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X11R7.4 - man page for pcreprecompile (x11r4 section 3)

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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.

       If  you	save  compiled	patterns to a file, you can copy them to a different host and run
       them there. This works even if the new host has the opposite  endianness  to  the  one  on
       which  the patterns were compiled. There may be a small performance penalty, but it should
       be insignificant. However, compiling regular expressions with one version of PCRE for  use
       with a different version is not guaranteed to work and may cause crashes.

SAVING A COMPILED PATTERN
       The  value  returned  by  pcre_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_fullinfo() with an argument of PCRE_INFO_SIZE. You can then save the data in
       any appropriate manner. Here is sample code 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 pattern 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 study data in a similar
       way to the compiled  pattern  itself.  When  studying  generates  additional  information,
       pcre_study()  returns  a  pointer to a pcre_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_extra block itself).
       The length of the study data can be obtained by calling pcre_fullinfo() with  an  argument
       of  PCRE_INFO_STUDYSIZE.  Remember  to check that pcre_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,
       you  pass its pointer to pcre_exec() or pcre_dfa_exec() in the usual way. This should work
       even on another host, and even if that host has the opposite endianness to the  one  where
       the pattern was compiled.

       However,  if you passed a pointer to custom character tables when the pattern was compiled
       (the tableptr argument of  pcre_compile()),  you  must  now  pass  a  similar  pointer  to
       pcre_exec()  or	pcre_dfa_exec(),  because  the value saved with the compiled pattern will
       obviously be nonsense. A field in a pcre_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 pcre_exec() 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_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_extra block to pcre_exec() or  pcre_dfa_exec()  in  the  usual
       way.

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. Recompiling  is  definitely  needed
       for release 7.2.

AUTHOR

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

REVISION

       Last updated: 13 June 2007
       Copyright (c) 1997-2007 University of Cambridge.

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