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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 pcrestack discussion of stack usage
96 pcretest description of the \fBpcretest\fP testing command
97 .sp
98 In addition, in the "man" and HTML formats, there is a short page for each
99 C library function, listing its arguments and results.
100 .
101 .
103 .rs
104 .sp
105 There are some size limitations in PCRE but it is hoped that they will never in
106 practice be relevant.
107 .P
108 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
109 compiled with the default internal linkage size of 2. If you want to process
110 regular expressions that are truly enormous, you can compile PCRE with an
111 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
112 distribution and the
113 .\" HREF
114 \fBpcrebuild\fP
115 .\"
116 documentation for details). In these cases the limit is substantially larger.
117 However, the speed of execution will be slower.
118 .P
119 All values in repeating quantifiers must be less than 65536. The maximum
120 compiled length of subpattern with an explicit repeat count is 30000 bytes. The
121 maximum number of capturing subpatterns is 65535.
122 .P
123 There is no limit to the number of non-capturing subpatterns, but the maximum
124 depth of nesting of all kinds of parenthesized subpattern, including capturing
125 subpatterns, assertions, and other types of subpattern, is 200.
126 .P
127 The maximum length of name for a named subpattern is 32, and the maximum number
128 of named subpatterns is 10000.
129 .P
130 The maximum length of a subject string is the largest positive number that an
131 integer variable can hold. However, when using the traditional matching
132 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
133 This means that the available stack space may limit the size of a subject
134 string that can be processed by certain patterns. For a discussion of stack
135 issues, see the
136 .\" HREF
137 \fBpcrestack\fP
138 .\"
139 documentation.
140 .sp
141 .\" HTML <a name="utf8support"></a>
142 .
143 .
145 .rs
146 .sp
147 From release 3.3, PCRE has had some support for character strings encoded in
148 the UTF-8 format. For release 4.0 this was greatly extended to cover most
149 common requirements, and in release 5.0 additional support for Unicode general
150 category properties was added.
151 .P
152 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
153 the code, and, in addition, you must call
154 .\" HREF
155 \fBpcre_compile()\fP
156 .\"
157 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
158 subject strings that are matched against it are treated as UTF-8 strings
159 instead of just strings of bytes.
160 .P
161 If you compile PCRE with UTF-8 support, but do not use it at run time, the
162 library will be a bit bigger, but the additional run time overhead is limited
163 to testing the PCRE_UTF8 flag in several places, so should not be very large.
164 .P
165 If PCRE is built with Unicode character property support (which implies UTF-8
166 support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
167 The available properties that can be tested are limited to the general
168 category properties such as Lu for an upper case letter or Nd for a decimal
169 number, the Unicode script names such as Arabic or Han, and the derived
170 properties Any and L&. A full list is given in the
171 .\" HREF
172 \fBpcrepattern\fP
173 .\"
174 documentation. Only the short names for properties are supported. For example,
175 \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
176 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
177 compatibility with Perl 5.6. PCRE does not support this.
178 .P
179 The following comments apply when PCRE is running in UTF-8 mode:
180 .P
181 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
182 are checked for validity on entry to the relevant functions. If an invalid
183 UTF-8 string is passed, an error return is given. In some situations, you may
184 already know that your strings are valid, and therefore want to skip these
185 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
186 at compile time or at run time, PCRE assumes that the pattern or subject it
187 is given (respectively) contains only valid UTF-8 codes. In this case, it does
188 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
189 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
190 may crash.
191 .P
192 2. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
193 UTF-8 character if the value is greater than 127.
194 .P
195 3. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
196 characters for values greater than \e177.
197 .P
198 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
199 bytes, for example: \ex{100}{3}.
200 .P
201 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
202 .P
203 6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
204 but its use can lead to some strange effects. This facility is not available in
205 the alternative matching function, \fBpcre_dfa_exec()\fP.
206 .P
207 7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
208 test characters of any code value, but the characters that PCRE recognizes as
209 digits, spaces, or word characters remain the same set as before, all with
210 values less than 256. This remains true even when PCRE includes Unicode
211 property support, because to do otherwise would slow down PCRE in many common
212 cases. If you really want to test for a wider sense of, say, "digit", you
213 must use Unicode property tests such as \ep{Nd}.
214 .P
215 8. Similarly, characters that match the POSIX named character classes are all
216 low-valued characters.
217 .P
218 9. Case-insensitive matching applies only to characters whose values are less
219 than 128, unless PCRE is built with Unicode property support. Even when Unicode
220 property support is available, PCRE still uses its own character tables when
221 checking the case of low-valued characters, so as not to degrade performance.
222 The Unicode property information is used only for characters with higher
223 values. Even when Unicode property support is available, PCRE supports
224 case-insensitive matching only when there is a one-to-one mapping between a
225 letter's cases. There are a small number of many-to-one mappings in Unicode;
226 these are not supported by PCRE.
227 .
229 .rs
230 .sp
231 Philip Hazel
232 .br
233 University Computing Service,
234 .br
235 Cambridge CB2 3QG, England.
236 .P
237 Putting an actual email address here seems to have been a spam magnet, so I've
238 taken it away. If you want to email me, use my initial and surname, separated
239 by a dot, at the domain ucs.cam.ac.uk.
240 .sp
241 .in 0
242 Last updated: 05 June 2006
243 .br
244 Copyright (c) 1997-2006 University of Cambridge.

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