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       PCRE - Perl-compatible regular expressions


       This  document  describes  the  optional	 features  of PCRE that can be
       selected when the library is compiled. It assumes use of the  configure
       script,	where the optional features are selected or deselected by pro‐
       viding options to configure before running the make  command.  However,
       the  same  options  can be selected in both Unix-like and non-Unix-like
       environments using the GUI facility of cmake-gui if you are using CMake
       instead of configure to build PCRE.

       There  is  a  lot more information about building PCRE in non-Unix-like
       environments in the file called NON_UNIX_USE, which is part of the PCRE
       distribution.  You  should consult this file as well as the README file
       if you are building in a non-Unix-like environment.

       The complete list of options for configure (which includes the standard
       ones  such  as  the  selection  of  the	installation directory) can be
       obtained by running

	 ./configure --help

       The following sections include  descriptions  of	 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 complemen‐
       tary option always exists as well, but as it specifies the default,  it
       is not described.


       By default, the configure script will search for a C++ compiler and C++
       header files. If it finds them, it automatically builds the C++ wrapper
       library for PCRE. You can disable this by adding


       to the configure command.


       To build PCRE with support for UTF-8 Unicode 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()
       or pcre_compile2() functions.

       If you set --enable-utf8 when compiling in an EBCDIC environment,  PCRE
       expects its input to be either ASCII or UTF-8 (depending on the runtime
       option). It is not possible to support both EBCDIC and UTF-8  codes  in
       the  same  version  of  the  library.  Consequently,  --enable-utf8 and
       --enable-ebcdic are mutually exclusive.


       UTF-8 support allows PCRE to process character values greater than  255
       in  the	strings that it handles. On its own, however, it does not pro‐
       vide any facilities for accessing the properties of such characters. If
       you  want  to  be able to use the pattern escapes \P, \p, and \X, which
       refer to Unicode character properties, you must add


       to the configure command. This implies UTF-8 support, even if you  have
       not explicitly requested it.

       Including  Unicode  property  support  adds around 30K of tables to the
       PCRE library. Only the general category properties such as  Lu  and  Nd
       are supported. Details are given in the pcrepattern documentation.


       By  default,  PCRE interprets the linefeed (LF) character as indicating
       the end of a line. This is the normal newline  character	 on  Unix-like
       systems.	 You  can compile PCRE to use carriage return (CR) instead, by


       to the  configure  command.  There  is  also  a	--enable-newline-is-lf
       option, which explicitly specifies linefeed as the newline character.

       Alternatively, you can specify that line endings are to be indicated by
       the two character sequence CRLF. If you want this, add


       to the configure command. There is a fourth option, specified by


       which causes PCRE to recognize any of the three sequences  CR,  LF,  or
       CRLF as indicating a line ending. Finally, a fifth option, specified by


       causes PCRE to recognize any Unicode newline sequence.

       Whatever	 line  ending convention is selected when PCRE is built can be
       overridden when the library functions are called. At build time	it  is
       conventional to use the standard for your operating system.


       By  default,  the  sequence \R in a pattern matches any Unicode newline
       sequence, whatever has been selected as the line	 ending	 sequence.  If
       you specify


       the  default  is changed so that \R matches only CR, LF, or CRLF. What‐
       ever is selected when PCRE is built can be overridden when the  library
       functions are called.


       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.


       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 truyl enormous patterns,
       so it is possible to compile PCRE to use three-byte or  four-byte  off‐
       sets 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.


       When matching with the pcre_exec() function, PCRE implements backtrack‐
       ing by making recursive calls to an internal function  called  match().
       In  environments	 where	the size of the stack is limited, this can se‐
       verely limit PCRE's operation. (The Unix environment does  not  usually
       suffer from this problem, but it may sometimes be necessary to increase
       the maximum stack size.	There is a discussion in the  pcrestack	 docu‐
       mentation.)  An alternative approach to recursion that uses memory from
       the heap to remember data, instead of using recursive  function	calls,
       has  been  implemented to work round the problem of limited stack size.
       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. By default these point to malloc() and free(), but  you
       can replace the pointers so that your own functions are used instead.

       Separate	 functions  are	 provided  rather  than	 using pcre_malloc and
       pcre_free 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  malloc()	 and free(). PCRE runs
       noticeably more slowly when built in this way. This option affects only
       the pcre_exec() function; it is not relevant for pcre_dfa_exec().


       Internally,  PCRE has a function called match(), which it calls repeat‐
       edly  (sometimes	 recursively)  when  matching  a  pattern   with   the
       pcre_exec()  function.  By controlling the maximum number of times this
       function may be called during a single matching operation, 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.  This  setting  has	 no  effect   on   the
       pcre_dfa_exec() matching function.

       In  some	 environments  it is desirable to limit the depth of recursive
       calls of match() more strictly than the total number of calls, in order
       to  restrict  the maximum amount of stack (or heap, if --disable-stack-
       for-recursion is specified) that is used. A second limit controls this;
       it  defaults  to	 the  value  that is set for --with-match-limit, which
       imposes no additional constraints. However, you can set a  lower	 limit
       by adding, for example,


       to  the	configure  command.  This  value can also be overridden at run


       PCRE uses fixed tables for processing characters whose code values  are
       less  than 256. By default, PCRE is built with a set of tables that are
       distributed in the file pcre_chartables.c.dist. These  tables  are  for
       ASCII codes only. If you add


       to  the	configure  command, the distributed tables are no longer used.
       Instead, a program called dftables is compiled and  run.	 This  outputs
       the source for new set of tables, created in the default locale of your
       C runtime system. (This method of replacing the tables does not work if
       you  are cross compiling, because dftables is run on the local host. If
       you need to create alternative tables when cross	 compiling,  you  will
       have to do so "by hand".)


       PCRE  assumes  by  default that it will run in an environment where the
       character code is ASCII (or Unicode, which is  a	 superset  of  ASCII).
       This  is	 the  case for most computer operating systems. PCRE can, how‐
       ever, be compiled to run in an EBCDIC environment by adding


       to the configure command. This setting implies --enable-rebuild-charta‐
       bles.  You  should  only	 use  it if you know that you are in an EBCDIC
       environment (for example,  an  IBM  mainframe  operating	 system).  The
       --enable-ebcdic option is incompatible with --enable-utf8.


       By default, pcregrep reads all files as plain text. You can build it so
       that it recognizes files whose names end in .gz or .bz2, and reads them
       with libz or libbz2, respectively, by adding one or both of


       to the configure command. These options naturally require that the rel‐
       evant libraries are installed on your system. Configuration  will  fail
       if they are not.


       If you add


       to  the	configure  command,  pcretest  is  linked with the libreadline
       library, and when its input is from a terminal, it reads it  using  the
       readline() function. This provides line-editing and history facilities.
       Note that libreadline is GPL-licensed, so if you distribute a binary of
       pcretest linked in this way, there may be licensing issues.

       Setting	this  option  causes  the -lreadline option to be added to the
       pcretest build. In many operating environments with  a  sytem-installed
       libreadline this is sufficient. However, in some environments (e.g.  if
       an unmodified distribution version of readline is in use),  some	 extra
       configuration  may  be necessary. The INSTALL file for libreadline says

	 "Readline uses the termcap functions, but does not link with the
	 termcap or curses library itself, allowing applications which link
	 with readline the to choose an appropriate library."

       If your environment has not been set up so that an appropriate  library
       is automatically included, you may need to add something like


       immediately before the configure command.


       pcreapi(3), pcre_config(3).


       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


       Last updated: 29 September 2009
       Copyright (c) 1997-2009 University of Cambridge.

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