pcreprecompile(3) — Linux manual page


PCREPRECOMPILE(3)       Library Functions Manual       PCREPRECOMPILE(3)

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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. 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
       different host and run them there. If the two hosts have
       different endianness (byte order), you should run the
       pcre[16|32]_pattern_to_host_byte_order() function on the new host
       before trying to match the pattern. The matching functions return
       PCRE_ERROR_BADENDIANNESS if they detect a pattern with the wrong

       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.


       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 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
       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 created cannot be saved because it is
       too dependent on the current environment. 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.
       Remember 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 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 pattern 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

       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


       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         top

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

REVISION         top

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

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular
       Expressions) project.  Information about the project can be found
       at ⟨http://www.pcre.org/⟩.  If you have a bug report for this
       manual page, see
       ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This page was
       obtained from the tarball pcre-8.45.tar.gz fetched from
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       2021-08-27.  If you discover any rendering problems in this HTML
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PCRE 8.34                   12 November 2013           PCREPRECOMPILE(3)

Pages that refer to this page: pcretest(1)pcreapi(3)