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1 .TH PCRE 3
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. The current implementation of PCRE (release 6.x) corresponds
10 approximately with Perl 5.8, including support for UTF-8 encoded strings and
11 Unicode general category properties. However, this support has to be explicitly
12 enabled; it is not the default.
13 .P
14 In addition to the Perl-compatible matching function, PCRE also contains an
15 alternative matching function that matches the same compiled patterns in a
16 different way. In certain circumstances, the alternative function has some
17 advantages. For a discussion of the two matching algorithms, see the
18 .\" HREF
19 \fBpcrematching\fP
20 .\"
21 page.
22 .P
23 PCRE is written in C and released as a C library. A number of people have
24 written wrappers and interfaces of various kinds. In particular, Google Inc.
25 have provided a comprehensive C++ wrapper. This is now included as part of the
26 PCRE distribution. The
27 .\" HREF
28 \fBpcrecpp\fP
29 .\"
30 page has details of this interface. Other people's contributions can be found
31 in the \fIContrib\fR directory at the primary FTP site, which is:
32 .sp
33 .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
34 .\" </a>
35 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
36 .P
37 Details of exactly which Perl regular expression features are and are not
38 supported by PCRE are given in separate documents. See the
39 .\" HREF
40 \fBpcrepattern\fR
41 .\"
42 and
43 .\" HREF
44 \fBpcrecompat\fR
45 .\"
46 pages.
47 .P
48 Some features of PCRE can be included, excluded, or changed when the library is
49 built. The
50 .\" HREF
51 \fBpcre_config()\fR
52 .\"
53 function makes it possible for a client to discover which features are
54 available. The features themselves are described in the
55 .\" HREF
56 \fBpcrebuild\fP
57 .\"
58 page. Documentation about building PCRE for various operating systems can be
59 found in the \fBREADME\fP file in the source distribution.
60 .P
61 The library contains a number of undocumented internal functions and data
62 tables that are used by more than one of the exported external functions, but
63 which are not intended for use by external callers. Their names all begin with
64 "_pcre_", which hopefully will not provoke any name clashes. In some
65 environments, it is possible to control which external symbols are exported
66 when a shared library is built, and in these cases the undocumented symbols are
67 not exported.
68 .
69 .
71 .rs
72 .sp
73 The user documentation for PCRE comprises a number of different sections. In
74 the "man" format, each of these is a separate "man page". In the HTML format,
75 each is a separate page, linked from the index page. In the plain text format,
76 all the sections are concatenated, for ease of searching. The sections are as
77 follows:
78 .sp
79 pcre this document
80 pcreapi details of PCRE's native C API
81 pcrebuild options for building PCRE
82 pcrecallout details of the callout feature
83 pcrecompat discussion of Perl compatibility
84 pcrecpp details of the C++ wrapper
85 pcregrep description of the \fBpcregrep\fP command
86 pcrematching discussion of the two matching algorithms
87 pcrepartial details of the partial matching facility
88 .\" JOIN
89 pcrepattern syntax and semantics of supported
90 regular expressions
91 pcreperform discussion of performance issues
92 pcreposix the POSIX-compatible C API
93 pcreprecompile details of saving and re-using precompiled patterns
94 pcresample discussion of the sample program
95 pcretest description of the \fBpcretest\fP testing command
96 .sp
97 In addition, in the "man" and HTML formats, there is a short page for each
98 C library function, listing its arguments and results.
99 .
100 .
102 .rs
103 .sp
104 There are some size limitations in PCRE but it is hoped that they will never in
105 practice be relevant.
106 .P
107 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
108 compiled with the default internal linkage size of 2. If you want to process
109 regular expressions that are truly enormous, you can compile PCRE with an
110 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
111 distribution and the
112 .\" HREF
113 \fBpcrebuild\fP
114 .\"
115 documentation for details). In these cases the limit is substantially larger.
116 However, the speed of execution will be slower.
117 .P
118 All values in repeating quantifiers must be less than 65536.
119 The maximum number of capturing subpatterns is 65535.
120 .P
121 There is no limit to the number of non-capturing subpatterns, but the maximum
122 depth of nesting of all kinds of parenthesized subpattern, including capturing
123 subpatterns, assertions, and other types of subpattern, is 200.
124 .P
125 The maximum length of a subject string is the largest positive number that an
126 integer variable can hold. However, when using the traditional matching
127 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
128 This means that the available stack space may limit the size of a subject
129 string that can be processed by certain patterns.
130 .sp
131 .\" HTML <a name="utf8support"></a>
132 .
133 .
135 .rs
136 .sp
137 From release 3.3, PCRE has had some support for character strings encoded in
138 the UTF-8 format. For release 4.0 this was greatly extended to cover most
139 common requirements, and in release 5.0 additional support for Unicode general
140 category properties was added.
141 .P
142 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
143 the code, and, in addition, you must call
144 .\" HREF
145 \fBpcre_compile()\fP
146 .\"
147 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
148 subject strings that are matched against it are treated as UTF-8 strings
149 instead of just strings of bytes.
150 .P
151 If you compile PCRE with UTF-8 support, but do not use it at run time, the
152 library will be a bit bigger, but the additional run time overhead is limited
153 to testing the PCRE_UTF8 flag in several places, so should not be very large.
154 .P
155 If PCRE is built with Unicode character property support (which implies UTF-8
156 support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
157 The available properties that can be tested are limited to the general
158 category properties such as Lu for an upper case letter or Nd for a decimal
159 number. A full list is given in the
160 .\" HREF
161 \fBpcrepattern\fP
162 .\"
163 documentation. The PCRE library is increased in size by about 90K when Unicode
164 property support is included.
165 .P
166 The following comments apply when PCRE is running in UTF-8 mode:
167 .P
168 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
169 are checked for validity on entry to the relevant functions. If an invalid
170 UTF-8 string is passed, an error return is given. In some situations, you may
171 already know that your strings are valid, and therefore want to skip these
172 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
173 at compile time or at run time, PCRE assumes that the pattern or subject it
174 is given (respectively) contains only valid UTF-8 codes. In this case, it does
175 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
176 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
177 may crash.
178 .P
179 2. In a pattern, the escape sequence \ex{...}, where the contents of the braces
180 is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
181 code number is the given hexadecimal number, for example: \ex{1234}. If a
182 non-hexadecimal digit appears between the braces, the item is not recognized.
183 This escape sequence can be used either as a literal, or within a character
184 class.
185 .P
186 3. The original hexadecimal escape sequence, \exhh, matches a two-byte UTF-8
187 character if the value is greater than 127.
188 .P
189 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
190 bytes, for example: \ex{100}{3}.
191 .P
192 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
193 .P
194 6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
195 but its use can lead to some strange effects. This facility is not available in
196 the alternative matching function, \fBpcre_dfa_exec()\fP.
197 .P
198 7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
199 test characters of any code value, but the characters that PCRE recognizes as
200 digits, spaces, or word characters remain the same set as before, all with
201 values less than 256. This remains true even when PCRE includes Unicode
202 property support, because to do otherwise would slow down PCRE in many common
203 cases. If you really want to test for a wider sense of, say, "digit", you
204 must use Unicode property tests such as \ep{Nd}.
205 .P
206 8. Similarly, characters that match the POSIX named character classes are all
207 low-valued characters.
208 .P
209 9. Case-insensitive matching applies only to characters whose values are less
210 than 128, unless PCRE is built with Unicode property support. Even when Unicode
211 property support is available, PCRE still uses its own character tables when
212 checking the case of low-valued characters, so as not to degrade performance.
213 The Unicode property information is used only for characters with higher
214 values.
215 .
217 .rs
218 .sp
219 Philip Hazel
220 .br
221 University Computing Service,
222 .br
223 Cambridge CB2 3QG, England.
224 .P
225 Putting an actual email address here seems to have been a spam magnet, so I've
226 taken it away. If you want to email me, use my initial and surname, separated
227 by a dot, at the domain ucs.cam.ac.uk.
228 .sp
229 .in 0
230 Last updated: 07 March 2005
231 .br
232 Copyright (c) 1997-2005 University of Cambridge.

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