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revision 209 by ph10, Tue Aug 7 09:22:06 2007 UTC revision 345 by ph10, Mon Apr 28 15:10:02 2008 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. (Certain features that appeared in Python and PCRE before they  differences. Certain features that appeared in Python and PCRE before they
10  appeared in Perl are also available using the Python syntax.)  appeared in Perl are also available using the Python syntax. There is also some
11    support for certain .NET and Oniguruma syntax items, and there is an option for
12    requesting some minor changes that give better JavaScript compatibility.
13  .P  .P
14  The current implementation of PCRE (release 7.x) corresponds approximately with  The current implementation of PCRE (release 7.x) corresponds approximately with
15  Perl 5.10, including support for UTF-8 encoded strings and Unicode general  Perl 5.10, including support for UTF-8 encoded strings and Unicode general
# Line 144  issues, see the Line 146  issues, see the
146  \fBpcrestack\fP  \fBpcrestack\fP
147  .\"  .\"
148  documentation.  documentation.
149  .sp  .
150  .\" HTML <a name="utf8support"></a>  .\" HTML <a name="utf8support"></a>
151  .  .
152  .  .
# Line 182  documentation. Only the short names for Line 184  documentation. Only the short names for
184  \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.  \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  Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
186  compatibility with Perl 5.6. PCRE does not support this.  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:  .\" HTML <a name="utf8strings"></a>
189  .P  .
190  1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects  .SS "Validity of UTF-8 strings"
191  are checked for validity on entry to the relevant functions. Note that the  .rs
192  check is for a syntactically valid UTF-8 byte string, as defined by RFC 2279.  .sp
193  It is \fInot\fP a check for a UTF-8 string of assigned or allowable Unicode  When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
194  code points. For example, the byte sequence \exED\exB2\ex94 is a valid UTF-8  are (by default) checked for validity on entry to the relevant functions. From
195  encoding of the code point U+DC94, and is not rejected by PCRE. However, that  release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
196  code point is in the "Low Surrogate Area" of Unicode, of which the Unicode  themselves derived from the Unicode specification. Earlier releases of PCRE
197  Standard says this: "The Low Surrogate Area does not contain any character  followed the rules of RFC 2279, which allows the full range of 31-bit values (0
198  assignments, consequently no character code charts or namelists are provided  to 0x7FFFFFFF). The current check allows only values in the range U+0 to
199  for this area. Surrogates are reserved for use with UTF-16 and then must be  U+10FFFF, excluding U+D800 to U+DFFF.
200  used in pairs."  .P
201  .P  The excluded code points are the "Low Surrogate Area" of Unicode, of which the
202  The reason for the UTF-8 check at the start is so that the rest of PCRE can  Unicode Standard says this: "The Low Surrogate Area does not contain any
203  assume that UTF-8 strings are well formed. There is no intention of  character assignments, consequently no character code charts or namelists are
204  interpreting the values of the code points, which would involve more processing  provided for this area. Surrogates are reserved for use with UTF-16 and then
205  and affect performance.  must be used in pairs." The code points that are encoded by UTF-16 pairs are
206  .P  available as independent code points in the UTF-8 encoding. (In other words,
207  If a syntactically invalid UTF-8 string is passed, an error return is given. In  the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
208  some situations, you may already know that your strings are valid, and  UTF-8.)
209  therefore want to skip these checks in order to improve performance. If you set  .P
210  the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that  If an invalid UTF-8 string is passed to PCRE, an error return
211  the pattern or subject it is given (respectively) contains only valid UTF-8  (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
212  codes. In this case, it does not diagnose an invalid UTF-8 string. If you pass  your strings are valid, and therefore want to skip these checks in order to
213  an invalid UTF-8 string to PCRE when PCRE_NO_UTF8_CHECK is set, the results are  improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
214  undefined. Your program may crash.  at run time, PCRE assumes that the pattern or subject it is given
215  .P  (respectively) contains only valid UTF-8 codes. In this case, it does not
216  2. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte  diagnose an invalid UTF-8 string.
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    If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
227    encoded in a UTF-8-like manner as per the old RFC, you can set
228    PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
229    situation, you will have to apply your own validity check.
230    .
231    .SS "General comments about UTF-8 mode"
232    .rs
233    .sp
234    1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
235  UTF-8 character if the value is greater than 127.  UTF-8 character if the value is greater than 127.
236  .P  .P
237  3. Octal numbers up to \e777 are recognized, and match two-byte UTF-8  2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
238  characters for values greater than \e177.  characters for values greater than \e177.
239  .P  .P
240  4. Repeat quantifiers apply to complete UTF-8 characters, not to individual  3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
241  bytes, for example: \ex{100}{3}.  bytes, for example: \ex{100}{3}.
242  .P  .P
243  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.
244  .P  .P
245  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,
246  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
247  the alternative matching function, \fBpcre_dfa_exec()\fP.  the alternative matching function, \fBpcre_dfa_exec()\fP.
248  .P  .P
249  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
250  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
251  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
252  values less than 256. This remains true even when PCRE includes Unicode  values less than 256. This remains true even when PCRE includes Unicode
# Line 234  property support, because to do otherwis Line 254  property support, because to do otherwis
254  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
255  must use Unicode property tests such as \ep{Nd}.  must use Unicode property tests such as \ep{Nd}.
256  .P  .P
257  8. Similarly, characters that match the POSIX named character classes are all  7. Similarly, characters that match the POSIX named character classes are all
258  low-valued characters.  low-valued characters.
259  .P  .P
260  9. However, the Perl 5.10 horizontal and vertical whitespace matching escapes  8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
261  (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.  (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
262  .P  .P
263  10. Case-insensitive matching applies only to characters whose values are less  9. Case-insensitive matching applies only to characters whose values are less
264  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
265  property support is available, PCRE still uses its own character tables when  property support is available, PCRE still uses its own character tables when
266  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.
# Line 269  two digits 10, at the domain cam.ac.uk. Line 289  two digits 10, at the domain cam.ac.uk.
289  .rs  .rs
290  .sp  .sp
291  .nf  .nf
292  Last updated: 07 August 2007  Last updated: 12 April 2008
293  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2008 University of Cambridge.
294  .fi  .fi

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