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

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