1PCRE(3) Library Functions Manual PCRE(3)
2
6 PCRE - Perl-compatible regular expressions (original API)
7
9 This document relates to PCRE releases that use the original API, with
11 library names libpcre, libpcre16, and libpcre32. January 2015 saw the
12 first release of a new API, known as PCRE2, with release numbers start‐
13 ing at 10.00 and library names libpcre2-8, libpcre2-16, and
14 libpcre2-32. The old libraries (now called PCRE1) are now at end of
15 life, and 8.45 is the final release. New projects are advised to use
16 the new PCRE2 libraries.
17
19 The PCRE library is a set of functions that implement regular expres‐
21 sion pattern matching using the same syntax and semantics as Perl, with
22 just a few differences. Some features that appeared in Python and PCRE
23 before they appeared in Perl are also available using the Python syn‐
24 tax, there is some support for one or two .NET and Oniguruma syntax
25 items, and there is an option for requesting some minor changes that
26 give better JavaScript compatibility.
27 Starting with release 8.30, it is possible to compile two separate PCRE
29 libraries: the original, which supports 8-bit character strings (in‐
30 cluding UTF-8 strings), and a second library that supports 16-bit char‐
31 acter strings (including UTF-16 strings). The build process allows ei‐
32 ther one or both to be built. The majority of the work to make this
33 possible was done by Zoltan Herczeg.
34 Starting with release 8.32 it is possible to compile a third separate
36 PCRE library that supports 32-bit character strings (including UTF-32
37 strings). The build process allows any combination of the 8-, 16- and
38 32-bit libraries. The work to make this possible was done by Christian
39 Persch.
40 The three libraries contain identical sets of functions, except that
42 the names in the 16-bit library start with pcre16_ instead of pcre_,
43 and the names in the 32-bit library start with pcre32_ instead of
44 pcre_. To avoid over-complication and reduce the documentation mainte‐
45 nance load, most of the documentation describes the 8-bit library, with
46 the differences for the 16-bit and 32-bit libraries described sepa‐
47 rately in the pcre16 and pcre32 pages. References to functions or
48 structures of the form pcre[16|32]_xxx should be read as meaning
49 "pcre_xxx when using the 8-bit library, pcre16_xxx when using the
50 16-bit library, or pcre32_xxx when using the 32-bit library".
51 The current implementation of PCRE corresponds approximately with Perl
53 5.12, including support for UTF-8/16/32 encoded strings and Unicode
54 general category properties. However, UTF-8/16/32 and Unicode support
55 has to be explicitly enabled; it is not the default. The Unicode tables
56 correspond to Unicode release 6.3.0.
57 In addition to the Perl-compatible matching function, PCRE contains an
59 alternative function that matches the same compiled patterns in a dif‐
60 ferent way. In certain circumstances, the alternative function has some
61 advantages. For a discussion of the two matching algorithms, see the
62 pcrematching page.
63 PCRE is written in C and released as a C library. A number of people
65 have written wrappers and interfaces of various kinds. In particular,
66 Google Inc. have provided a comprehensive C++ wrapper for the 8-bit
67 library. This is now included as part of the PCRE distribution. The
68 pcrecpp page has details of this interface. Other people's contribu‐
69 tions can be found in the Contrib directory at the primary FTP site,
70 which is:
71 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
73 Details of exactly which Perl regular expression features are and are
75 not supported by PCRE are given in separate documents. See the pcrepat‐
76 tern and pcrecompat pages. There is a syntax summary in the pcresyntax
77 page.
78 Some features of PCRE can be included, excluded, or changed when the
80 library is built. The pcre_config() function makes it possible for a
81 client to discover which features are available. The features them‐
82 selves are described in the pcrebuild page. Documentation about build‐
83 ing PCRE for various operating systems can be found in the README and
84 NON-AUTOTOOLS_BUILD files in the source distribution.
85 The libraries contains a number of undocumented internal functions and
87 data tables that are used by more than one of the exported external
88 functions, but which are not intended for use by external callers.
89 Their names all begin with "_pcre_" or "_pcre16_" or "_pcre32_", which
90 hopefully will not provoke any name clashes. In some environments, it
91 is possible to control which external symbols are exported when a
92 shared library is built, and in these cases the undocumented symbols
93 are not exported.
94
96 If you are using PCRE in a non-UTF application that permits users to
98 supply arbitrary patterns for compilation, you should be aware of a
99 feature that allows users to turn on UTF support from within a pattern,
100 provided that PCRE was built with UTF support. For example, an 8-bit
101 pattern that begins with "(*UTF8)" or "(*UTF)" turns on UTF-8 mode,
102 which interprets patterns and subjects as strings of UTF-8 characters
103 instead of individual 8-bit characters. This causes both the pattern
104 and any data against which it is matched to be checked for UTF-8 valid‐
105 ity. If the data string is very long, such a check might use suffi‐
106 ciently many resources as to cause your application to lose perfor‐
107 mance.
108 One way of guarding against this possibility is to use the
110 pcre_fullinfo() function to check the compiled pattern's options for
111 UTF. Alternatively, from release 8.33, you can set the PCRE_NEVER_UTF
112 option at compile time. This causes a compile time error if a pattern
113 contains a UTF-setting sequence.
114 If your application is one that supports UTF, be aware that validity
116 checking can take time. If the same data string is to be matched many
117 times, you can use the PCRE_NO_UTF[8|16|32]_CHECK option for the second
118 and subsequent matches to save redundant checks.
119 Another way that performance can be hit is by running a pattern that
121 has a very large search tree against a string that will never match.
122 Nested unlimited repeats in a pattern are a common example. PCRE pro‐
123 vides some protection against this: see the PCRE_EXTRA_MATCH_LIMIT fea‐
124 ture in the pcreapi page.
125
127 The user documentation for PCRE comprises a number of different sec‐
129 tions. In the "man" format, each of these is a separate "man page". In
130 the HTML format, each is a separate page, linked from the index page.
131 In the plain text format, the descriptions of the pcregrep and pcretest
132 programs are in files called pcregrep.txt and pcretest.txt, respec‐
133 tively. The remaining sections, except for the pcredemo section (which
134 is a program listing), are concatenated in pcre.txt, for ease of
135 searching. The sections are as follows:
136 pcre this document
138 pcre-config show PCRE installation configuration information
139 pcre16 details of the 16-bit library
140 pcre32 details of the 32-bit library
141 pcreapi details of PCRE's native C API
142 pcrebuild building PCRE
143 pcrecallout details of the callout feature
144 pcrecompat discussion of Perl compatibility
145 pcrecpp details of the C++ wrapper for the 8-bit library
146 pcredemo a demonstration C program that uses PCRE
147 pcregrep description of the pcregrep command (8-bit only)
148 pcrejit discussion of the just-in-time optimization support
149 pcrelimits details of size and other limits
150 pcrematching discussion of the two matching algorithms
151 pcrepartial details of the partial matching facility
152 pcrepattern syntax and semantics of supported
153 regular expressions
154 pcreperform discussion of performance issues
155 pcreposix the POSIX-compatible C API for the 8-bit library
156 pcreprecompile details of saving and re-using precompiled patterns
157 pcresample discussion of the pcredemo program
158 pcrestack discussion of stack usage
159 pcresyntax quick syntax reference
160 pcretest description of the pcretest testing command
161 pcreunicode discussion of Unicode and UTF-8/16/32 support
162 In the "man" and HTML formats, there is also a short page for each C
164 library function, listing its arguments and results.
165
167 Philip Hazel
169 University Computing Service
170 Cambridge CB2 3QH, England.
171 Putting an actual email address here seems to have been a spam magnet,
173 so I've taken it away. If you want to email me, use my two initials,
174 followed by the two digits 10, at the domain cam.ac.uk.
175
177 Last updated: 14 June 2021
179 Copyright (c) 1997-2021 University of Cambridge.
180PCRE 8.45 14 June 2021 PCRE(3)