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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 77 differences. The current implementation of PCRE (release 6.x) corresponds
10 nigel 75 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 nigel 77 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 nigel 75 PCRE is written in C and released as a C library. A number of people have
24 nigel 77 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 nigel 75 .sp
33 nigel 63 .\" 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 nigel 75 .P
37 nigel 63 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 nigel 75 .P
48 nigel 63 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 nigel 75 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 nigel 77 .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 nigel 75 .
66     .
67     .SH "USER DOCUMENTATION"
68 nigel 63 .rs
69     .sp
70 nigel 75 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 nigel 63 pcre this document
77 nigel 77 pcreapi details of PCRE's native C API
78 nigel 63 pcrebuild options for building PCRE
79     pcrecallout details of the callout feature
80     pcrecompat discussion of Perl compatibility
81 nigel 77 pcrecpp details of the C++ wrapper
82 nigel 75 pcregrep description of the \fBpcregrep\fP command
83 nigel 77 pcrematching discussion of the two matching algorithms
84 nigel 75 pcrepartial details of the partial matching facility
85     .\" JOIN
86 nigel 63 pcrepattern syntax and semantics of supported
87     regular expressions
88     pcreperform discussion of performance issues
89 nigel 77 pcreposix the POSIX-compatible C API
90 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
91 nigel 63 pcresample discussion of the sample program
92 nigel 75 pcretest description of the \fBpcretest\fP testing command
93     .sp
94 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
95 nigel 77 C library function, listing its arguments and results.
96 nigel 75 .
97     .
98 nigel 41 .SH LIMITATIONS
99 nigel 63 .rs
100     .sp
101 nigel 41 There are some size limitations in PCRE but it is hoped that they will never in
102     practice be relevant.
103 nigel 75 .P
104 nigel 63 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 nigel 75 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
108 nigel 63 distribution and the
109     .\" HREF
110 nigel 75 \fBpcrebuild\fP
111 nigel 63 .\"
112 nigel 75 documentation for details). In these cases the limit is substantially larger.
113 nigel 63 However, the speed of execution will be slower.
114 nigel 75 .P
115 nigel 41 All values in repeating quantifiers must be less than 65536.
116 nigel 63 The maximum number of capturing subpatterns is 65535.
117 nigel 75 .P
118 nigel 53 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 nigel 41 subpatterns, assertions, and other types of subpattern, is 200.
121 nigel 75 .P
122 nigel 41 The maximum length of a subject string is the largest positive number that an
123 nigel 77 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 nigel 75 .sp
128 nigel 63 .\" HTML <a name="utf8support"></a>
129 nigel 75 .
130     .
131     .SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
132 nigel 63 .rs
133     .sp
134 nigel 75 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 nigel 63 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 nigel 75 \fBpcre_compile()\fP
143 nigel 63 .\"
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 nigel 75 .P
148 nigel 49 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 nigel 75 .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 nigel 63 The following comments apply when PCRE is running in UTF-8 mode:
164 nigel 75 .P
165 nigel 71 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 nigel 75 .P
176     2. In a pattern, the escape sequence \ex{...}, where the contents of the braces
177 nigel 49 is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
178 nigel 75 code number is the given hexadecimal number, for example: \ex{1234}. If a
179 nigel 63 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 nigel 75 .P
183     3. The original hexadecimal escape sequence, \exhh, matches a two-byte UTF-8
184 nigel 63 character if the value is greater than 127.
185 nigel 75 .P
186 nigel 63 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
187 nigel 75 bytes, for example: \ex{100}{3}.
188     .P
189 nigel 63 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
190 nigel 75 .P
191     6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
192 nigel 77 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 nigel 75 .P
195     7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
196 nigel 63 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 nigel 75 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 nigel 41 .SH AUTHOR
214 nigel 63 .rs
215     .sp
216 nigel 77 Philip Hazel
217 nigel 41 .br
218     University Computing Service,
219     .br
220     Cambridge CB2 3QG, England.
221 nigel 77 .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 nigel 75 .sp
226 nigel 63 .in 0
227 nigel 77 Last updated: 07 March 2005
228 nigel 41 .br
229 nigel 77 Copyright (c) 1997-2005 University of Cambridge.

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