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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 507 by ph10, Wed Mar 10 16:08:01 2010 UTC
# Line 18  man page, in case the conversion went wr Line 18  man page, in case the conversion went wr
18  <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>  <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
19  <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>  <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
20  <li><a name="TOC5" href="#SEC5">AUTHOR</a>  <li><a name="TOC5" href="#SEC5">AUTHOR</a>
21    <li><a name="TOC6" href="#SEC6">REVISION</a>
22  </ul>  </ul>
23  <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>  <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
24  <P>  <P>
25  The PCRE library is a set of functions that implement regular expression  The PCRE library is a set of functions that implement regular expression
26  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
27  differences. The current implementation of PCRE (release 6.x) corresponds  differences. Some features that appeared in Python and PCRE before they
28  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 some
29  Unicode general category properties. However, this support has to be explicitly  support for one or two .NET and Oniguruma syntax items, and there is an option
30  enabled; it is not the default.  for requesting some minor changes that give better JavaScript compatibility.
31  </P>  </P>
32  <P>  <P>
33  In addition to the Perl-compatible matching function, PCRE also contains an  The current implementation of PCRE corresponds approximately with Perl 5.10,
34  alternative matching function that matches the same compiled patterns in a  including support for UTF-8 encoded strings and Unicode general category
35  different way. In certain circumstances, the alternative function has some  properties. However, UTF-8 and Unicode support has to be explicitly enabled; it
36  advantages. For a discussion of the two matching algorithms, see the  is not the default. The Unicode tables correspond to Unicode release 5.2.0.
37    </P>
38    <P>
39    In addition to the Perl-compatible matching function, PCRE contains an
40    alternative function that matches the same compiled patterns in a different
41    way. In certain circumstances, the alternative function has some advantages.
42    For a discussion of the two matching algorithms, see the
43  <a href="pcrematching.html"><b>pcrematching</b></a>  <a href="pcrematching.html"><b>pcrematching</b></a>
44  page.  page.
45  </P>  </P>
# Line 52  supported by PCRE are given in separate Line 59  supported by PCRE are given in separate
59  <a href="pcrepattern.html"><b>pcrepattern</b></a>  <a href="pcrepattern.html"><b>pcrepattern</b></a>
60  and  and
61  <a href="pcrecompat.html"><b>pcrecompat</b></a>  <a href="pcrecompat.html"><b>pcrecompat</b></a>
62  pages.  pages. There is a syntax summary in the
63    <a href="pcresyntax.html"><b>pcresyntax</b></a>
64    page.
65  </P>  </P>
66  <P>  <P>
67  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
# Line 62  function makes it possible for a client Line 71  function makes it possible for a client
71  available. The features themselves are described in the  available. The features themselves are described in the
72  <a href="pcrebuild.html"><b>pcrebuild</b></a>  <a href="pcrebuild.html"><b>pcrebuild</b></a>
73  page. Documentation about building PCRE for various operating systems can be  page. Documentation about building PCRE for various operating systems can be
74  found in the <b>README</b> file in the source distribution.  found in the <b>README</b> and <b>NON-UNIX-USE</b> files in the source
75    distribution.
76  </P>  </P>
77  <P>  <P>
78  The library contains a number of undocumented internal functions and data  The library contains a number of undocumented internal functions and data
# Line 78  not exported. Line 88  not exported.
88  The user documentation for PCRE comprises a number of different sections. In  The user documentation for PCRE comprises a number of different sections. In
89  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,
90  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,
91  all the sections are concatenated, for ease of searching. The sections are as  all the sections, except the <b>pcredemo</b> section, are concatenated, for ease
92  follows:  of searching. The sections are as follows:
93  <pre>  <pre>
94    pcre              this document    pcre              this document
95      pcre-config       show PCRE installation configuration information
96    pcreapi           details of PCRE's native C API    pcreapi           details of PCRE's native C API
97    pcrebuild         options for building PCRE    pcrebuild         options for building PCRE
98    pcrecallout       details of the callout feature    pcrecallout       details of the callout feature
99    pcrecompat        discussion of Perl compatibility    pcrecompat        discussion of Perl compatibility
100    pcrecpp           details of the C++ wrapper    pcrecpp           details of the C++ wrapper
101      pcredemo          a demonstration C program that uses PCRE
102    pcregrep          description of the <b>pcregrep</b> command    pcregrep          description of the <b>pcregrep</b> command
103    pcrematching      discussion of the two matching algorithms    pcrematching      discussion of the two matching algorithms
104    pcrepartial       details of the partial matching facility    pcrepartial       details of the partial matching facility
# Line 94  follows: Line 106  follows:
106    pcreperform       discussion of performance issues    pcreperform       discussion of performance issues
107    pcreposix         the POSIX-compatible C API    pcreposix         the POSIX-compatible C API
108    pcreprecompile    details of saving and re-using precompiled patterns    pcreprecompile    details of saving and re-using precompiled patterns
109    pcresample        discussion of the sample program    pcresample        discussion of the pcredemo program
110    pcrestack         discussion of stack usage    pcrestack         discussion of stack usage
111      pcresyntax        quick syntax reference
112    pcretest          description of the <b>pcretest</b> testing command    pcretest          description of the <b>pcretest</b> testing command
113  </pre>  </pre>
114  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
# Line 114  internal linkage size of 3 or 4 (see the Line 127  internal linkage size of 3 or 4 (see the
127  distribution and the  distribution and the
128  <a href="pcrebuild.html"><b>pcrebuild</b></a>  <a href="pcrebuild.html"><b>pcrebuild</b></a>
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  <P>  <P>
133  All values in repeating quantifiers must be less than 65536. The maximum  All values in repeating quantifiers must be less than 65536.
 compiled length of subpattern with an explicit repeat count is 30000 bytes. The  
 maximum number of capturing subpatterns is 65535.  
134  </P>  </P>
135  <P>  <P>
136  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
137  depth of nesting of all kinds of parenthesized subpattern, including capturing  no more than 65535 capturing subpatterns.
 subpatterns, assertions, and other types of subpattern, is 200.  
138  </P>  </P>
139  <P>  <P>
140  The maximum length of name for a named subpattern is 32, and the maximum number  The maximum length of name for a named subpattern is 32 characters, and the
141  of named subpatterns is 10000.  maximum number of named subpatterns is 10000.
142  </P>  </P>
143  <P>  <P>
144  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
# Line 151  category properties was added. Line 161  category properties was added.
161  In order process UTF-8 strings, you must build PCRE to include UTF-8 support in  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  the code, and, in addition, you must call
163  <a href="pcre_compile.html"><b>pcre_compile()</b></a>  <a href="pcre_compile.html"><b>pcre_compile()</b></a>
164  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
165  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
166  instead of just strings of bytes.  strings that are matched against it are treated as UTF-8 strings instead of
167    strings of 1-byte characters.
168  </P>  </P>
169  <P>  <P>
170  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
171  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
172  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.
173  </P>  </P>
174  <P>  <P>
175  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
# Line 172  documentation. Only the short names for Line 183  documentation. Only the short names for
183  \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.  \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
184  Furthermore, in Perl, many properties may optionally be prefixed by "Is", for  Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
185  compatibility with Perl 5.6. PCRE does not support this.  compatibility with Perl 5.6. PCRE does not support this.
186  </P>  <a name="utf8strings"></a></P>
187  <P>  <br><b>
188  The following comments apply when PCRE is running in UTF-8 mode:  Validity of UTF-8 strings
189  </P>  </b><br>
190  <P>  <P>
191  1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects  When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
192  are checked for validity on entry to the relevant functions. If an invalid  are (by default) checked for validity on entry to the relevant functions. From
193  UTF-8 string is passed, an error return is given. In some situations, you may  release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
194  already know that your strings are valid, and therefore want to skip these  themselves derived from the Unicode specification. Earlier releases of PCRE
195  checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag  followed the rules of RFC 2279, which allows the full range of 31-bit values (0
196  at compile time or at run time, PCRE assumes that the pattern or subject it  to 0x7FFFFFFF). The current check allows only values in the range U+0 to
197  is given (respectively) contains only valid UTF-8 codes. In this case, it does  U+10FFFF, excluding U+D800 to U+DFFF.
198  not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to  </P>
199  PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program  <P>
200  may crash.  The excluded code points are the "Low Surrogate Area" of Unicode, of which the
201  </P>  Unicode Standard says this: "The Low Surrogate Area does not contain any
202    character assignments, consequently no character code charts or namelists are
203    provided for this area. Surrogates are reserved for use with UTF-16 and then
204    must be used in pairs." The code points that are encoded by UTF-16 pairs are
205    available as independent code points in the UTF-8 encoding. (In other words,
206    the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
207    UTF-8.)
208    </P>
209    <P>
210    If an invalid UTF-8 string is passed to PCRE, an error return
211    (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
212    your strings are valid, and therefore want to skip these checks in order to
213    improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
214    at run time, PCRE assumes that the pattern or subject it is given
215    (respectively) contains only valid UTF-8 codes. In this case, it does not
216    diagnose an invalid UTF-8 string.
217    </P>
218    <P>
219    If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
220    happens depends on why the string is invalid. If the string conforms to the
221    "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
222    in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
223    test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
224    rules of RFC 2279. However, if the string does not even conform to RFC 2279,
225    the result is undefined. Your program may crash.
226    </P>
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    </P>
233    <br><b>
234    General comments about UTF-8 mode
235    </b><br>
236  <P>  <P>
237  2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte  1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
238  UTF-8 character if the value is greater than 127.  UTF-8 character if the value is greater than 127.
239  </P>  </P>
240  <P>  <P>
241  3. Octal numbers up to \777 are recognized, and match two-byte UTF-8  2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
242  characters for values greater than \177.  characters for values greater than \177.
243  </P>  </P>
244  <P>  <P>
245  4. Repeat quantifiers apply to complete UTF-8 characters, not to individual  3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
246  bytes, for example: \x{100}{3}.  bytes, for example: \x{100}{3}.
247  </P>  </P>
248  <P>  <P>
249  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.
250  </P>  </P>
251  <P>  <P>
252  6. The escape sequence \C can be used to match a single byte in UTF-8 mode,  5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
253  but its use can lead to some strange effects. This facility is not available in  but its use can lead to some strange effects. This facility is not available in
254  the alternative matching function, <b>pcre_dfa_exec()</b>.  the alternative matching function, <b>pcre_dfa_exec()</b>.
255  </P>  </P>
256  <P>  <P>
257  7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly  6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
258  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
259  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
260  values less than 256. This remains true even when PCRE includes Unicode  values less than 256. This remains true even when PCRE includes Unicode
261  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
262  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
263  must use Unicode property tests such as \p{Nd}.  must use Unicode property tests such as \p{Nd}. Note that this also applies to
264    \b, because it is defined in terms of \w and \W.
265  </P>  </P>
266  <P>  <P>
267  8. Similarly, characters that match the POSIX named character classes are all  7. Similarly, characters that match the POSIX named character classes are all
268  low-valued characters.  low-valued characters.
269  </P>  </P>
270  <P>  <P>
271    8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
272    (\h, \H, \v, and \V) do match all the appropriate Unicode characters.
273    </P>
274    <P>
275  9. Case-insensitive matching applies only to characters whose values are less  9. Case-insensitive matching applies only to characters whose values are less
276  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
277  property support is available, PCRE still uses its own character tables when  property support is available, PCRE still uses its own character tables when
# Line 236  these are not supported by PCRE. Line 286  these are not supported by PCRE.
286  <P>  <P>
287  Philip Hazel  Philip Hazel
288  <br>  <br>
289  University Computing Service,  University Computing Service
290    <br>
291    Cambridge CB2 3QH, England.
292  <br>  <br>
 Cambridge CB2 3QG, England.  
293  </P>  </P>
294  <P>  <P>
295  Putting an actual email address here seems to have been a spam magnet, so I've  Putting an actual email address here seems to have been a spam magnet, so I've
296  taken it away. If you want to email me, use my initial and surname, separated  taken it away. If you want to email me, use my two initials, followed by the
297  by a dot, at the domain ucs.cam.ac.uk.  two digits 10, at the domain cam.ac.uk.
298  Last updated: 05 June 2006  </P>
299    <br><a name="SEC6" href="#TOC1">REVISION</a><br>
300    <P>
301    Last updated: 01 March 2010
302    <br>
303    Copyright &copy; 1997-2010 University of Cambridge.
304  <br>  <br>
 Copyright &copy; 1997-2006 University of Cambridge.  
305  <p>  <p>
306  Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.
307  </p>  </p>

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