1PCRE(3) Library Functions Manual PCRE(3)
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6 PCRE - Perl-compatible regular expressions
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9 The PCRE library is a set of functions that implement regular expres‐
11 sion pattern matching using the same syntax and semantics as Perl, with
12 just a few differences. Certain features that appeared in Python and
13 PCRE before they appeared in Perl are also available using the Python
14 syntax. There is also some support for certain .NET and Oniguruma syn‐
15 tax items, and there is an option for requesting some minor changes
16 that give better JavaScript compatibility.
17 The current implementation of PCRE (release 7.x) corresponds approxi‐
19 mately with Perl 5.10, including support for UTF-8 encoded strings and
20 Unicode general category properties. However, UTF-8 and Unicode support
21 has to be explicitly enabled; it is not the default. The Unicode tables
22 correspond to Unicode release 5.0.0.
23 In addition to the Perl-compatible matching function, PCRE contains an
25 alternative matching function that matches the same compiled patterns
26 in a different way. In certain circumstances, the alternative function
27 has some advantages. For a discussion of the two matching algorithms,
28 see the pcrematching page.
29 PCRE is written in C and released as a C library. A number of people
31 have written wrappers and interfaces of various kinds. In particular,
32 Google Inc. have provided a comprehensive C++ wrapper. This is now
33 included as part of the PCRE distribution. The pcrecpp page has details
34 of this interface. Other people's contributions can be found in the
35 Contrib directory at the primary FTP site, which is:
36 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
38 Details of exactly which Perl regular expression features are and are
40 not supported by PCRE are given in separate documents. See the pcrepat‐
41 tern and pcrecompat pages. There is a syntax summary in the pcresyntax
42 page.
43 Some features of PCRE can be included, excluded, or changed when the
45 library is built. The pcre_config() function makes it possible for a
46 client to discover which features are available. The features them‐
47 selves are described in the pcrebuild page. Documentation about build‐
48 ing PCRE for various operating systems can be found in the README file
49 in the source distribution.
50 The library contains a number of undocumented internal functions and
52 data tables that are used by more than one of the exported external
53 functions, but which are not intended for use by external callers.
54 Their names all begin with "_pcre_", which hopefully will not provoke
55 any name clashes. In some environments, it is possible to control which
56 external symbols are exported when a shared library is built, and in
57 these cases the undocumented symbols are not exported.
58
60 The user documentation for PCRE comprises a number of different sec‐
62 tions. In the "man" format, each of these is a separate "man page". In
63 the HTML format, each is a separate page, linked from the index page.
64 In the plain text format, all the sections are concatenated, for ease
65 of searching. The sections are as follows:
66 pcre this document
68 pcre-config show PCRE installation configuration information
69 pcreapi details of PCRE's native C API
70 pcrebuild options for building PCRE
71 pcrecallout details of the callout feature
72 pcrecompat discussion of Perl compatibility
73 pcrecpp details of the C++ wrapper
74 pcregrep description of the pcregrep command
75 pcrematching discussion of the two matching algorithms
76 pcrepartial details of the partial matching facility
77 pcrepattern syntax and semantics of supported
78 regular expressions
79 pcresyntax quick syntax reference
80 pcreperform discussion of performance issues
81 pcreposix the POSIX-compatible C API
82 pcreprecompile details of saving and re-using precompiled patterns
83 pcresample discussion of the sample program
84 pcrestack discussion of stack usage
85 pcretest description of the pcretest testing command
86 In addition, in the "man" and HTML formats, there is a short page for
88 each C library function, listing its arguments and results.
89
91 There are some size limitations in PCRE but it is hoped that they will
93 never in practice be relevant.
94 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
96 is compiled with the default internal linkage size of 2. If you want to
97 process regular expressions that are truly enormous, you can compile
98 PCRE with an internal linkage size of 3 or 4 (see the README file in
99 the source distribution and the pcrebuild documentation for details).
100 In these cases the limit is substantially larger. However, the speed
101 of execution is slower.
102 All values in repeating quantifiers must be less than 65536.
104 There is no limit to the number of parenthesized subpatterns, but there
106 can be no more than 65535 capturing subpatterns.
107 The maximum length of name for a named subpattern is 32 characters, and
109 the maximum number of named subpatterns is 10000.
110 The maximum length of a subject string is the largest positive number
112 that an integer variable can hold. However, when using the traditional
113 matching function, PCRE uses recursion to handle subpatterns and indef‐
114 inite repetition. This means that the available stack space may limit
115 the size of a subject string that can be processed by certain patterns.
116 For a discussion of stack issues, see the pcrestack documentation.
117
119 From release 3.3, PCRE has had some support for character strings
121 encoded in the UTF-8 format. For release 4.0 this was greatly extended
122 to cover most common requirements, and in release 5.0 additional sup‐
123 port for Unicode general category properties was added.
124 In order process UTF-8 strings, you must build PCRE to include UTF-8
126 support in the code, and, in addition, you must call pcre_compile()
127 with the PCRE_UTF8 option flag. When you do this, both the pattern and
128 any subject strings that are matched against it are treated as UTF-8
129 strings instead of just strings of bytes.
130 If you compile PCRE with UTF-8 support, but do not use it at run time,
132 the library will be a bit bigger, but the additional run time overhead
133 is limited to testing the PCRE_UTF8 flag occasionally, so should not be
134 very big.
135 If PCRE is built with Unicode character property support (which implies
137 UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup‐
138 ported. The available properties that can be tested are limited to the
139 general category properties such as Lu for an upper case letter or Nd
140 for a decimal number, the Unicode script names such as Arabic or Han,
141 and the derived properties Any and L&. A full list is given in the
142 pcrepattern documentation. Only the short names for properties are sup‐
143 ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let‐
144 ter}, is not supported. Furthermore, in Perl, many properties may
145 optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
146 does not support this.
147 Validity of UTF-8 strings
149 When you set the PCRE_UTF8 flag, the strings passed as patterns and
151 subjects are (by default) checked for validity on entry to the relevant
152 functions. From release 7.3 of PCRE, the check is according the rules
153 of RFC 3629, which are themselves derived from the Unicode specifica‐
154 tion. Earlier releases of PCRE followed the rules of RFC 2279, which
155 allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current
156 check allows only values in the range U+0 to U+10FFFF, excluding U+D800
157 to U+DFFF.
158 The excluded code points are the "Low Surrogate Area" of Unicode, of
160 which the Unicode Standard says this: "The Low Surrogate Area does not
161 contain any character assignments, consequently no character code
162 charts or namelists are provided for this area. Surrogates are reserved
163 for use with UTF-16 and then must be used in pairs." The code points
164 that are encoded by UTF-16 pairs are available as independent code
165 points in the UTF-8 encoding. (In other words, the whole surrogate
166 thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
167 If an invalid UTF-8 string is passed to PCRE, an error return is given.
169 At compile time, the only additional information is the offset to the
170 first byte of the failing character. The runtime functions (pcre_exec()
171 and pcre_dfa_exec()), pass back this information as well as a more
172 detailed reason code if the caller has provided memory in which to do
173 this.
174 In some situations, you may already know that your strings are valid,
176 and therefore want to skip these checks in order to improve perfor‐
177 mance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or at run
178 time, PCRE assumes that the pattern or subject it is given (respec‐
179 tively) contains only valid UTF-8 codes. In this case, it does not
180 diagnose an invalid UTF-8 string.
181 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
183 what happens depends on why the string is invalid. If the string con‐
184 forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
185 string of characters in the range 0 to 0x7FFFFFFF. In other words,
186 apart from the initial validity test, PCRE (when in UTF-8 mode) handles
187 strings according to the more liberal rules of RFC 2279. However, if
188 the string does not even conform to RFC 2279, the result is undefined.
189 Your program may crash.
190 If you want to process strings of values in the full range 0 to
192 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
193 set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
194 this situation, you will have to apply your own validity check.
195 General comments about UTF-8 mode
197 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
199 two-byte UTF-8 character if the value is greater than 127.
200 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
202 characters for values greater than \177.
203 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi‐
205 vidual bytes, for example: \x{100}{3}.
206 4. The dot metacharacter matches one UTF-8 character instead of a sin‐
208 gle byte.
209 5. The escape sequence \C can be used to match a single byte in UTF-8
211 mode, but its use can lead to some strange effects. This facility is
212 not available in the alternative matching function, pcre_dfa_exec().
213 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
215 test characters of any code value, but the characters that PCRE recog‐
216 nizes as digits, spaces, or word characters remain the same set as
217 before, all with values less than 256. This remains true even when PCRE
218 includes Unicode property support, because to do otherwise would slow
219 down PCRE in many common cases. If you really want to test for a wider
220 sense of, say, "digit", you must use Unicode property tests such as
221 \p{Nd}.
222 7. Similarly, characters that match the POSIX named character classes
224 are all low-valued characters.
225 8. However, the Perl 5.10 horizontal and vertical white space matching
227 escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char‐
228 acters.
229 9. Case-insensitive matching applies only to characters whose values
231 are less than 128, unless PCRE is built with Unicode property support.
232 Even when Unicode property support is available, PCRE still uses its
233 own character tables when checking the case of low-valued characters,
234 so as not to degrade performance. The Unicode property information is
235 used only for characters with higher values. Even when Unicode property
236 support is available, PCRE supports case-insensitive matching only when
237 there is a one-to-one mapping between a letter's cases. There are a
238 small number of many-to-one mappings in Unicode; these are not sup‐
239 ported by PCRE.
240
242 Philip Hazel
244 University Computing Service
245 Cambridge CB2 3QH, England.
246 Putting an actual email address here seems to have been a spam magnet,
248 so I've taken it away. If you want to email me, use my two initials,
249 followed by the two digits 10, at the domain cam.ac.uk.
250
252 Last updated: 12 April 2008
254 Copyright (c) 1997-2011 University of Cambridge.
255 PCRE(3)