1PCRE2(3) Library Functions Manual PCRE2(3)
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6 PCRE2 - Perl-compatible regular expressions (revised API)
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9 PCRE2 is the name used for a revised API for the PCRE library, which is
11 a set of functions, written in C, that implement regular expression
12 pattern matching using the same syntax and semantics as Perl, with just
13 a few differences. Some features that appeared in Python and the origi‐
14 nal PCRE before they appeared in Perl are also available using the
15 Python syntax. There is also some support for one or two .NET and Onig‐
16 uruma syntax items, and there are options for requesting some minor
17 changes that give better ECMAScript (aka JavaScript) compatibility.
18 The source code for PCRE2 can be compiled to support 8-bit, 16-bit, or
20 32-bit code units, which means that up to three separate libraries may
21 be installed. The original work to extend PCRE to 16-bit and 32-bit
22 code units was done by Zoltan Herczeg and Christian Persch, respec‐
23 tively. In all three cases, strings can be interpreted either as one
24 character per code unit, or as UTF-encoded Unicode, with support for
25 Unicode general category properties. Unicode support is optional at
26 build time (but is the default). However, processing strings as UTF
27 code units must be enabled explicitly at run time. The version of Uni‐
28 code in use can be discovered by running
29 pcre2test -C
31 The three libraries contain identical sets of functions, with names
33 ending in _8, _16, or _32, respectively (for example, pcre2_com‐
34 pile_8()). However, by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or
35 32, a program that uses just one code unit width can be written using
36 generic names such as pcre2_compile(), and the documentation is written
37 assuming that this is the case.
38 In addition to the Perl-compatible matching function, PCRE2 contains an
40 alternative function that matches the same compiled patterns in a dif‐
41 ferent way. In certain circumstances, the alternative function has some
42 advantages. For a discussion of the two matching algorithms, see the
43 pcre2matching page.
44 Details of exactly which Perl regular expression features are and are
46 not supported by PCRE2 are given in separate documents. See the
47 pcre2pattern and pcre2compat pages. There is a syntax summary in the
48 pcre2syntax page.
49 Some features of PCRE2 can be included, excluded, or changed when the
51 library is built. The pcre2_config() function makes it possible for a
52 client to discover which features are available. The features them‐
53 selves are described in the pcre2build page. Documentation about build‐
54 ing PCRE2 for various operating systems can be found in the README and
55 NON-AUTOTOOLS_BUILD files in the source distribution.
56 The libraries contains a number of undocumented internal functions and
58 data tables that are used by more than one of the exported external
59 functions, but which are not intended for use by external callers.
60 Their names all begin with "_pcre2", which hopefully will not provoke
61 any name clashes. In some environments, it is possible to control which
62 external symbols are exported when a shared library is built, and in
63 these cases the undocumented symbols are not exported.
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66 If you are using PCRE2 in a non-UTF application that permits users to
68 supply arbitrary patterns for compilation, you should be aware of a
69 feature that allows users to turn on UTF support from within a pattern.
70 For example, an 8-bit pattern that begins with "(*UTF)" turns on UTF-8
71 mode, which interprets patterns and subjects as strings of UTF-8 code
72 units instead of individual 8-bit characters. This causes both the pat‐
73 tern and any data against which it is matched to be checked for UTF-8
74 validity. If the data string is very long, such a check might use suf‐
75 ficiently many resources as to cause your application to lose perfor‐
76 mance.
77 One way of guarding against this possibility is to use the pcre2_pat‐
79 tern_info() function to check the compiled pattern's options for
80 PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option when
81 calling pcre2_compile(). This causes an compile time error if a pattern
82 contains a UTF-setting sequence.
83 The use of Unicode properties for character types such as \d can also
85 be enabled from within the pattern, by specifying "(*UCP)". This fea‐
86 ture can be disallowed by setting the PCRE2_NEVER_UCP option.
87 If your application is one that supports UTF, be aware that validity
89 checking can take time. If the same data string is to be matched many
90 times, you can use the PCRE2_NO_UTF_CHECK option for the second and
91 subsequent matches to avoid running redundant checks.
92 The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead
94 to problems, because it may leave the current matching point in the
95 middle of a multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C
96 option can be used by an application to lock out the use of \C, causing
97 a compile-time error if it is encountered. It is also possible to build
98 PCRE2 with the use of \C permanently disabled.
99 Another way that performance can be hit is by running a pattern that
101 has a very large search tree against a string that will never match.
102 Nested unlimited repeats in a pattern are a common example. PCRE2 pro‐
103 vides some protection against this: see the pcre2_set_match_limit()
104 function in the pcre2api page.
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107 The user documentation for PCRE2 comprises a number of different sec‐
109 tions. In the "man" format, each of these is a separate "man page". In
110 the HTML format, each is a separate page, linked from the index page.
111 In the plain text format, the descriptions of the pcre2grep and
112 pcre2test programs are in files called pcre2grep.txt and pcre2test.txt,
113 respectively. The remaining sections, except for the pcre2demo section
114 (which is a program listing), and the short pages for individual func‐
115 tions, are concatenated in pcre2.txt, for ease of searching. The sec‐
116 tions are as follows:
117 pcre2 this document
119 pcre2-config show PCRE2 installation configuration information
120 pcre2api details of PCRE2's native C API
121 pcre2build building PCRE2
122 pcre2callout details of the callout feature
123 pcre2compat discussion of Perl compatibility
124 pcre2demo a demonstration C program that uses PCRE2
125 pcre2grep description of the pcre2grep command (8-bit only)
126 pcre2jit discussion of just-in-time optimization support
127 pcre2limits details of size and other limits
128 pcre2matching discussion of the two matching algorithms
129 pcre2partial details of the partial matching facility
130 pcre2pattern syntax and semantics of supported regular
131 expression patterns
132 pcre2perform discussion of performance issues
133 pcre2posix the POSIX-compatible C API for the 8-bit library
134 pcre2sample discussion of the pcre2demo program
135 pcre2stack discussion of stack usage
136 pcre2syntax quick syntax reference
137 pcre2test description of the pcre2test command
138 pcre2unicode discussion of Unicode and UTF support
139 In the "man" and HTML formats, there is also a short page for each C
141 library function, listing its arguments and results.
142
144 Philip Hazel
146 University Computing Service
147 Cambridge, England.
148 Putting an actual email address here is a spam magnet. If you want to
150 email me, use my two initials, followed by the two digits 10, at the
151 domain cam.ac.uk.
152
154 Last updated: 16 October 2015
156 Copyright (c) 1997-2015 University of Cambridge.
157PCRE2 10.21 16 October 2015 PCRE2(3)