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.