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1 .TH PCRE 3 "08 January 2014" "PCRE 8.35"
3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 The PCRE library is a set of functions that implement regular expression
8 pattern matching using the same syntax and semantics as Perl, with just a few
9 differences. Some features that appeared in Python and PCRE before they
10 appeared in Perl are also available using the Python syntax, there is some
11 support for one or two .NET and Oniguruma syntax items, and there is an option
12 for requesting some minor changes that give better JavaScript compatibility.
13 .P
14 Starting with release 8.30, it is possible to compile two separate PCRE
15 libraries: the original, which supports 8-bit character strings (including
16 UTF-8 strings), and a second library that supports 16-bit character strings
17 (including UTF-16 strings). The build process allows either one or both to be
18 built. The majority of the work to make this possible was done by Zoltan
19 Herczeg.
20 .P
21 Starting with release 8.32 it is possible to compile a third separate PCRE
22 library that supports 32-bit character strings (including UTF-32 strings). The
23 build process allows any combination of the 8-, 16- and 32-bit libraries. The
24 work to make this possible was done by Christian Persch.
25 .P
26 The three libraries contain identical sets of functions, except that the names
27 in the 16-bit library start with \fBpcre16_\fP instead of \fBpcre_\fP, and the
28 names in the 32-bit library start with \fBpcre32_\fP instead of \fBpcre_\fP. To
29 avoid over-complication and reduce the documentation maintenance load, most of
30 the documentation describes the 8-bit library, with the differences for the
31 16-bit and 32-bit libraries described separately in the
32 .\" HREF
33 \fBpcre16\fP
34 and
35 .\" HREF
36 \fBpcre32\fP
37 .\"
38 pages. References to functions or structures of the form \fIpcre[16|32]_xxx\fP
39 should be read as meaning "\fIpcre_xxx\fP when using the 8-bit library,
40 \fIpcre16_xxx\fP when using the 16-bit library, or \fIpcre32_xxx\fP when using
41 the 32-bit library".
42 .P
43 The current implementation of PCRE corresponds approximately with Perl 5.12,
44 including support for UTF-8/16/32 encoded strings and Unicode general category
45 properties. However, UTF-8/16/32 and Unicode support has to be explicitly
46 enabled; it is not the default. The Unicode tables correspond to Unicode
47 release 6.3.0.
48 .P
49 In addition to the Perl-compatible matching function, PCRE contains an
50 alternative function that matches the same compiled patterns in a different
51 way. In certain circumstances, the alternative function has some advantages.
52 For a discussion of the two matching algorithms, see the
53 .\" HREF
54 \fBpcrematching\fP
55 .\"
56 page.
57 .P
58 PCRE is written in C and released as a C library. A number of people have
59 written wrappers and interfaces of various kinds. In particular, Google Inc.
60 have provided a comprehensive C++ wrapper for the 8-bit library. This is now
61 included as part of the PCRE distribution. The
62 .\" HREF
63 \fBpcrecpp\fP
64 .\"
65 page has details of this interface. Other people's contributions can be found
66 in the \fIContrib\fP directory at the primary FTP site, which is:
67 .sp
68 .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
69 .\" </a>
70 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
71 .\"
72 .P
73 Details of exactly which Perl regular expression features are and are not
74 supported by PCRE are given in separate documents. See the
75 .\" HREF
76 \fBpcrepattern\fP
77 .\"
78 and
79 .\" HREF
80 \fBpcrecompat\fP
81 .\"
82 pages. There is a syntax summary in the
83 .\" HREF
84 \fBpcresyntax\fP
85 .\"
86 page.
87 .P
88 Some features of PCRE can be included, excluded, or changed when the library is
89 built. The
90 .\" HREF
91 \fBpcre_config()\fP
92 .\"
93 function makes it possible for a client to discover which features are
94 available. The features themselves are described in the
95 .\" HREF
96 \fBpcrebuild\fP
97 .\"
98 page. Documentation about building PCRE for various operating systems can be
99 found in the
100 .\" HTML <a href="README.txt">
101 .\" </a>
102 \fBREADME\fP
103 .\"
104 and
105 .\" HTML <a href="NON-AUTOTOOLS-BUILD.txt">
106 .\" </a>
108 .\"
109 files in the source distribution.
110 .P
111 The libraries contains a number of undocumented internal functions and data
112 tables that are used by more than one of the exported external functions, but
113 which are not intended for use by external callers. Their names all begin with
114 "_pcre_" or "_pcre16_" or "_pcre32_", which hopefully will not provoke any name
115 clashes. In some environments, it is possible to control which external symbols
116 are exported when a shared library is built, and in these cases the
117 undocumented symbols are not exported.
118 .
119 .
121 .rs
122 .sp
123 If you are using PCRE in a non-UTF application that permits users to supply
124 arbitrary patterns for compilation, you should be aware of a feature that
125 allows users to turn on UTF support from within a pattern, provided that PCRE
126 was built with UTF support. For example, an 8-bit pattern that begins with
127 "(*UTF8)" or "(*UTF)" turns on UTF-8 mode, which interprets patterns and
128 subjects as strings of UTF-8 characters instead of individual 8-bit characters.
129 This causes both the pattern and any data against which it is matched to be
130 checked for UTF-8 validity. If the data string is very long, such a check might
131 use sufficiently many resources as to cause your application to lose
132 performance.
133 .P
134 One way of guarding against this possibility is to use the
135 \fBpcre_fullinfo()\fP function to check the compiled pattern's options for UTF.
136 Alternatively, from release 8.33, you can set the PCRE_NEVER_UTF option at
137 compile time. This causes an compile time error if a pattern contains a
138 UTF-setting sequence.
139 .P
140 If your application is one that supports UTF, be aware that validity checking
141 can take time. If the same data string is to be matched many times, you can use
142 the PCRE_NO_UTF[8|16|32]_CHECK option for the second and subsequent matches to
143 save redundant checks.
144 .P
145 Another way that performance can be hit is by running a pattern that has a very
146 large search tree against a string that will never match. Nested unlimited
147 repeats in a pattern are a common example. PCRE provides some protection
148 against this: see the PCRE_EXTRA_MATCH_LIMIT feature in the
149 .\" HREF
150 \fBpcreapi\fP
151 .\"
152 page.
153 .
154 .
156 .rs
157 .sp
158 The user documentation for PCRE comprises a number of different sections. In
159 the "man" format, each of these is a separate "man page". In the HTML format,
160 each is a separate page, linked from the index page. In the plain text format,
161 the descriptions of the \fBpcregrep\fP and \fBpcretest\fP programs are in files
162 called \fBpcregrep.txt\fP and \fBpcretest.txt\fP, respectively. The remaining
163 sections, except for the \fBpcredemo\fP section (which is a program listing),
164 are concatenated in \fBpcre.txt\fP, for ease of searching. The sections are as
165 follows:
166 .sp
167 pcre this document
168 pcre-config show PCRE installation configuration information
169 pcre16 details of the 16-bit library
170 pcre32 details of the 32-bit library
171 pcreapi details of PCRE's native C API
172 pcrebuild building PCRE
173 pcrecallout details of the callout feature
174 pcrecompat discussion of Perl compatibility
175 pcrecpp details of the C++ wrapper for the 8-bit library
176 pcredemo a demonstration C program that uses PCRE
177 pcregrep description of the \fBpcregrep\fP command (8-bit only)
178 pcrejit discussion of the just-in-time optimization support
179 pcrelimits details of size and other limits
180 pcrematching discussion of the two matching algorithms
181 pcrepartial details of the partial matching facility
182 .\" JOIN
183 pcrepattern syntax and semantics of supported
184 regular expressions
185 pcreperform discussion of performance issues
186 pcreposix the POSIX-compatible C API for the 8-bit library
187 pcreprecompile details of saving and re-using precompiled patterns
188 pcresample discussion of the pcredemo program
189 pcrestack discussion of stack usage
190 pcresyntax quick syntax reference
191 pcretest description of the \fBpcretest\fP testing command
192 pcreunicode discussion of Unicode and UTF-8/16/32 support
193 .sp
194 In the "man" and HTML formats, there is also a short page for each C library
195 function, listing its arguments and results.
196 .
197 .
199 .rs
200 .sp
201 .nf
202 Philip Hazel
203 University Computing Service
204 Cambridge CB2 3QH, England.
205 .fi
206 .P
207 Putting an actual email address here seems to have been a spam magnet, so I've
208 taken it away. If you want to email me, use my two initials, followed by the
209 two digits 10, at the domain cam.ac.uk.
210 .
211 .
213 .rs
214 .sp
215 .nf
216 Last updated: 08 January 2014
217 Copyright (c) 1997-2014 University of Cambridge.
218 .fi


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