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Mon Jul 30 11:13:54 2007 UTC (14 years, 3 months ago) by ph10
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Expand the documentation about the NULLWSLIMIT error.
1 .TH PCRE 3
3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 The PCRE library is a set of functions that implement regular expression
8 pattern matching using the same syntax and semantics as Perl, with just a few
9 differences. (Certain features that appeared in Python and PCRE before they
10 appeared in Perl are also available using the Python syntax.)
11 .P
12 The current implementation of PCRE (release 7.x) corresponds approximately with
13 Perl 5.10, including support for UTF-8 encoded strings and Unicode general
14 category properties. However, UTF-8 and Unicode support has to be explicitly
15 enabled; it is not the default. The Unicode tables correspond to Unicode
16 release 5.0.0.
17 .P
18 In addition to the Perl-compatible matching function, PCRE contains an
19 alternative matching function that matches the same compiled patterns in a
20 different way. In certain circumstances, the alternative function has some
21 advantages. For a discussion of the two matching algorithms, see the
22 .\" HREF
23 \fBpcrematching\fP
24 .\"
25 page.
26 .P
27 PCRE is written in C and released as a C library. A number of people have
28 written wrappers and interfaces of various kinds. In particular, Google Inc.
29 have provided a comprehensive C++ wrapper. This is now included as part of the
30 PCRE distribution. The
31 .\" HREF
32 \fBpcrecpp\fP
33 .\"
34 page has details of this interface. Other people's contributions can be found
35 in the \fIContrib\fR directory at the primary FTP site, which is:
36 .sp
37 .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
38 .\" </a>
39 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
40 .P
41 Details of exactly which Perl regular expression features are and are not
42 supported by PCRE are given in separate documents. See the
43 .\" HREF
44 \fBpcrepattern\fR
45 .\"
46 and
47 .\" HREF
48 \fBpcrecompat\fR
49 .\"
50 pages.
51 .P
52 Some features of PCRE can be included, excluded, or changed when the library is
53 built. The
54 .\" HREF
55 \fBpcre_config()\fR
56 .\"
57 function makes it possible for a client to discover which features are
58 available. The features themselves are described in the
59 .\" HREF
60 \fBpcrebuild\fP
61 .\"
62 page. Documentation about building PCRE for various operating systems can be
63 found in the \fBREADME\fP file in the source distribution.
64 .P
65 The library contains a number of undocumented internal functions and data
66 tables that are used by more than one of the exported external functions, but
67 which are not intended for use by external callers. Their names all begin with
68 "_pcre_", which hopefully will not provoke any name clashes. In some
69 environments, it is possible to control which external symbols are exported
70 when a shared library is built, and in these cases the undocumented symbols are
71 not exported.
72 .
73 .
75 .rs
76 .sp
77 The user documentation for PCRE comprises a number of different sections. In
78 the "man" format, each of these is a separate "man page". In the HTML format,
79 each is a separate page, linked from the index page. In the plain text format,
80 all the sections are concatenated, for ease of searching. The sections are as
81 follows:
82 .sp
83 pcre this document
84 pcre-config show PCRE installation configuration information
85 pcreapi details of PCRE's native C API
86 pcrebuild options for building PCRE
87 pcrecallout details of the callout feature
88 pcrecompat discussion of Perl compatibility
89 pcrecpp details of the C++ wrapper
90 pcregrep description of the \fBpcregrep\fP command
91 pcrematching discussion of the two matching algorithms
92 pcrepartial details of the partial matching facility
93 .\" JOIN
94 pcrepattern syntax and semantics of supported
95 regular expressions
96 pcreperform discussion of performance issues
97 pcreposix the POSIX-compatible C API
98 pcreprecompile details of saving and re-using precompiled patterns
99 pcresample discussion of the sample program
100 pcrestack discussion of stack usage
101 pcretest description of the \fBpcretest\fP testing command
102 .sp
103 In addition, in the "man" and HTML formats, there is a short page for each
104 C library function, listing its arguments and results.
105 .
106 .
108 .rs
109 .sp
110 There are some size limitations in PCRE but it is hoped that they will never in
111 practice be relevant.
112 .P
113 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
114 compiled with the default internal linkage size of 2. If you want to process
115 regular expressions that are truly enormous, you can compile PCRE with an
116 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
117 distribution and the
118 .\" HREF
119 \fBpcrebuild\fP
120 .\"
121 documentation for details). In these cases the limit is substantially larger.
122 However, the speed of execution is slower.
123 .P
124 All values in repeating quantifiers must be less than 65536. The maximum
125 compiled length of subpattern with an explicit repeat count is 30000 bytes. The
126 maximum number of capturing subpatterns is 65535.
127 .P
128 There is no limit to the number of parenthesized subpatterns, but there can be
129 no more than 65535 capturing subpatterns.
130 .P
131 If a non-capturing subpattern with an unlimited repetition quantifier can match
132 an empty string, there is a limit of 1000 on the number of times it can be
133 repeated while not matching an empty string - if it does match an empty
134 string, the loop is immediately broken.
135 .P
136 The maximum length of name for a named subpattern is 32 characters, and the
137 maximum number of named subpatterns is 10000.
138 .P
139 The maximum length of a subject string is the largest positive number that an
140 integer variable can hold. However, when using the traditional matching
141 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
142 This means that the available stack space may limit the size of a subject
143 string that can be processed by certain patterns. For a discussion of stack
144 issues, see the
145 .\" HREF
146 \fBpcrestack\fP
147 .\"
148 documentation.
149 .sp
150 .\" HTML <a name="utf8support"></a>
151 .
152 .
154 .rs
155 .sp
156 From release 3.3, PCRE has had some support for character strings encoded in
157 the UTF-8 format. For release 4.0 this was greatly extended to cover most
158 common requirements, and in release 5.0 additional support for Unicode general
159 category properties was added.
160 .P
161 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
162 the code, and, in addition, you must call
163 .\" HREF
164 \fBpcre_compile()\fP
165 .\"
166 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
167 subject strings that are matched against it are treated as UTF-8 strings
168 instead of just strings of bytes.
169 .P
170 If you compile PCRE with UTF-8 support, but do not use it at run time, the
171 library will be a bit bigger, but the additional run time overhead is limited
172 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
173 .P
174 If PCRE is built with Unicode character property support (which implies UTF-8
175 support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
176 The available properties that can be tested are limited to the general
177 category properties such as Lu for an upper case letter or Nd for a decimal
178 number, the Unicode script names such as Arabic or Han, and the derived
179 properties Any and L&. A full list is given in the
180 .\" HREF
181 \fBpcrepattern\fP
182 .\"
183 documentation. Only the short names for properties are supported. For example,
184 \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
185 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
186 compatibility with Perl 5.6. PCRE does not support this.
187 .P
188 The following comments apply when PCRE is running in UTF-8 mode:
189 .P
190 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
191 are checked for validity on entry to the relevant functions. If an invalid
192 UTF-8 string is passed, an error return is given. In some situations, you may
193 already know that your strings are valid, and therefore want to skip these
194 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
195 at compile time or at run time, PCRE assumes that the pattern or subject it
196 is given (respectively) contains only valid UTF-8 codes. In this case, it does
197 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
198 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
199 may crash.
200 .P
201 2. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
202 UTF-8 character if the value is greater than 127.
203 .P
204 3. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
205 characters for values greater than \e177.
206 .P
207 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
208 bytes, for example: \ex{100}{3}.
209 .P
210 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
211 .P
212 6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
213 but its use can lead to some strange effects. This facility is not available in
214 the alternative matching function, \fBpcre_dfa_exec()\fP.
215 .P
216 7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
217 test characters of any code value, but the characters that PCRE recognizes as
218 digits, spaces, or word characters remain the same set as before, all with
219 values less than 256. This remains true even when PCRE includes Unicode
220 property support, because to do otherwise would slow down PCRE in many common
221 cases. If you really want to test for a wider sense of, say, "digit", you
222 must use Unicode property tests such as \ep{Nd}.
223 .P
224 8. Similarly, characters that match the POSIX named character classes are all
225 low-valued characters.
226 .P
227 9. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
228 (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
229 .P
230 10. Case-insensitive matching applies only to characters whose values are less
231 than 128, unless PCRE is built with Unicode property support. Even when Unicode
232 property support is available, PCRE still uses its own character tables when
233 checking the case of low-valued characters, so as not to degrade performance.
234 The Unicode property information is used only for characters with higher
235 values. Even when Unicode property support is available, PCRE supports
236 case-insensitive matching only when there is a one-to-one mapping between a
237 letter's cases. There are a small number of many-to-one mappings in Unicode;
238 these are not supported by PCRE.
239 .
240 .
242 .rs
243 .sp
244 .nf
245 Philip Hazel
246 University Computing Service
247 Cambridge CB2 3QH, England.
248 .fi
249 .P
250 Putting an actual email address here seems to have been a spam magnet, so I've
251 taken it away. If you want to email me, use my two initials, followed by the
252 two digits 10, at the domain cam.ac.uk.
253 .
254 .
256 .rs
257 .sp
258 .nf
259 Last updated: 30 July 2007
260 Copyright (c) 1997-2007 University of Cambridge.
261 .fi


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