <P>
156 Another way that performance can be hit is by running a pattern that has a very
157 large search tree against a string that will never match. Nested unlimited
158 repeats in a pattern are a common example. PCRE provides some protection
159 against this: see the PCRE_EXTRA_MATCH_LIMIT feature in the
160 <a href="pcreapi.html"><b>pcreapi</b></a>
161 page.
162 </P>
163 <br><a name="SEC4" href="#TOC1">USER DOCUMENTATION</a><br>
164 <P>
165 The user documentation for PCRE comprises a number of different sections. In
166 the "man" format, each of these is a separate "man page". In the HTML format,
167 each is a separate page, linked from the index page. In the plain text format,
168 the descriptions of the <b>pcregrep</b> and <b>pcretest</b> programs are in files
169 called <b>pcregrep.txt</b> and <b>pcretest.txt</b>, respectively. The remaining
170 sections, except for the <b>pcredemo</b> section (which is a program listing),
171 are concatenated in <b>pcre.txt</b>, for ease of searching. The sections are as
172 follows:
173 <pre>
174 pcre this document
175 pcre-config show PCRE installation configuration information
176 pcre16 details of the 16-bit library
177 pcre32 details of the 32-bit library
178 pcreapi details of PCRE's native C API
179 pcrebuild building PCRE
180 pcrecallout details of the callout feature
181 pcrecompat discussion of Perl compatibility
182 pcrecpp details of the C++ wrapper for the 8-bit library
183 pcredemo a demonstration C program that uses PCRE
184 pcregrep description of the <b>pcregrep</b> command (8-bit only)
185 pcrejit discussion of the just-in-time optimization support
186 pcrelimits details of size and other limits
187 pcrematching discussion of the two matching algorithms
188 pcrepartial details of the partial matching facility
189 pcrepattern syntax and semantics of supported regular expressions
190 pcreperform discussion of performance issues
191 pcreposix the POSIX-compatible C API for the 8-bit library
192 pcreprecompile details of saving and re-using precompiled patterns
193 pcresample discussion of the pcredemo program
194 pcrestack discussion of stack usage
195 pcresyntax quick syntax reference
196 pcretest description of the <b>pcretest</b> testing command
197 pcreunicode discussion of Unicode and UTF-8/16/32 support
198 </pre>
199 In the "man" and HTML formats, there is also a short page for each C library
200 function, listing its arguments and results.
201 </P>
202 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
203 <P>
204 Philip Hazel
205 <br>
206 University Computing Service
207 <br>
208 Cambridge CB2 3QH, England.
209 <br>
210 </P>
211 <P>
212 Putting an actual email address here seems to have been a spam magnet, so I've
213 taken it away. If you want to email me, use my two initials, followed by the
214 two digits 10, at the domain cam.ac.uk.
215 </P>
216 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
217 <P>
218 Last updated: 10 February 2015
219 <br>
220 Copyright &copy; 1997-2015 University of Cambridge.
221 <br>
222 <p>
223 Return to the <a href="index.html">PCRE index page</a>.
224 </p>

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