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Sun Sep 11 14:31:21 2011 UTC (3 years ago) by ph10
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Final source and document tidies for 8.20-RC1.

1 .TH PCREPRECOMPILE 3
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH "SAVING AND RE-USING PRECOMPILED PCRE PATTERNS"
5 .rs
6 .sp
7 If you are running an application that uses a large number of regular
8 expression patterns, it may be useful to store them in a precompiled form
9 instead of having to compile them every time the application is run.
10 If you are not using any private character tables (see the
11 .\" HREF
12 \fBpcre_maketables()\fP
13 .\"
14 documentation), this is relatively straightforward. If you are using private
15 tables, it is a little bit more complicated. However, if you are using the
16 just-in-time optimization feature of \fBpcre_study()\fP, it is not possible to
17 save and reload the JIT data.
18 .P
19 If you save compiled patterns to a file, you can copy them to a different host
20 and run them there. This works even if the new host has the opposite endianness
21 to the one on which the patterns were compiled. There may be a small
22 performance penalty, but it should be insignificant. However, compiling regular
23 expressions with one version of PCRE for use with a different version is not
24 guaranteed to work and may cause crashes, and saving and restoring a compiled
25 pattern loses any JIT optimization data.
26 .
27 .
28 .SH "SAVING A COMPILED PATTERN"
29 .rs
30 .sp
31 The value returned by \fBpcre_compile()\fP points to a single block of memory
32 that holds the compiled pattern and associated data. You can find the length of
33 this block in bytes by calling \fBpcre_fullinfo()\fP with an argument of
34 PCRE_INFO_SIZE. You can then save the data in any appropriate manner. Here is
35 sample code that compiles a pattern and writes it to a file. It assumes that
36 the variable \fIfd\fP refers to a file that is open for output:
37 .sp
38 int erroroffset, rc, size;
39 char *error;
40 pcre *re;
41 .sp
42 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
43 if (re == NULL) { ... handle errors ... }
44 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
45 if (rc < 0) { ... handle errors ... }
46 rc = fwrite(re, 1, size, fd);
47 if (rc != size) { ... handle errors ... }
48 .sp
49 In this example, the bytes that comprise the compiled pattern are copied
50 exactly. Note that this is binary data that may contain any of the 256 possible
51 byte values. On systems that make a distinction between binary and non-binary
52 data, be sure that the file is opened for binary output.
53 .P
54 If you want to write more than one pattern to a file, you will have to devise a
55 way of separating them. For binary data, preceding each pattern with its length
56 is probably the most straightforward approach. Another possibility is to write
57 out the data in hexadecimal instead of binary, one pattern to a line.
58 .P
59 Saving compiled patterns in a file is only one possible way of storing them for
60 later use. They could equally well be saved in a database, or in the memory of
61 some daemon process that passes them via sockets to the processes that want
62 them.
63 .P
64 If the pattern has been studied, it is also possible to save the normal study
65 data in a similar way to the compiled pattern itself. However, if the
66 PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is created cannot
67 be saved because it is too dependent on the current environment. When studying
68 generates additional information, \fBpcre_study()\fP returns a pointer to a
69 \fBpcre_extra\fP data block. Its format is defined in the
70 .\" HTML <a href="pcreapi.html#extradata">
71 .\" </a>
72 section on matching a pattern
73 .\"
74 in the
75 .\" HREF
76 \fBpcreapi\fP
77 .\"
78 documentation. The \fIstudy_data\fP field points to the binary study data, and
79 this is what you must save (not the \fBpcre_extra\fP block itself). The length
80 of the study data can be obtained by calling \fBpcre_fullinfo()\fP with an
81 argument of PCRE_INFO_STUDYSIZE. Remember to check that \fBpcre_study()\fP did
82 return a non-NULL value before trying to save the study data.
83 .
84 .
85 .SH "RE-USING A PRECOMPILED PATTERN"
86 .rs
87 .sp
88 Re-using a precompiled pattern is straightforward. Having reloaded it into main
89 memory, you pass its pointer to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP in
90 the usual way. This should work even on another host, and even if that host has
91 the opposite endianness to the one where the pattern was compiled.
92 .P
93 However, if you passed a pointer to custom character tables when the pattern
94 was compiled (the \fItableptr\fP argument of \fBpcre_compile()\fP), you must
95 now pass a similar pointer to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP,
96 because the value saved with the compiled pattern will obviously be nonsense. A
97 field in a \fBpcre_extra()\fP block is used to pass this data, as described in
98 the
99 .\" HTML <a href="pcreapi.html#extradata">
100 .\" </a>
101 section on matching a pattern
102 .\"
103 in the
104 .\" HREF
105 \fBpcreapi\fP
106 .\"
107 documentation.
108 .P
109 If you did not provide custom character tables when the pattern was compiled,
110 the pointer in the compiled pattern is NULL, which causes \fBpcre_exec()\fP to
111 use PCRE's internal tables. Thus, you do not need to take any special action at
112 run time in this case.
113 .P
114 If you saved study data with the compiled pattern, you need to create your own
115 \fBpcre_extra\fP data block and set the \fIstudy_data\fP field to point to the
116 reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA bit in the
117 \fIflags\fP field to indicate that study data is present. Then pass the
118 \fBpcre_extra\fP block to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP in the
119 usual way. If the pattern was studied for just-in-time optimization, that data
120 cannot be saved, and so is lost by a save/restore cycle.
121 .
122 .
123 .SH "COMPATIBILITY WITH DIFFERENT PCRE RELEASES"
124 .rs
125 .sp
126 In general, it is safest to recompile all saved patterns when you update to a
127 new PCRE release, though not all updates actually require this.
128 .
129 .
130 .
131 .SH AUTHOR
132 .rs
133 .sp
134 .nf
135 Philip Hazel
136 University Computing Service
137 Cambridge CB2 3QH, England.
138 .fi
139 .
140 .
141 .SH REVISION
142 .rs
143 .sp
144 .nf
145 Last updated: 26 August 2011
146 Copyright (c) 1997-2011 University of Cambridge.
147 .fi

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