If  you  are running an application that uses a large number of regular
       expression patterns, it may be useful to store them  in  a  precompiled
       form  instead  of  having to compile them every time the application is
       run.  If you are not  using  any  private  character  tables  (see  the
       pcre_maketables()  documentation),  this is relatively straightforward.
       If you are using private tables, it is a little bit  more  complicated.
       However,  if you are using the just-in-time optimization feature, it is
       not possible to save and reload the JIT data.

       If you save compiled patterns to a file, you can copy them to a differ-
       ent host and run them there. If the two hosts have different endianness
       (byte order), you should run the  pcre[16]_pattern_to_host_byte_order()
       function on the new host before trying to match the pattern. The match-
       ing functions return PCRE_ERROR_BADENDIANNESS if they detect a  pattern
       with the wrong endianness.

       Compiling  regular  expressions with one version of PCRE for use with a
       different version is not guaranteed to work and may cause crashes,  and
       saving  and  restoring  a  compiled  pattern loses any JIT optimization


       The value returned by pcre[16]_compile() points to a  single  block  of
       memory  that  holds  the  compiled pattern and associated data. You can
       find the length of this block in bytes by  calling  pcre[16]_fullinfo()
       with  an  argument of PCRE_INFO_SIZE. You can then save the data in any
       appropriate manner. Here is sample code for the 8-bit library that com-
       piles  a  pattern and writes it to a file. It assumes that the variable
       fd refers to a file that is open for output:

         int erroroffset, rc, size;
         char *error;
         pcre *re;

         re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
         if (re == NULL) { ... handle errors ... }
         rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
         if (rc < 0) { ... handle errors ... }
         rc = fwrite(re, 1, size, fd);
         if (rc != size) { ... handle errors ... }

       In this example, the bytes  that  comprise  the  compiled  pattern  are
       copied  exactly.  Note that this is binary data that may contain any of
       the 256 possible byte  values.  On  systems  that  make  a  distinction
       between binary and non-binary data, be sure that the file is opened for
       binary output.

       If you want to write more than one pattern to a file, you will have  to
       devise  a  way of separating them. For binary data, preceding each pat-
       tern with its length is probably  the  most  straightforward  approach.
       ronment.    When    studying    generates    additional    information,
       pcre[16]_study() returns a pointer to a pcre[16]_extra data block.  Its
       format  is  defined in the section on matching a pattern in the pcreapi
       documentation. The study_data field points to the  binary  study  data,
       and  this  is what you must save (not the pcre[16]_extra block itself).
       The  length  of  the  study   data   can   be   obtained   by   calling
       pcre[16]_fullinfo()  with  an argument of PCRE_INFO_STUDYSIZE. Remember
       to check that pcre[16]_study() did return a non-NULL value before  try-
       ing to save the study data.


       Re-using  a  precompiled pattern is straightforward. Having reloaded it
       into main memory, called pcre[16]_pattern_to_host_byte_order() if  nec-
       essary,  you pass its pointer to pcre[16]_exec() or pcre[16]_dfa_exec()
       in the usual way.

       However, if you passed a pointer to custom character  tables  when  the
       pattern was compiled (the tableptr argument of pcre[16]_compile()), you
       must   now   pass   a   similar   pointer   to    pcre[16]_exec()    or
       pcre[16]_dfa_exec(),  because the value saved with the compiled pattern
       will obviously be nonsense. A field in a pcre[16]_extra() block is used
       to pass this data, as described in the section on matching a pattern in
       the pcreapi documentation.

       If you did not provide custom character tables  when  the  pattern  was
       compiled, the pointer in the compiled pattern is NULL, which causes the
       matching functions to use PCRE's internal tables. Thus, you do not need
       to take any special action at run time in this case.

       If  you  saved study data with the compiled pattern, you need to create
       your own pcre[16]_extra data block and  set  the  study_data  field  to
       point   to   the   reloaded   study   data.   You  must  also  set  the
       PCRE_EXTRA_STUDY_DATA bit in the flags field  to  indicate  that  study
       data  is  present.  Then  pass the pcre[16]_extra block to the matching
       function in the usual way. If the pattern was studied for  just-in-time
       optimization,  that  data  cannot  be  saved,  and  so  is  lost  by  a
       save/restore cycle.


       In general, it is safest to  recompile  all  saved  patterns  when  you
       update  to  a new PCRE release, though not all updates actually require


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


       Last updated: 10 January 2012
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