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revision 75 by nigel, Sat Feb 24 21:40:37 2007 UTC revision 429 by ph10, Tue Sep 1 16:10:16 2009 UTC
# Line 6  PCRE - Perl-compatible regular expressio Line 6  PCRE - Perl-compatible regular expressio
6  .sp  .sp
7  The PCRE library is a set of functions that implement regular expression  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  pattern matching using the same syntax and semantics as Perl, with just a few
9  differences. The current implementation of PCRE (release 5.x) corresponds  differences. Certain features that appeared in Python and PCRE before they
10  approximately with Perl 5.8, including support for UTF-8 encoded strings and  appeared in Perl are also available using the Python syntax. There is also some
11  Unicode general category properties. However, this support has to be explicitly  support for certain .NET and Oniguruma syntax items, and there is an option for
12  enabled; it is not the default.  requesting some minor changes that give better JavaScript compatibility.
13    .P
14    The current implementation of PCRE (release 8.xx) corresponds approximately
15    with Perl 5.10, including support for UTF-8 encoded strings and Unicode general
16    category properties. However, UTF-8 and Unicode support has to be explicitly
17    enabled; it is not the default. The Unicode tables correspond to Unicode
18    release 5.1.
19    .P
20    In addition to the Perl-compatible matching function, PCRE contains an
21    alternative matching function that matches the same compiled patterns in a
22    different way. In certain circumstances, the alternative function has some
23    advantages. For a discussion of the two matching algorithms, see the
24    .\" HREF
25    \fBpcrematching\fP
26    .\"
27    page.
28  .P  .P
29  PCRE is written in C and released as a C library. A number of people have  PCRE is written in C and released as a C library. A number of people have
30  written wrappers and interfaces of various kinds. A C++ class is included in  written wrappers and interfaces of various kinds. In particular, Google Inc.
31  these contributions, which can be found in the \fIContrib\fR directory at the  have provided a comprehensive C++ wrapper. This is now included as part of the
32  primary FTP site, which is:  PCRE distribution. The
33    .\" HREF
34    \fBpcrecpp\fP
35    .\"
36    page has details of this interface. Other people's contributions can be found
37    in the \fIContrib\fR directory at the primary FTP site, which is:
38  .sp  .sp
39  .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">  .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
40  .\" </a>  .\" </a>
# Line 29  and Line 49  and
49  .\" HREF  .\" HREF
50  \fBpcrecompat\fR  \fBpcrecompat\fR
51  .\"  .\"
52  pages.  pages. There is a syntax summary in the
53    .\" HREF
54    \fBpcresyntax\fR
55    .\"
56    page.
57  .P  .P
58  Some features of PCRE can be included, excluded, or changed when the library is  Some features of PCRE can be included, excluded, or changed when the library is
59  built. The  built. The
# Line 43  available. The features themselves are d Line 67  available. The features themselves are d
67  .\"  .\"
68  page. Documentation about building PCRE for various operating systems can be  page. Documentation about building PCRE for various operating systems can be
69  found in the \fBREADME\fP file in the source distribution.  found in the \fBREADME\fP file in the source distribution.
70    .P
71    The library contains a number of undocumented internal functions and data
72    tables that are used by more than one of the exported external functions, but
73    which are not intended for use by external callers. Their names all begin with
74    "_pcre_", which hopefully will not provoke any name clashes. In some
75    environments, it is possible to control which external symbols are exported
76    when a shared library is built, and in these cases the undocumented symbols are
77    not exported.
78  .  .
79  .  .
80  .SH "USER DOCUMENTATION"  .SH "USER DOCUMENTATION"
# Line 51  found in the \fBREADME\fP file in the so Line 83  found in the \fBREADME\fP file in the so
83  The user documentation for PCRE comprises a number of different sections. In  The user documentation for PCRE comprises a number of different sections. In
84  the "man" format, each of these is a separate "man page". In the HTML format,  the "man" format, each of these is a separate "man page". In the HTML format,
85  each is a separate page, linked from the index page. In the plain text format,  each is a separate page, linked from the index page. In the plain text format,
86  all the sections are concatenated, for ease of searching. The sections are as  all the sections, except the \fBpcredemo\fP section, are concatenated, for ease
87  follows:  of searching. The sections are as follows:
88  .sp  .sp
89    pcre              this document    pcre              this document
90    pcreapi           details of PCRE's native API    pcre-config       show PCRE installation configuration information
91      pcreapi           details of PCRE's native C API
92    pcrebuild         options for building PCRE    pcrebuild         options for building PCRE
93    pcrecallout       details of the callout feature    pcrecallout       details of the callout feature
94    pcrecompat        discussion of Perl compatibility    pcrecompat        discussion of Perl compatibility
95      pcrecpp           details of the C++ wrapper
96      pcredemo          a demonstration C program that uses PCRE
97    pcregrep          description of the \fBpcregrep\fP command    pcregrep          description of the \fBpcregrep\fP command
98      pcrematching      discussion of the two matching algorithms
99    pcrepartial       details of the partial matching facility    pcrepartial       details of the partial matching facility
100  .\" JOIN  .\" JOIN
101    pcrepattern       syntax and semantics of supported    pcrepattern       syntax and semantics of supported
102                        regular expressions                        regular expressions
103      pcresyntax        quick syntax reference
104    pcreperform       discussion of performance issues    pcreperform       discussion of performance issues
105    pcreposix         the POSIX-compatible API    pcreposix         the POSIX-compatible C API
106    pcreprecompile    details of saving and re-using precompiled patterns    pcreprecompile    details of saving and re-using precompiled patterns
107    pcresample        discussion of the sample program    pcresample        discussion of the pcredemo program
108      pcrestack         discussion of stack usage
109    pcretest          description of the \fBpcretest\fP testing command    pcretest          description of the \fBpcretest\fP testing command
110  .sp  .sp
111  In addition, in the "man" and HTML formats, there is a short page for each  In addition, in the "man" and HTML formats, there is a short page for each
112  library function, listing its arguments and results.  C library function, listing its arguments and results.
113  .  .
114  .  .
115  .SH LIMITATIONS  .SH LIMITATIONS
# Line 89  distribution and the Line 127  distribution and the
127  \fBpcrebuild\fP  \fBpcrebuild\fP
128  .\"  .\"
129  documentation for details). In these cases the limit is substantially larger.  documentation for details). In these cases the limit is substantially larger.
130  However, the speed of execution will be slower.  However, the speed of execution is slower.
131  .P  .P
132  All values in repeating quantifiers must be less than 65536.  All values in repeating quantifiers must be less than 65536.
 The maximum number of capturing subpatterns is 65535.  
133  .P  .P
134  There is no limit to the number of non-capturing subpatterns, but the maximum  There is no limit to the number of parenthesized subpatterns, but there can be
135  depth of nesting of all kinds of parenthesized subpattern, including capturing  no more than 65535 capturing subpatterns.
136  subpatterns, assertions, and other types of subpattern, is 200.  .P
137    The maximum length of name for a named subpattern is 32 characters, and the
138    maximum number of named subpatterns is 10000.
139  .P  .P
140  The maximum length of a subject string is the largest positive number that an  The maximum length of a subject string is the largest positive number that an
141  integer variable can hold. However, PCRE uses recursion to handle subpatterns  integer variable can hold. However, when using the traditional matching
142  and indefinite repetition. This means that the available stack space may limit  function, PCRE uses recursion to handle subpatterns and indefinite repetition.
143  the size of a subject string that can be processed by certain patterns.  This means that the available stack space may limit the size of a subject
144  .sp  string that can be processed by certain patterns. For a discussion of stack
145    issues, see the
146    .\" HREF
147    \fBpcrestack\fP
148    .\"
149    documentation.
150    .
151  .\" HTML <a name="utf8support"></a>  .\" HTML <a name="utf8support"></a>
152  .  .
153  .  .
# Line 119  the code, and, in addition, you must cal Line 164  the code, and, in addition, you must cal
164  .\" HREF  .\" HREF
165  \fBpcre_compile()\fP  \fBpcre_compile()\fP
166  .\"  .\"
167  with the PCRE_UTF8 option flag. When you do this, both the pattern and any  with the PCRE_UTF8 option flag, or the pattern must start with the sequence
168  subject strings that are matched against it are treated as UTF-8 strings  (*UTF8). When either of these is the case, both the pattern and any subject
169  instead of just strings of bytes.  strings that are matched against it are treated as UTF-8 strings instead of
170    just strings of bytes.
171  .P  .P
172  If you compile PCRE with UTF-8 support, but do not use it at run time, the  If you compile PCRE with UTF-8 support, but do not use it at run time, the
173  library will be a bit bigger, but the additional run time overhead is limited  library will be a bit bigger, but the additional run time overhead is limited
174  to testing the PCRE_UTF8 flag in several places, so should not be very large.  to testing the PCRE_UTF8 flag occasionally, so should not be very big.
175  .P  .P
176  If PCRE is built with Unicode character property support (which implies UTF-8  If PCRE is built with Unicode character property support (which implies UTF-8
177  support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.  support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
178  The available properties that can be tested are limited to the general  The available properties that can be tested are limited to the general
179  category properties such as Lu for an upper case letter or Nd for a decimal  category properties such as Lu for an upper case letter or Nd for a decimal
180  number. A full list is given in the  number, the Unicode script names such as Arabic or Han, and the derived
181    properties Any and L&. A full list is given in the
182  .\" HREF  .\" HREF
183  \fBpcrepattern\fP  \fBpcrepattern\fP
184  .\"  .\"
185  documentation. The PCRE library is increased in size by about 90K when Unicode  documentation. Only the short names for properties are supported. For example,
186  property support is included.  \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
187  .P  Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
188  The following comments apply when PCRE is running in UTF-8 mode:  compatibility with Perl 5.6. PCRE does not support this.
189  .P  .
190  1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects  .\" HTML <a name="utf8strings"></a>
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  .SS "Validity of UTF-8 strings"
193  already know that your strings are valid, and therefore want to skip these  .rs
194  checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag  .sp
195  at compile time or at run time, PCRE assumes that the pattern or subject it  When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
196  is given (respectively) contains only valid UTF-8 codes. In this case, it does  are (by default) checked for validity on entry to the relevant functions. From
197  not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to  release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
198  PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program  themselves derived from the Unicode specification. Earlier releases of PCRE
199  may crash.  followed the rules of RFC 2279, which allows the full range of 31-bit values (0
200  .P  to 0x7FFFFFFF). The current check allows only values in the range U+0 to
201  2. In a pattern, the escape sequence \ex{...}, where the contents of the braces  U+10FFFF, excluding U+D800 to U+DFFF.
202  is a string of hexadecimal digits, is interpreted as a UTF-8 character whose  .P
203  code number is the given hexadecimal number, for example: \ex{1234}. If a  The excluded code points are the "Low Surrogate Area" of Unicode, of which the
204  non-hexadecimal digit appears between the braces, the item is not recognized.  Unicode Standard says this: "The Low Surrogate Area does not contain any
205  This escape sequence can be used either as a literal, or within a character  character assignments, consequently no character code charts or namelists are
206  class.  provided for this area. Surrogates are reserved for use with UTF-16 and then
207    must be used in pairs." The code points that are encoded by UTF-16 pairs are
208    available as independent code points in the UTF-8 encoding. (In other words,
209    the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
210    UTF-8.)
211    .P
212    If an invalid UTF-8 string is passed to PCRE, an error return
213    (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
214    your strings are valid, and therefore want to skip these checks in order to
215    improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
216    at run time, PCRE assumes that the pattern or subject it is given
217    (respectively) contains only valid UTF-8 codes. In this case, it does not
218    diagnose an invalid UTF-8 string.
219    .P
220    If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
221    happens depends on why the string is invalid. If the string conforms to the
222    "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
223    in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
224    test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
225    rules of RFC 2279. However, if the string does not even conform to RFC 2279,
226    the result is undefined. Your program may crash.
227    .P
228    If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
229    encoded in a UTF-8-like manner as per the old RFC, you can set
230    PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
231    situation, you will have to apply your own validity check.
232    .
233    .SS "General comments about UTF-8 mode"
234    .rs
235    .sp
236    1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
237    UTF-8 character if the value is greater than 127.
238  .P  .P
239  3. The original hexadecimal escape sequence, \exhh, matches a two-byte UTF-8  2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
240  character if the value is greater than 127.  characters for values greater than \e177.
241  .P  .P
242  4. Repeat quantifiers apply to complete UTF-8 characters, not to individual  3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
243  bytes, for example: \ex{100}{3}.  bytes, for example: \ex{100}{3}.
244  .P  .P
245  5. The dot metacharacter matches one UTF-8 character instead of a single byte.  4. The dot metacharacter matches one UTF-8 character instead of a single byte.
246  .P  .P
247  6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,  5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
248  but its use can lead to some strange effects.  but its use can lead to some strange effects. This facility is not available in
249    the alternative matching function, \fBpcre_dfa_exec()\fP.
250  .P  .P
251  7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly  6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
252  test characters of any code value, but the characters that PCRE recognizes as  test characters of any code value, but the characters that PCRE recognizes as
253  digits, spaces, or word characters remain the same set as before, all with  digits, spaces, or word characters remain the same set as before, all with
254  values less than 256. This remains true even when PCRE includes Unicode  values less than 256. This remains true even when PCRE includes Unicode
255  property support, because to do otherwise would slow down PCRE in many common  property support, because to do otherwise would slow down PCRE in many common
256  cases. If you really want to test for a wider sense of, say, "digit", you  cases. If you really want to test for a wider sense of, say, "digit", you
257  must use Unicode property tests such as \ep{Nd}.  must use Unicode property tests such as \ep{Nd}. Note that this also applies to
258    \eb, because it is defined in terms of \ew and \eW.
259  .P  .P
260  8. Similarly, characters that match the POSIX named character classes are all  7. Similarly, characters that match the POSIX named character classes are all
261  low-valued characters.  low-valued characters.
262  .P  .P
263    8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
264    (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
265    .P
266  9. Case-insensitive matching applies only to characters whose values are less  9. Case-insensitive matching applies only to characters whose values are less
267  than 128, unless PCRE is built with Unicode property support. Even when Unicode  than 128, unless PCRE is built with Unicode property support. Even when Unicode
268  property support is available, PCRE still uses its own character tables when  property support is available, PCRE still uses its own character tables when
269  checking the case of low-valued characters, so as not to degrade performance.  checking the case of low-valued characters, so as not to degrade performance.
270  The Unicode property information is used only for characters with higher  The Unicode property information is used only for characters with higher
271  values.  values. Even when Unicode property support is available, PCRE supports
272    case-insensitive matching only when there is a one-to-one mapping between a
273    letter's cases. There are a small number of many-to-one mappings in Unicode;
274    these are not supported by PCRE.
275    .
276  .  .
277  .SH AUTHOR  .SH AUTHOR
278  .rs  .rs
279  .sp  .sp
280  Philip Hazel <ph10@cam.ac.uk>  .nf
281  .br  Philip Hazel
282  University Computing Service,  University Computing Service
283  .br  Cambridge CB2 3QH, England.
284  Cambridge CB2 3QG, England.  .fi
285  .br  .P
286  Phone: +44 1223 334714  Putting an actual email address here seems to have been a spam magnet, so I've
287  .sp  taken it away. If you want to email me, use my two initials, followed by the
288  .in 0  two digits 10, at the domain cam.ac.uk.
289  Last updated: 09 September 2004  .
290  .br  .
291  Copyright (c) 1997-2004 University of Cambridge.  .SH REVISION
292    .rs
293    .sp
294    .nf
295    Last updated: 01 September 2009
296    Copyright (c) 1997-2009 University of Cambridge.
297    .fi

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