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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 91 pcrestack discussion of stack usage
96 nigel 75 pcretest description of the \fBpcretest\fP testing command
97     .sp
98 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
99 nigel 77 C library function, listing its arguments and results.
100 nigel 75 .
101     .
102 nigel 41 .SH LIMITATIONS
103 nigel 63 .rs
104     .sp
105 nigel 41 There are some size limitations in PCRE but it is hoped that they will never in
106     practice be relevant.
107 nigel 75 .P
108 nigel 63 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
109     compiled with the default internal linkage size of 2. If you want to process
110     regular expressions that are truly enormous, you can compile PCRE with an
111 nigel 75 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
112 nigel 63 distribution and the
113     .\" HREF
114 nigel 75 \fBpcrebuild\fP
115 nigel 63 .\"
116 nigel 75 documentation for details). In these cases the limit is substantially larger.
117 nigel 63 However, the speed of execution will be slower.
118 nigel 75 .P
119 nigel 91 All values in repeating quantifiers must be less than 65536. The maximum
120     compiled length of subpattern with an explicit repeat count is 30000 bytes. The
121     maximum number of capturing subpatterns is 65535.
122 nigel 75 .P
123 nigel 53 There is no limit to the number of non-capturing subpatterns, but the maximum
124     depth of nesting of all kinds of parenthesized subpattern, including capturing
125 nigel 41 subpatterns, assertions, and other types of subpattern, is 200.
126 nigel 75 .P
127 nigel 91 The maximum length of name for a named subpattern is 32, and the maximum number
128     of named subpatterns is 10000.
129     .P
130 nigel 41 The maximum length of a subject string is the largest positive number that an
131 nigel 77 integer variable can hold. However, when using the traditional matching
132     function, PCRE uses recursion to handle subpatterns and indefinite repetition.
133     This means that the available stack space may limit the size of a subject
134 nigel 91 string that can be processed by certain patterns. For a discussion of stack
135     issues, see the
136     .\" HREF
137     \fBpcrestack\fP
138     .\"
139     documentation.
140 nigel 75 .sp
141 nigel 63 .\" HTML <a name="utf8support"></a>
142 nigel 75 .
143     .
145 nigel 63 .rs
146     .sp
147 nigel 75 From release 3.3, PCRE has had some support for character strings encoded in
148     the UTF-8 format. For release 4.0 this was greatly extended to cover most
149     common requirements, and in release 5.0 additional support for Unicode general
150     category properties was added.
151     .P
152 nigel 63 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
153     the code, and, in addition, you must call
154     .\" HREF
155 nigel 75 \fBpcre_compile()\fP
156 nigel 63 .\"
157     with the PCRE_UTF8 option flag. When you do this, both the pattern and any
158     subject strings that are matched against it are treated as UTF-8 strings
159     instead of just strings of bytes.
160 nigel 75 .P
161 nigel 49 If you compile PCRE with UTF-8 support, but do not use it at run time, the
162     library will be a bit bigger, but the additional run time overhead is limited
163     to testing the PCRE_UTF8 flag in several places, so should not be very large.
164 nigel 75 .P
165     If PCRE is built with Unicode character property support (which implies UTF-8
166     support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
167     The available properties that can be tested are limited to the general
168     category properties such as Lu for an upper case letter or Nd for a decimal
169 nigel 87 number, the Unicode script names such as Arabic or Han, and the derived
170     properties Any and L&. A full list is given in the
171 nigel 75 .\" HREF
172     \fBpcrepattern\fP
173     .\"
174 nigel 87 documentation. Only the short names for properties are supported. For example,
175     \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
176     Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
177     compatibility with Perl 5.6. PCRE does not support this.
178 nigel 75 .P
179 nigel 63 The following comments apply when PCRE is running in UTF-8 mode:
180 nigel 75 .P
181 nigel 71 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
182     are checked for validity on entry to the relevant functions. If an invalid
183     UTF-8 string is passed, an error return is given. In some situations, you may
184     already know that your strings are valid, and therefore want to skip these
185     checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
186     at compile time or at run time, PCRE assumes that the pattern or subject it
187     is given (respectively) contains only valid UTF-8 codes. In this case, it does
188     not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
189     PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
190     may crash.
191 nigel 75 .P
192 nigel 87 2. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
193     UTF-8 character if the value is greater than 127.
194 nigel 75 .P
195 nigel 91 3. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
196     characters for values greater than \e177.
197     .P
198     4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
199 nigel 75 bytes, for example: \ex{100}{3}.
200     .P
201 nigel 91 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
202 nigel 75 .P
203 nigel 91 6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
204 nigel 77 but its use can lead to some strange effects. This facility is not available in
205     the alternative matching function, \fBpcre_dfa_exec()\fP.
206 nigel 75 .P
207 nigel 91 7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
208 nigel 63 test characters of any code value, but the characters that PCRE recognizes as
209     digits, spaces, or word characters remain the same set as before, all with
210 nigel 75 values less than 256. This remains true even when PCRE includes Unicode
211     property support, because to do otherwise would slow down PCRE in many common
212     cases. If you really want to test for a wider sense of, say, "digit", you
213     must use Unicode property tests such as \ep{Nd}.
214     .P
215 nigel 91 8. Similarly, characters that match the POSIX named character classes are all
216 nigel 75 low-valued characters.
217     .P
218 nigel 91 9. Case-insensitive matching applies only to characters whose values are less
219 nigel 75 than 128, unless PCRE is built with Unicode property support. Even when Unicode
220     property support is available, PCRE still uses its own character tables when
221     checking the case of low-valued characters, so as not to degrade performance.
222     The Unicode property information is used only for characters with higher
223 nigel 87 values. Even when Unicode property support is available, PCRE supports
224     case-insensitive matching only when there is a one-to-one mapping between a
225     letter's cases. There are a small number of many-to-one mappings in Unicode;
226     these are not supported by PCRE.
227 nigel 75 .
228 nigel 41 .SH AUTHOR
229 nigel 63 .rs
230     .sp
231 nigel 77 Philip Hazel
232 nigel 41 .br
233     University Computing Service,
234     .br
235     Cambridge CB2 3QG, England.
236 nigel 77 .P
237     Putting an actual email address here seems to have been a spam magnet, so I've
238     taken it away. If you want to email me, use my initial and surname, separated
239     by a dot, at the domain ucs.cam.ac.uk.
240 nigel 75 .sp
241 nigel 63 .in 0
242 nigel 91 Last updated: 05 June 2006
243 nigel 41 .br
244 nigel 87 Copyright (c) 1997-2006 University of Cambridge.

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