/[pcre]/code/trunk/doc/html/pcre.html

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-revision 83 by nigel,
Sat Feb 24 21:41:06 2007 UTC
+revision 211 by ph10,
Thu Aug  9 09:52:43 2007 UTC
 Line 18 
 man page, in case the conversion went wr
  <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
  <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
  <li><a name="TOC5" href="#SEC5">AUTHOR</a>
+ <li><a name="TOC6" href="#SEC6">REVISION</a>
  </ul>
  <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
  <P>
  The PCRE library is a set of functions that implement regular expression
  pattern matching using the same syntax and semantics as Perl, with just a few
- differences. The current implementation of PCRE (release 6.x) corresponds
+ differences. (Certain features that appeared in Python and PCRE before they
- approximately with Perl 5.8, including support for UTF-8 encoded strings and
+ appeared in Perl are also available using the Python syntax.)
- Unicode general category properties. However, this support has to be explicitly
- enabled; it is not the default.
  </P>
  <P>
- In addition to the Perl-compatible matching function, PCRE also contains an
+ The current implementation of PCRE (release 7.x) corresponds approximately with
+ Perl 5.10, including support for UTF-8 encoded strings and Unicode general
+ category properties. However, UTF-8 and Unicode support has to be explicitly
+ enabled; it is not the default. The Unicode tables correspond to Unicode
+ release 5.0.0.
+ </P>
+ <P>
+ In addition to the Perl-compatible matching function, PCRE contains an
  alternative matching function that matches the same compiled patterns in a
  different way. In certain circumstances, the alternative function has some
  advantages. For a discussion of the two matching algorithms, see the
-Line 52 
 supported by PCRE are given in separate
+Line 58 
 supported by PCRE are given in separate
  <a href="pcrepattern.html"><b>pcrepattern</b></a>
  and
  <a href="pcrecompat.html"><b>pcrecompat</b></a>
- pages.
+ pages. There is a syntax summary in the
+ <a href="pcresyntax.html"><b>pcresyntax</b></a>
+ page.
  </P>
  <P>
  Some features of PCRE can be included, excluded, or changed when the library is
-Line 82 
 all the sections are concatenated, for e
+Line 90 
 all the sections are concatenated, for e
  follows:
  <pre>
    pcre              this document
+   pcre-config       show PCRE installation configuration information
    pcreapi           details of PCRE's native C API
    pcrebuild         options for building PCRE
    pcrecallout       details of the callout feature
-Line 91 
 follows:
+Line 100 
 follows:
    pcrematching      discussion of the two matching algorithms
    pcrepartial       details of the partial matching facility
    pcrepattern       syntax and semantics of supported regular expressions
+   pcresyntax        quick syntax reference
    pcreperform       discussion of performance issues
    pcreposix         the POSIX-compatible C API
    pcreprecompile    details of saving and re-using precompiled patterns
    pcresample        discussion of the sample program
+   pcrestack         discussion of stack usage
    pcretest          description of the <b>pcretest</b> testing command
  </pre>
  In addition, in the "man" and HTML formats, there is a short page for each
-Line 113 
 internal linkage size of 3 or 4 (see the
+Line 124 
 internal linkage size of 3 or 4 (see the
  distribution and the
  <a href="pcrebuild.html"><b>pcrebuild</b></a>
  documentation for details). In these cases the limit is substantially larger.
- However, the speed of execution will be slower.
+ However, the speed of execution is slower.
  </P>
  <P>
  All values in repeating quantifiers must be less than 65536.
- The maximum number of capturing subpatterns is 65535.
  </P>
  <P>
- 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
- 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.
+ </P>
+ <P>
+ The maximum length of name for a named subpattern is 32 characters, and the
+ maximum number of named subpatterns is 10000.
  </P>
  <P>
  The maximum length of a subject string is the largest positive number that an
  integer variable can hold. However, when using the traditional matching
  function, PCRE uses recursion to handle subpatterns and indefinite repetition.
  This means that the available stack space may limit the size of a subject
- string that can be processed by certain patterns.
+ string that can be processed by certain patterns. For a discussion of stack
+ issues, see the
+ <a href="pcrestack.html"><b>pcrestack</b></a>
+ documentation.
  <a name="utf8support"></a></P>
  <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
  <P>
-Line 149 
 instead of just strings of bytes.
+Line 165 
 instead of just strings of bytes.
  <P>
  If you compile PCRE with UTF-8 support, but do not use it at run time, the
  library will be a bit bigger, but the additional run time overhead is limited
- 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.
  </P>
  <P>
  If PCRE is built with Unicode character property support (which implies UTF-8
  support), the escape sequences \p{..}, \P{..}, and \X are supported.
  The available properties that can be tested are limited to the general
  category properties such as Lu for an upper case letter or Nd for a decimal
- number. A full list is given in the
+ number, the Unicode script names such as Arabic or Han, and the derived
+ properties Any and L&. A full list is given in the
  <a href="pcrepattern.html"><b>pcrepattern</b></a>
- documentation. The PCRE library is increased in size by about 90K when Unicode
+ documentation. Only the short names for properties are supported. For example,
- property support is included.
+ \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
- </P>
+ Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
- <P>
+ compatibility with Perl 5.6. PCRE does not support this.
- The following comments apply when PCRE is running in UTF-8 mode:
+ <a name="utf8strings"></a></P>
- </P>
+ <br><b>
- <P>
+ Validity of UTF-8 strings
-. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
+ </b><br>
- are checked for validity on entry to the relevant functions. If an invalid
+ <P>
- UTF-8 string is passed, an error return is given. In some situations, you may
+ When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
- already know that your strings are valid, and therefore want to skip these
+ are (by default) checked for validity on entry to the relevant functions. From
- checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
+ release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
- at compile time or at run time, PCRE assumes that the pattern or subject it
+ themselves derived from the Unicode specification. Earlier releases of PCRE
- is given (respectively) contains only valid UTF-8 codes. In this case, it does
+ followed the rules of RFC 2279, which allows the full range of 31-bit values (0
- not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
+ to 0x7FFFFFFF). The current check allows only values in the range U+0 to
- PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
+ U+10FFFF, excluding U+D800 to U+DFFF.
- may crash.
+ </P>
+ <P>
+ The excluded code points are the "Low Surrogate Area" of Unicode, of which the
+ Unicode Standard says this: "The Low Surrogate Area does not contain any
+ character assignments, consequently no character code charts or namelists are
+ provided for this area. Surrogates are reserved for use with UTF-16 and then
+ must be used in pairs." The code points that are encoded by UTF-16 pairs are
+ available as independent code points in the UTF-8 encoding. (In other words,
+ the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
+ UTF-8.)
+ </P>
+ <P>
+ If an invalid UTF-8 string is passed to PCRE, an error return
+ (PCRE_ERROR_BADUTF8) 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.
+ </P>
+ <P>
+ If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
+ happens depends on why the string is invalid. If the string conforms to the
+ "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
+ in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
+ test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
+ rules of RFC 2279. However, if the string does not even conform to RFC 2279,
+ the result is undefined. Your program may crash.
+ </P>
+ <P>
+ If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
+ encoded in a UTF-8-like manner as per the old RFC, you can set
+ PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
+ situation, you will have to apply your own validity check.
+ </P>
+ <br><b>
+ General comments about UTF-8 mode
+ </b><br>
+ <P>
+. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
+ UTF-8 character if the value is greater than 127.
  </P>
  <P>
-. In a pattern, the escape sequence \x{...}, where the contents of the braces
+. Octal numbers up to \777 are recognized, and match two-byte UTF-8
- is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
+ characters for values greater than \177.
- 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>
-. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8
+. Repeat quantifiers apply to complete UTF-8 characters, not to individual
- character if the value is greater than 127.
- </P>
- <P>
-. Repeat quantifiers apply to complete UTF-8 characters, not to individual
  bytes, for example: \x{100}{3}.
  </P>
  <P>
 . The dot metacharacter matches one UTF-8 character instead of a single byte.
  </P>
  <P>
 . The escape sequence \C can be used to match a single byte in UTF-8 mode,
  but its use can lead to some strange effects. This facility is not available in
  the alternative matching function, <b>pcre_dfa_exec()</b>.
  </P>
  <P>
 . The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
  test characters of any code value, but the characters that PCRE recognizes as
  digits, spaces, or word characters remain the same set as before, all with
  values less than 256. This remains true even when PCRE includes Unicode
-Line 210 
 cases. If you really want to test for a
+Line 259 
 cases. If you really want to test for a
  must use Unicode property tests such as \p{Nd}.
  </P>
  <P>
 . Similarly, characters that match the POSIX named character classes are all
  low-valued characters.
  </P>
  <P>
+. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
+ (\h, \H, \v, and \V) do match all the appropriate Unicode characters.
+ </P>
+ <P>
 . Case-insensitive matching applies only to characters whose values are less
  than 128, unless PCRE is built with Unicode property support. Even when Unicode
  property support is available, PCRE still uses its own character tables when
  checking the case of low-valued characters, so as not to degrade performance.
  The Unicode property information is used only for characters with higher
- values.
+ values. Even when Unicode property support is available, PCRE supports
+ case-insensitive matching only when there is a one-to-one mapping between a
+ letter's cases. There are a small number of many-to-one mappings in Unicode;
+ these are not supported by PCRE.
  </P>
  <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
  <P>
  Philip Hazel
  <br>
- University Computing Service,
+ University Computing Service
+ <br>
+ Cambridge CB2 3QH, England.
  <br>
- Cambridge CB2 3QG, England.
  </P>
  <P>
  Putting an actual email address here seems to have been a spam magnet, so I've
- 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
- by a dot, at the domain ucs.cam.ac.uk.
+ two digits 10, at the domain cam.ac.uk.
- Last updated: 07 March 2005
+ </P>
+ <br><a name="SEC6" href="#TOC1">REVISION</a><br>
+ <P>
+ Last updated: 09 August 2007
+ <br>
+ Copyright &copy; 1997-2007 University of Cambridge.
  <br>
- Copyright &copy; 1997-2005 University of Cambridge.
  <p>
  Return to the <a href="index.html">PCRE index page</a>.
  </p>

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