pcreprecompile man page on FreeBSD

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       PCRE - Perl-compatible regular expressions


       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 differ‐
       ent  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 insignifi‐
       cant. However, compiling regular expressions with one version  of  PCRE
       for  use	 with  a  different  version is not guaranteed to work and may
       cause crashes.


       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	 argu‐
       ment  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

	 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 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 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 documen‐

       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.


       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: 17 November 2010
       Copyright (c) 1997-2010 University of Cambridge.


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