pcreprecompile man page on Hurd

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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]_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
       data.

SAVING A COMPILED PATTERN

       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.
       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]_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

       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.

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: 10 January 2012
       Copyright (c) 1997-2012 University of Cambridge.

PCRE 8.30			10 January 2012		     PCREPRECOMPILE(3)
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