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PCRE(3)								       PCRE(3)

       PCRE - Perl-compatible regular expressions


       This  document  describes  the  optional	 features  of PCRE that can be
       selected when the library is compiled. They are all selected, or	 dese‐
       lected,	by  providing  options	to  the	 configure script which is run
       before the make command. The complete list  of  options	for  configure
       (which  includes the standard ones such as the selection of the instal‐
       lation directory) can be obtained by running

	 ./configure --help

       The following sections describe certain options whose names begin  with
       --enable	 or  --disable. These settings specify changes to the defaults
       for the configure command. Because of the  way  that  configure	works,
       --enable	 and  --disable	 always	 come  in  pairs, so the complementary
       option always exists as well, but as it specifies the  default,	it  is
       not described.


       To build PCRE with support for UTF-8 character strings, add


       to  the	configure  command.  Of	 itself, this does not make PCRE treat
       strings as UTF-8. As well as compiling PCRE with this option, you  also
       have  have to set the PCRE_UTF8 option when you call the pcre_compile()


       By default, PCRE treats character 10 (linefeed) as the newline  charac‐
       ter. This is the normal newline character on Unix-like systems. You can
       compile PCRE to use character 13 (carriage return) instead by adding


       to the configure command. For completeness there is  also  a  --enable-
       newline-is-lf  option,  which explicitly specifies linefeed as the new‐
       line character.


       The PCRE building process uses libtool to build both shared and	static
       Unix  libraries by default. You can suppress one of these by adding one


       to the configure command, as required.


       When PCRE is called through the POSIX interface (see the pcreposix doc‐
       umentation),  additional	 working  storage  is required for holding the
       pointers to capturing substrings because PCRE requires  three  integers
       per  substring,	whereas	 the POSIX interface provides only two. If the
       number of expected substrings is small, the wrapper function uses space
       on the stack, because this is faster than using malloc() for each call.
       The default threshold above which the stack is no longer used is 10; it
       can be changed by adding a setting such as


       to the configure command.


       Internally,  PCRE  has a function called match() which it calls repeat‐
       edly (possibly recursively) when performing a  matching	operation.  By
       limiting	 the  number of times this function may be called, a limit can
       be placed on the resources used by a single call	 to  pcre_exec().  The
       limit  can be changed at run time, as described in the pcreapi documen‐
       tation. The default is 10 million, but this can be changed by adding  a
       setting such as


       to the configure command.


       Within  a  compiled  pattern,  offset values are used to point from one
       part to another (for example, from an opening parenthesis to an	alter‐
       nation  metacharacter).	By  default two-byte values are used for these
       offsets, leading to a maximum size for a	 compiled  pattern  of	around
       64K.  This  is sufficient to handle all but the most gigantic patterns.
       Nevertheless, some people do want to process enormous patterns,	so  it
       is  possible  to compile PCRE to use three-byte or four-byte offsets by
       adding a setting such as


       to the configure command. The value given must be 2,  3,	 or  4.	 Using
       longer  offsets slows down the operation of PCRE because it has to load
       additional bytes when handling them.

       If you build PCRE with an increased link size, test 2 (and  test	 5  if
       you  are using UTF-8) will fail. Part of the output of these tests is a
       representation of the compiled pattern, and this changes with the  link


       PCRE  implements	 backtracking while matching by making recursive calls
       to an internal function called match(). In environments where the  size
       of the stack is limited, this can severely limit PCRE's operation. (The
       Unix environment does not usually suffer from this problem.) An	alter‐
       native  approach	 that  uses  memory  from  the	heap to remember data,
       instead of using recursive function calls, has been implemented to work
       round  this  problem. If you want to build a version of PCRE that works
       this way, add


       to the configure command. With this configuration, PCRE	will  use  the
       pcre_stack_malloc  and pcre_stack_free variables to call memory manage‐
       ment functions. Separate functions are provided because	the  usage  is
       very  predictable:  the	block sizes requested are always the same, and
       the blocks are always freed in reverse order. A calling	program	 might
       be  able	 to implement optimized functions that perform better than the
       standard malloc() and  free()  functions.  PCRE	runs  noticeably  more
       slowly when built in this way.


       PCRE  assumes  by  default that it will run in an environment where the
       character code is ASCII (or UTF-8, which is a superset of ASCII).  PCRE
       can, however, be compiled to run in an EBCDIC environment by adding


       to the configure command.

Last updated: 09 December 2003
Copyright (c) 1997-2003 University of Cambridge.


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