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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 5.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 PCRE is written in C and released as a C library. A number of people have
15 written wrappers and interfaces of various kinds. A C++ class is included in
16 these contributions, which can be found in the \fIContrib\fR directory at the
17 primary FTP site, which is:
18 .sp
19 .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
20 .\" </a>
21 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
22 .P
23 Details of exactly which Perl regular expression features are and are not
24 supported by PCRE are given in separate documents. See the
25 .\" HREF
26 \fBpcrepattern\fR
27 .\"
28 and
29 .\" HREF
30 \fBpcrecompat\fR
31 .\"
32 pages.
33 .P
34 Some features of PCRE can be included, excluded, or changed when the library is
35 built. The
36 .\" HREF
37 \fBpcre_config()\fR
38 .\"
39 function makes it possible for a client to discover which features are
40 available. The features themselves are described in the
41 .\" HREF
42 \fBpcrebuild\fP
43 .\"
44 page. Documentation about building PCRE for various operating systems can be
45 found in the \fBREADME\fP file in the source distribution.
46 .
47 .
49 .rs
50 .sp
51 The user documentation for PCRE comprises a number of different sections. In
52 the "man" format, each of these is a separate "man page". In the HTML format,
53 each is a separate page, linked from the index page. In the plain text format,
54 all the sections are concatenated, for ease of searching. The sections are as
55 follows:
56 .sp
57 pcre this document
58 pcreapi details of PCRE's native API
59 pcrebuild options for building PCRE
60 pcrecallout details of the callout feature
61 pcrecompat discussion of Perl compatibility
62 pcregrep description of the \fBpcregrep\fP command
63 pcrepartial details of the partial matching facility
64 .\" JOIN
65 pcrepattern syntax and semantics of supported
66 regular expressions
67 pcreperform discussion of performance issues
68 pcreposix the POSIX-compatible API
69 pcreprecompile details of saving and re-using precompiled patterns
70 pcresample discussion of the sample program
71 pcretest description of the \fBpcretest\fP testing command
72 .sp
73 In addition, in the "man" and HTML formats, there is a short page for each
74 library function, listing its arguments and results.
75 .
76 .
78 .rs
79 .sp
80 There are some size limitations in PCRE but it is hoped that they will never in
81 practice be relevant.
82 .P
83 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
84 compiled with the default internal linkage size of 2. If you want to process
85 regular expressions that are truly enormous, you can compile PCRE with an
86 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
87 distribution and the
88 .\" HREF
89 \fBpcrebuild\fP
90 .\"
91 documentation for details). In these cases the limit is substantially larger.
92 However, the speed of execution will be slower.
93 .P
94 All values in repeating quantifiers must be less than 65536.
95 The maximum number of capturing subpatterns is 65535.
96 .P
97 There is no limit to the number of non-capturing subpatterns, but the maximum
98 depth of nesting of all kinds of parenthesized subpattern, including capturing
99 subpatterns, assertions, and other types of subpattern, is 200.
100 .P
101 The maximum length of a subject string is the largest positive number that an
102 integer variable can hold. However, PCRE uses recursion to handle subpatterns
103 and indefinite repetition. This means that the available stack space may limit
104 the size of a subject string that can be processed by certain patterns.
105 .sp
106 .\" HTML <a name="utf8support"></a>
107 .
108 .
110 .rs
111 .sp
112 From release 3.3, PCRE has had some support for character strings encoded in
113 the UTF-8 format. For release 4.0 this was greatly extended to cover most
114 common requirements, and in release 5.0 additional support for Unicode general
115 category properties was added.
116 .P
117 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
118 the code, and, in addition, you must call
119 .\" HREF
120 \fBpcre_compile()\fP
121 .\"
122 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
123 subject strings that are matched against it are treated as UTF-8 strings
124 instead of just strings of bytes.
125 .P
126 If you compile PCRE with UTF-8 support, but do not use it at run time, the
127 library will be a bit bigger, but the additional run time overhead is limited
128 to testing the PCRE_UTF8 flag in several places, so should not be very large.
129 .P
130 If PCRE is built with Unicode character property support (which implies UTF-8
131 support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
132 The available properties that can be tested are limited to the general
133 category properties such as Lu for an upper case letter or Nd for a decimal
134 number. A full list is given in the
135 .\" HREF
136 \fBpcrepattern\fP
137 .\"
138 documentation. The PCRE library is increased in size by about 90K when Unicode
139 property support is included.
140 .P
141 The following comments apply when PCRE is running in UTF-8 mode:
142 .P
143 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
144 are checked for validity on entry to the relevant functions. If an invalid
145 UTF-8 string is passed, an error return is given. In some situations, you may
146 already know that your strings are valid, and therefore want to skip these
147 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
148 at compile time or at run time, PCRE assumes that the pattern or subject it
149 is given (respectively) contains only valid UTF-8 codes. In this case, it does
150 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
151 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
152 may crash.
153 .P
154 2. In a pattern, the escape sequence \ex{...}, where the contents of the braces
155 is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
156 code number is the given hexadecimal number, for example: \ex{1234}. If a
157 non-hexadecimal digit appears between the braces, the item is not recognized.
158 This escape sequence can be used either as a literal, or within a character
159 class.
160 .P
161 3. The original hexadecimal escape sequence, \exhh, matches a two-byte UTF-8
162 character if the value is greater than 127.
163 .P
164 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
165 bytes, for example: \ex{100}{3}.
166 .P
167 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
168 .P
169 6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
170 but its use can lead to some strange effects.
171 .P
172 7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
173 test characters of any code value, but the characters that PCRE recognizes as
174 digits, spaces, or word characters remain the same set as before, all with
175 values less than 256. This remains true even when PCRE includes Unicode
176 property support, because to do otherwise would slow down PCRE in many common
177 cases. If you really want to test for a wider sense of, say, "digit", you
178 must use Unicode property tests such as \ep{Nd}.
179 .P
180 8. Similarly, characters that match the POSIX named character classes are all
181 low-valued characters.
182 .P
183 9. Case-insensitive matching applies only to characters whose values are less
184 than 128, unless PCRE is built with Unicode property support. Even when Unicode
185 property support is available, PCRE still uses its own character tables when
186 checking the case of low-valued characters, so as not to degrade performance.
187 The Unicode property information is used only for characters with higher
188 values.
189 .
191 .rs
192 .sp
193 Philip Hazel <ph10@cam.ac.uk>
194 .br
195 University Computing Service,
196 .br
197 Cambridge CB2 3QG, England.
198 .br
199 Phone: +44 1223 334714
200 .sp
201 .in 0
202 Last updated: 09 September 2004
203 .br
204 Copyright (c) 1997-2004 University of Cambridge.

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