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# 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 5.x) corresponds  differences. Certain 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 also some
29  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
30  enabled; it is not the default.  requesting some minor changes that give better JavaScript compatibility.
31    </P>
32    <P>
33    The current implementation of PCRE (release 7.x) corresponds approximately with
34    Perl 5.10, including support for UTF-8 encoded strings and Unicode general
35    category properties. However, UTF-8 and Unicode support has to be explicitly
36    enabled; it is not the default. The Unicode tables correspond to Unicode
37    release 5.1.
38    </P>
39    <P>
40    In addition to the Perl-compatible matching function, PCRE contains an
41    alternative matching function that matches the same compiled patterns in a
42    different way. In certain circumstances, the alternative function has some
43    advantages. For a discussion of the two matching algorithms, see the
44    <a href="pcrematching.html"><b>pcrematching</b></a>
45    page.
46  </P>  </P>
47  <P>  <P>
48  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
49  written wrappers and interfaces of various kinds. A C++ class is included in  written wrappers and interfaces of various kinds. In particular, Google Inc.
50  these contributions, which can be found in the <i>Contrib</i> directory at the  have provided a comprehensive C++ wrapper. This is now included as part of the
51  primary FTP site, which is:  PCRE distribution. The
52    <a href="pcrecpp.html"><b>pcrecpp</b></a>
53    page has details of this interface. Other people's contributions can be found
54    in the <i>Contrib</i> directory at the primary FTP site, which is:
55  <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>  <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
56  </P>  </P>
57  <P>  <P>
# Line 41  supported by PCRE are given in separate Line 60  supported by PCRE are given in separate
60  <a href="pcrepattern.html"><b>pcrepattern</b></a>  <a href="pcrepattern.html"><b>pcrepattern</b></a>
61  and  and
62  <a href="pcrecompat.html"><b>pcrecompat</b></a>  <a href="pcrecompat.html"><b>pcrecompat</b></a>
63  pages.  pages. There is a syntax summary in the
64    <a href="pcresyntax.html"><b>pcresyntax</b></a>
65    page.
66  </P>  </P>
67  <P>  <P>
68  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 53  available. The features themselves are d Line 74  available. The features themselves are d
74  page. Documentation about building PCRE for various operating systems can be  page. Documentation about building PCRE for various operating systems can be
75  found in the <b>README</b> file in the source distribution.  found in the <b>README</b> file in the source distribution.
76  </P>  </P>
77    <P>
78    The library contains a number of undocumented internal functions and data
79    tables that are used by more than one of the exported external functions, but
80    which are not intended for use by external callers. Their names all begin with
81    "_pcre_", which hopefully will not provoke any name clashes. In some
82    environments, it is possible to control which external symbols are exported
83    when a shared library is built, and in these cases the undocumented symbols are
84    not exported.
85    </P>
86  <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>  <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
87  <P>  <P>
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
# Line 62  all the sections are concatenated, for e Line 92  all the sections are concatenated, for e
92  follows:  follows:
93  <pre>  <pre>
94    pcre              this document    pcre              this document
95    pcreapi           details of PCRE's native API    pcre-config       show PCRE installation configuration information
96      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
101    pcregrep          description of the <b>pcregrep</b> command    pcregrep          description of the <b>pcregrep</b> command
102      pcrematching      discussion of the two matching algorithms
103    pcrepartial       details of the partial matching facility    pcrepartial       details of the partial matching facility
104    pcrepattern       syntax and semantics of supported regular expressions    pcrepattern       syntax and semantics of supported regular expressions
105      pcresyntax        quick syntax reference
106    pcreperform       discussion of performance issues    pcreperform       discussion of performance issues
107    pcreposix         the POSIX-compatible 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 sample program
110      pcrestack         discussion of stack usage
111    pcretest          description of the <b>pcretest</b> testing command    pcretest          description of the <b>pcretest</b> testing command
112  </pre>  </pre>
113  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
114  library function, listing its arguments and results.  C library function, listing its arguments and results.
115  </P>  </P>
116  <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>  <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
117  <P>  <P>
# Line 91  internal linkage size of 3 or 4 (see the Line 126  internal linkage size of 3 or 4 (see the
126  distribution and the  distribution and the
127  <a href="pcrebuild.html"><b>pcrebuild</b></a>  <a href="pcrebuild.html"><b>pcrebuild</b></a>
128  documentation for details). In these cases the limit is substantially larger.  documentation for details). In these cases the limit is substantially larger.
129  However, the speed of execution will be slower.  However, the speed of execution is slower.
130  </P>  </P>
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  <P>  <P>
135  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
136  depth of nesting of all kinds of parenthesized subpattern, including capturing  no more than 65535 capturing subpatterns.
137  subpatterns, assertions, and other types of subpattern, is 200.  </P>
138    <P>
139    The maximum length of name for a named subpattern is 32 characters, and the
140    maximum number of named subpatterns is 10000.
141  </P>  </P>
142  <P>  <P>
143  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
144  integer variable can hold. However, PCRE uses recursion to handle subpatterns  integer variable can hold. However, when using the traditional matching
145  and indefinite repetition. This means that the available stack space may limit  function, PCRE uses recursion to handle subpatterns and indefinite repetition.
146  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
147    string that can be processed by certain patterns. For a discussion of stack
148    issues, see the
149    <a href="pcrestack.html"><b>pcrestack</b></a>
150    documentation.
151  <a name="utf8support"></a></P>  <a name="utf8support"></a></P>
152  <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>  <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
153  <P>  <P>
# Line 119  category properties was added. Line 160  category properties was added.
160  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
161  the code, and, in addition, you must call  the code, and, in addition, you must call
162  <a href="pcre_compile.html"><b>pcre_compile()</b></a>  <a href="pcre_compile.html"><b>pcre_compile()</b></a>
163  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
164  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
165  instead of just strings of bytes.  strings that are matched against it are treated as UTF-8 strings instead of
166    just strings of bytes.
167  </P>  </P>
168  <P>  <P>
169  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
170  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
171  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.
172  </P>  </P>
173  <P>  <P>
174  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
175  support), the escape sequences \p{..}, \P{..}, and \X are supported.  support), the escape sequences \p{..}, \P{..}, and \X are supported.
176  The available properties that can be tested are limited to the general  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  category properties such as Lu for an upper case letter or Nd for a decimal
178  number. A full list is given in the  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  <a href="pcrepattern.html"><b>pcrepattern</b></a>  <a href="pcrepattern.html"><b>pcrepattern</b></a>
181  documentation. The PCRE library is increased in size by about 90K when Unicode  documentation. Only the short names for properties are supported. For example,
182  property support is included.  \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
183    Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
184    compatibility with Perl 5.6. PCRE does not support this.
185    <a name="utf8strings"></a></P>
186    <br><b>
187    Validity of UTF-8 strings
188    </b><br>
189    <P>
190    When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
191    are (by default) checked for validity on entry to the relevant functions. From
192    release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
193    themselves derived from the Unicode specification. Earlier releases of PCRE
194    followed the rules of RFC 2279, which allows the full range of 31-bit values (0
195    to 0x7FFFFFFF). The current check allows only values in the range U+0 to
196    U+10FFFF, excluding U+D800 to U+DFFF.
197    </P>
198    <P>
199    The excluded code points are the "Low Surrogate Area" of Unicode, of which the
200    Unicode Standard says this: "The Low Surrogate Area does not contain any
201    character assignments, consequently no character code charts or namelists are
202    provided for this area. Surrogates are reserved for use with UTF-16 and then
203    must be used in pairs." The code points that are encoded by UTF-16 pairs are
204    available as independent code points in the UTF-8 encoding. (In other words,
205    the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
206    UTF-8.)
207    </P>
208    <P>
209    If an invalid UTF-8 string is passed to PCRE, an error return
210    (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
211    your strings are valid, and therefore want to skip these checks in order to
212    improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
213    at run time, PCRE assumes that the pattern or subject it is given
214    (respectively) contains only valid UTF-8 codes. In this case, it does not
215    diagnose an invalid UTF-8 string.
216    </P>
217    <P>
218    If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
219    happens depends on why the string is invalid. If the string conforms to the
220    "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
221    in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
222    test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
223    rules of RFC 2279. However, if the string does not even conform to RFC 2279,
224    the result is undefined. Your program may crash.
225    </P>
226    <P>
227    If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
228    encoded in a UTF-8-like manner as per the old RFC, you can set
229    PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
230    situation, you will have to apply your own validity check.
231    </P>
232    <br><b>
233    General comments about UTF-8 mode
234    </b><br>
235    <P>
236    1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
237    UTF-8 character if the value is greater than 127.
238  </P>  </P>
239  <P>  <P>
240  The following comments apply when PCRE is running in UTF-8 mode:  2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
241    characters for values greater than \177.
242  </P>  </P>
243  <P>  <P>
244  1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects  3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
 are checked for validity on entry to the relevant functions. If an invalid  
 UTF-8 string is passed, an error return is given. In some situations, you may  
 already know that your strings are valid, and therefore want to skip these  
 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag  
 at compile time or at run time, PCRE assumes that the pattern or subject it  
 is given (respectively) contains only valid UTF-8 codes. In this case, it does  
 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to  
 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program  
 may crash.  
 </P>  
 <P>  
 2. In a pattern, the escape sequence \x{...}, where the contents of the braces  
 is a string of hexadecimal digits, is interpreted as a UTF-8 character whose  
 code number is the given hexadecimal number, for example: \x{1234}. If a  
 non-hexadecimal digit appears between the braces, the item is not recognized.  
 This escape sequence can be used either as a literal, or within a character  
 class.  
 </P>  
 <P>  
 3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8  
 character if the value is greater than 127.  
 </P>  
 <P>  
 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual  
245  bytes, for example: \x{100}{3}.  bytes, for example: \x{100}{3}.
246  </P>  </P>
247  <P>  <P>
248  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.
249  </P>  </P>
250  <P>  <P>
251  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,
252  but its use can lead to some strange effects.  but its use can lead to some strange effects. This facility is not available in
253    the alternative matching function, <b>pcre_dfa_exec()</b>.
254  </P>  </P>
255  <P>  <P>
256  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
257  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
258  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
259  values less than 256. This remains true even when PCRE includes Unicode  values less than 256. This remains true even when PCRE includes Unicode
260  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
261  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
262  must use Unicode property tests such as \p{Nd}.  must use Unicode property tests such as \p{Nd}. Note that this also applies to
263    \b, because it is defined in terms of \w and \W.
264  </P>  </P>
265  <P>  <P>
266  8. Similarly, characters that match the POSIX named character classes are all  7. Similarly, characters that match the POSIX named character classes are all
267  low-valued characters.  low-valued characters.
268  </P>  </P>
269  <P>  <P>
270    8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
271    (\h, \H, \v, and \V) do match all the appropriate Unicode characters.
272    </P>
273    <P>
274  9. Case-insensitive matching applies only to characters whose values are less  9. Case-insensitive matching applies only to characters whose values are less
275  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
276  property support is available, PCRE still uses its own character tables when  property support is available, PCRE still uses its own character tables when
277  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.
278  The Unicode property information is used only for characters with higher  The Unicode property information is used only for characters with higher
279  values.  values. Even when Unicode property support is available, PCRE supports
280    case-insensitive matching only when there is a one-to-one mapping between a
281    letter's cases. There are a small number of many-to-one mappings in Unicode;
282    these are not supported by PCRE.
283  </P>  </P>
284  <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>  <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
285  <P>  <P>
286  Philip Hazel &#60;ph10@cam.ac.uk&#62;  Philip Hazel
287    <br>
288    University Computing Service
289  <br>  <br>
290  University Computing Service,  Cambridge CB2 3QH, England.
291  <br>  <br>
292  Cambridge CB2 3QG, England.  </P>
293    <P>
294    Putting an actual email address here seems to have been a spam magnet, so I've
295    taken it away. If you want to email me, use my two initials, followed by the
296    two digits 10, at the domain cam.ac.uk.
297    </P>
298    <br><a name="SEC6" href="#TOC1">REVISION</a><br>
299    <P>
300    Last updated: 11 April 2009
301  <br>  <br>
302  Phone: +44 1223 334714  Copyright &copy; 1997-2009 University of Cambridge.
 Last updated: 09 September 2004  
303  <br>  <br>
 Copyright &copy; 1997-2004 University of Cambridge.  
304  <p>  <p>
305  Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.
306  </p>  </p>

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