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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  providing  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 complementary option always exists as well, but as it specifies the default, it is not


       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 provide 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 adding


       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. Whatever 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 of


       to the configure command, as required.


       When PCRE is called  through  the  POSIX  interface  (see  the  pcreposix  documentation),
       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 alternation 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 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.


       When matching with the  pcre_exec()  function,  PCRE  implements  backtracking  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, but it may sometimes be necessary to increase the
       maximum stack size.  There is a discussion in the pcrestack documentation.) 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 management functions. By default these  point  to
       malloc()  and free(), but you can replace the pointers so that your own functions are used

       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 repeatedly (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 documentation. 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

       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 time.


       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, however, be compiled to  run  in  an  EBCDIC  environment  by


       to  the  configure  command.  This setting implies --enable-rebuild-chartables. 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-


       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 relevant 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.