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1 nigel 41 .TH PCRE 3
2     .SH NAME
3 nigel 63 PCRE - Perl-compatible regular expressions
4 nigel 75 .SH INTRODUCTION
5 nigel 63 .rs
6     .sp
7 nigel 41 The PCRE library is a set of functions that implement regular expression
8 nigel 63 pattern matching using the same syntax and semantics as Perl, with just a few
9 nigel 75 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 nigel 63 .\" 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 nigel 75 .P
23 nigel 63 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 nigel 75 .P
34 nigel 63 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 nigel 75 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     .
48     .SH "USER DOCUMENTATION"
49 nigel 63 .rs
50     .sp
51 nigel 75 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 nigel 63 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 nigel 75 pcregrep description of the \fBpcregrep\fP command
63     pcrepartial details of the partial matching facility
64     .\" JOIN
65 nigel 63 pcrepattern syntax and semantics of supported
66     regular expressions
67     pcreperform discussion of performance issues
68     pcreposix the POSIX-compatible API
69 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
70 nigel 63 pcresample discussion of the sample program
71 nigel 75 pcretest description of the \fBpcretest\fP testing command
72     .sp
73 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
74     library function, listing its arguments and results.
75 nigel 75 .
76     .
77 nigel 41 .SH LIMITATIONS
78 nigel 63 .rs
79     .sp
80 nigel 41 There are some size limitations in PCRE but it is hoped that they will never in
81     practice be relevant.
82 nigel 75 .P
83 nigel 63 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 nigel 75 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
87 nigel 63 distribution and the
88     .\" HREF
89 nigel 75 \fBpcrebuild\fP
90 nigel 63 .\"
91 nigel 75 documentation for details). In these cases the limit is substantially larger.
92 nigel 63 However, the speed of execution will be slower.
93 nigel 75 .P
94 nigel 41 All values in repeating quantifiers must be less than 65536.
95 nigel 63 The maximum number of capturing subpatterns is 65535.
96 nigel 75 .P
97 nigel 53 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 nigel 41 subpatterns, assertions, and other types of subpattern, is 200.
100 nigel 75 .P
101 nigel 41 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 nigel 75 .sp
106 nigel 63 .\" HTML <a name="utf8support"></a>
107 nigel 75 .
108     .
109     .SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
110 nigel 63 .rs
111     .sp
112 nigel 75 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 nigel 63 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 nigel 75 \fBpcre_compile()\fP
121 nigel 63 .\"
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 nigel 75 .P
126 nigel 49 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 nigel 75 .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 nigel 63 The following comments apply when PCRE is running in UTF-8 mode:
142 nigel 75 .P
143 nigel 71 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 nigel 75 .P
154     2. In a pattern, the escape sequence \ex{...}, where the contents of the braces
155 nigel 49 is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
156 nigel 75 code number is the given hexadecimal number, for example: \ex{1234}. If a
157 nigel 63 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 nigel 75 .P
161     3. The original hexadecimal escape sequence, \exhh, matches a two-byte UTF-8
162 nigel 63 character if the value is greater than 127.
163 nigel 75 .P
164 nigel 63 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
165 nigel 75 bytes, for example: \ex{100}{3}.
166     .P
167 nigel 63 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
168 nigel 75 .P
169     6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
170 nigel 63 but its use can lead to some strange effects.
171 nigel 75 .P
172     7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
173 nigel 63 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 nigel 75 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     .
190 nigel 41 .SH AUTHOR
191 nigel 63 .rs
192     .sp
193 nigel 41 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 nigel 75 .sp
201 nigel 63 .in 0
202 nigel 75 Last updated: 09 September 2004
203 nigel 41 .br
204 nigel 75 Copyright (c) 1997-2004 University of Cambridge.

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