Introduction to Library Functions PCREPRECOMPILE(3)
NAME
PCRE - Perl-compatible regular expressions
SAVING AND RE-USING PRECOMPILED PCRE PATERNS
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 pcremaketables() documen-
tation), 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 PATERN
The value returned by pcrecompile() points to a single
block of memory that holds the compiled pattern and associ-
ated data. You can find the length of this block in bytes by
calling pcrefullinfo() with an argument of PCREINFOSIZE.
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 = pcrecompile("my pattern", 0, &error, &erroroffset,
NUL);
if (re == NUL) { ... handle errors ... }
rc = pcrefullinfo(re, NUL, PCREINFOSIZE, &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 pat-
tern 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
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Introduction to Library Functions PCREPRECOMPILE(3)
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,
pcrestudy() returns a pointer to a pcreextra data block.
Its format is defined in the section on matching a pattern
in the pcreapi documentation. The studydata field points to
the binary study data, and this is what you must save (not
the pcreextra block itself). The length of the study data
can be obtained by calling pcrefullinfo() with an argument
of PCREINFOSTUDYSIZE. Remember to check that pcrestudy()
did return a non-NUL value before trying to save the study
data.
RE-USING A PRECOMPILED PATERN
Re-using a precompiled pattern is straightforward. Having
reloaded it into main memory, you pass its pointer to
pcreexec() or pcredfaexec() 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 com-
piled.
However, if you passed a pointer to custom character tables
when the pattern was compiled (the tableptr argument of
pcrecompile()), you must now pass a similar pointer to
pcreexec() or pcredfaexec(), because the value saved with
the compiled pattern will obviously be nonsense. A field in
a pcreextra() block is used to pass this data, as described
in the section on matching a pattern in the pcreapi documen-
tation.
If you did not provide custom character tables when the pat-
tern was compiled, the pointer in the compiled pattern is
NUL, which causes pcreexec() 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 pcreextra data block and set the
studydata field to point to the reloaded study data. You
must also set the PCREXTRASTUDYDATA bit in the flags
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Introduction to Library Functions PCREPRECOMPILE(3)
field to indicate that study data is present. Then pass the
pcreextra block to pcreexec() or pcredfaexec() in the
usual way.
COMPATIBILITY WITH DIFERENT 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.
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWpcre
Interface Stability Uncommitted
NOTES
Source for PCRE is available on http:/opensolaris.org.
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