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1 nigel 41 .TH PCRE 3
2     .SH NAME
3 nigel 63 PCRE - Perl-compatible regular expressions
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 nigel 83 "_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 nigel 75 .
69     .
71 nigel 63 .rs
72     .sp
73 nigel 75 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 nigel 63 pcre this document
80 nigel 77 pcreapi details of PCRE's native C API
81 nigel 63 pcrebuild options for building PCRE
82     pcrecallout details of the callout feature
83     pcrecompat discussion of Perl compatibility
84 nigel 77 pcrecpp details of the C++ wrapper
85 nigel 75 pcregrep description of the \fBpcregrep\fP command
86 nigel 77 pcrematching discussion of the two matching algorithms
87 nigel 75 pcrepartial details of the partial matching facility
88     .\" JOIN
89 nigel 63 pcrepattern syntax and semantics of supported
90     regular expressions
91     pcreperform discussion of performance issues
92 nigel 77 pcreposix the POSIX-compatible C API
93 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
94 nigel 63 pcresample discussion of the sample program
95 nigel 75 pcretest description of the \fBpcretest\fP testing command
96     .sp
97 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
98 nigel 77 C library function, listing its arguments and results.
99 nigel 75 .
100     .
101 nigel 41 .SH LIMITATIONS
102 nigel 63 .rs
103     .sp
104 nigel 41 There are some size limitations in PCRE but it is hoped that they will never in
105     practice be relevant.
106 nigel 75 .P
107 nigel 63 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
108     compiled with the default internal linkage size of 2. If you want to process
109     regular expressions that are truly enormous, you can compile PCRE with an
110 nigel 75 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
111 nigel 63 distribution and the
112     .\" HREF
113 nigel 75 \fBpcrebuild\fP
114 nigel 63 .\"
115 nigel 75 documentation for details). In these cases the limit is substantially larger.
116 nigel 63 However, the speed of execution will be slower.
117 nigel 75 .P
118 nigel 41 All values in repeating quantifiers must be less than 65536.
119 nigel 63 The maximum number of capturing subpatterns is 65535.
120 nigel 75 .P
121 nigel 53 There is no limit to the number of non-capturing subpatterns, but the maximum
122     depth of nesting of all kinds of parenthesized subpattern, including capturing
123 nigel 41 subpatterns, assertions, and other types of subpattern, is 200.
124 nigel 75 .P
125 nigel 41 The maximum length of a subject string is the largest positive number that an
126 nigel 77 integer variable can hold. However, when using the traditional matching
127     function, PCRE uses recursion to handle subpatterns and indefinite repetition.
128     This means that the available stack space may limit the size of a subject
129     string that can be processed by certain patterns.
130 nigel 75 .sp
131 nigel 63 .\" HTML <a name="utf8support"></a>
132 nigel 75 .
133     .
135 nigel 63 .rs
136     .sp
137 nigel 75 From release 3.3, PCRE has had some support for character strings encoded in
138     the UTF-8 format. For release 4.0 this was greatly extended to cover most
139     common requirements, and in release 5.0 additional support for Unicode general
140     category properties was added.
141     .P
142 nigel 63 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
143     the code, and, in addition, you must call
144     .\" HREF
145 nigel 75 \fBpcre_compile()\fP
146 nigel 63 .\"
147     with the PCRE_UTF8 option flag. When you do this, both the pattern and any
148     subject strings that are matched against it are treated as UTF-8 strings
149     instead of just strings of bytes.
150 nigel 75 .P
151 nigel 49 If you compile PCRE with UTF-8 support, but do not use it at run time, the
152     library will be a bit bigger, but the additional run time overhead is limited
153     to testing the PCRE_UTF8 flag in several places, so should not be very large.
154 nigel 75 .P
155     If PCRE is built with Unicode character property support (which implies UTF-8
156     support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
157     The available properties that can be tested are limited to the general
158     category properties such as Lu for an upper case letter or Nd for a decimal
159 nigel 87 number, the Unicode script names such as Arabic or Han, and the derived
160     properties Any and L&. A full list is given in the
161 nigel 75 .\" HREF
162     \fBpcrepattern\fP
163     .\"
164 nigel 87 documentation. Only the short names for properties are supported. For example,
165     \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
166     Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
167     compatibility with Perl 5.6. PCRE does not support this.
168 nigel 75 .P
169 nigel 63 The following comments apply when PCRE is running in UTF-8 mode:
170 nigel 75 .P
171 nigel 71 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
172     are checked for validity on entry to the relevant functions. If an invalid
173     UTF-8 string is passed, an error return is given. In some situations, you may
174     already know that your strings are valid, and therefore want to skip these
175     checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
176     at compile time or at run time, PCRE assumes that the pattern or subject it
177     is given (respectively) contains only valid UTF-8 codes. In this case, it does
178     not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
179     PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
180     may crash.
181 nigel 75 .P
182 nigel 87 2. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
183     UTF-8 character if the value is greater than 127.
184 nigel 75 .P
185 nigel 87 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
186 nigel 75 bytes, for example: \ex{100}{3}.
187     .P
188 nigel 87 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
189 nigel 75 .P
190 nigel 87 5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
191 nigel 77 but its use can lead to some strange effects. This facility is not available in
192     the alternative matching function, \fBpcre_dfa_exec()\fP.
193 nigel 75 .P
194 nigel 87 6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
195 nigel 63 test characters of any code value, but the characters that PCRE recognizes as
196     digits, spaces, or word characters remain the same set as before, all with
197 nigel 75 values less than 256. This remains true even when PCRE includes Unicode
198     property support, because to do otherwise would slow down PCRE in many common
199     cases. If you really want to test for a wider sense of, say, "digit", you
200     must use Unicode property tests such as \ep{Nd}.
201     .P
202 nigel 87 7. Similarly, characters that match the POSIX named character classes are all
203 nigel 75 low-valued characters.
204     .P
205 nigel 87 8. Case-insensitive matching applies only to characters whose values are less
206 nigel 75 than 128, unless PCRE is built with Unicode property support. Even when Unicode
207     property support is available, PCRE still uses its own character tables when
208     checking the case of low-valued characters, so as not to degrade performance.
209     The Unicode property information is used only for characters with higher
210 nigel 87 values. Even when Unicode property support is available, PCRE supports
211     case-insensitive matching only when there is a one-to-one mapping between a
212     letter's cases. There are a small number of many-to-one mappings in Unicode;
213     these are not supported by PCRE.
214 nigel 75 .
215 nigel 41 .SH AUTHOR
216 nigel 63 .rs
217     .sp
218 nigel 77 Philip Hazel
219 nigel 41 .br
220     University Computing Service,
221     .br
222     Cambridge CB2 3QG, England.
223 nigel 77 .P
224     Putting an actual email address here seems to have been a spam magnet, so I've
225     taken it away. If you want to email me, use my initial and surname, separated
226     by a dot, at the domain ucs.cam.ac.uk.
227 nigel 75 .sp
228 nigel 63 .in 0
229 nigel 87 Last updated: 24 January 2006
230 nigel 41 .br
231 nigel 87 Copyright (c) 1997-2006 University of Cambridge.

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