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pcreprecompile(3)                                            pcreprecompile(3)




NAME

       PCRE - Perl-compatible regular expressions


SAVING AND RE-USING PRECOMPILED PCRE PATTERNS


       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|32]_pat-
       tern_to_host_byte_order()  function  on  the  new host before trying to
       match the pattern. The matching functions return  PCRE_ERROR_BADENDIAN-
       NESS 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
       data.


SAVING A COMPILED PATTERN


       The value returned by pcre[16|32]_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|32]_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  compiles  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.
       Another  possibility is to write out the data in hexadecimal instead of
       binary, one pattern to a line.

       Saving compiled patterns in a file is only one possible way of  storing
       them  for later use. They could equally well be saved in a database, or
       in the memory of some daemon process that passes them  via  sockets  to
       the processes that want them.

       If the pattern has been studied, it is also possible to save the normal
       study data in a similar way to the compiled pattern itself. However, if
       the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre-
       ated cannot be saved because it is too dependent on the  current  envi-
       ronment.    When    studying    generates    additional    information,
       pcre[16|32]_study() returns  a  pointer  to  a  pcre[16|32]_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|32]_extra
       block itself). The length of the study data can be obtained by  calling
       pcre[16|32]_fullinfo()  with an argument of PCRE_INFO_STUDYSIZE. Remem-
       ber to check that  pcre[16|32]_study()  did  return  a  non-NULL  value
       before trying to save the study data.


RE-USING A PRECOMPILED PATTERN


       Re-using  a  precompiled pattern is straightforward. Having reloaded it
       into main memory,  called  pcre[16|32]_pattern_to_host_byte_order()  if
       necessary,    you   pass   its   pointer   to   pcre[16|32]_exec()   or
       pcre[16|32]_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|32]_compile()),
       you  must  now  pass  a  similar  pointer  to   pcre[16|32]_exec()   or
       pcre[16|32]_dfa_exec(),  because the value saved with the compiled pat-
       tern will obviously be nonsense. A field in a pcre[16|32]_extra() block
       is  used  to  pass this data, as described in the section on matching a
       pattern in the pcreapi documentation.

       Warning: The tables that pcre_exec() and pcre_dfa_exec()  use  must  be
       the same as those that were used when the pattern was compiled. If this
       is not the case, the behaviour is undefined.

       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|32]_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|32]_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.


COMPATIBILITY WITH DIFFERENT PCRE RELEASES


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


AUTHOR


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


REVISION


       Last updated: 12 November 2013
       Copyright (c) 1997-2013 University of Cambridge.



PCRE 8.34                      12 November 2013              pcreprecompile(3)

pcre 8.34 - Generated Sun Jan 12 19:10:51 CST 2014