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Documentation.

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, it is not possible to save and reload the
17 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. If the two hosts have different endianness (byte order),
21 you should run the \fBpcre[16]_pattern_to_host_byte_order()\fP function on the
22 new host before trying to match the pattern. The matching functions return
23 PCRE_ERROR_BADENDIANNESS if they detect a pattern with the wrong endianness.
24 .P
25 Compiling regular expressions with one version of PCRE for use with a different
26 version is not guaranteed to work and may cause crashes, and saving and
27 restoring a compiled pattern loses any JIT optimization data.
28 .
29 .
30 .SH "SAVING A COMPILED PATTERN"
31 .rs
32 .sp
33 The value returned by \fBpcre[16]_compile()\fP points to a single block of
34 memory that holds the compiled pattern and associated data. You can find the
35 length of this block in bytes by calling \fBpcre[16]_fullinfo()\fP with an
36 argument of PCRE_INFO_SIZE. You can then save the data in any appropriate
37 manner. Here is sample code for the 8-bit library that compiles a pattern and
38 writes it to a file. It assumes that the variable \fIfd\fP refers to a file
39 that is open for output:
40 .sp
41 int erroroffset, rc, size;
42 char *error;
43 pcre *re;
44 .sp
45 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
46 if (re == NULL) { ... handle errors ... }
47 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
48 if (rc < 0) { ... handle errors ... }
49 rc = fwrite(re, 1, size, fd);
50 if (rc != size) { ... handle errors ... }
51 .sp
52 In this example, the bytes that comprise the compiled pattern are copied
53 exactly. Note that this is binary data that may contain any of the 256 possible
54 byte values. On systems that make a distinction between binary and non-binary
55 data, be sure that the file is opened for binary output.
56 .P
57 If you want to write more than one pattern to a file, you will have to devise a
58 way of separating them. For binary data, preceding each pattern with its length
59 is probably the most straightforward approach. Another possibility is to write
60 out the data in hexadecimal instead of binary, one pattern to a line.
61 .P
62 Saving compiled patterns in a file is only one possible way of storing them for
63 later use. They could equally well be saved in a database, or in the memory of
64 some daemon process that passes them via sockets to the processes that want
65 them.
66 .P
67 If the pattern has been studied, it is also possible to save the normal study
68 data in a similar way to the compiled pattern itself. However, if the
69 PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is created cannot
70 be saved because it is too dependent on the current environment. When studying
71 generates additional information, \fBpcre[16]_study()\fP returns a pointer to a
72 \fBpcre[16]_extra\fP data block. Its format is defined in the
73 .\" HTML <a href="pcreapi.html#extradata">
74 .\" </a>
75 section on matching a pattern
76 .\"
77 in the
78 .\" HREF
79 \fBpcreapi\fP
80 .\"
81 documentation. The \fIstudy_data\fP field points to the binary study data, and
82 this is what you must save (not the \fBpcre[16]_extra\fP block itself). The
83 length of the study data can be obtained by calling \fBpcre[16]_fullinfo()\fP
84 with an argument of PCRE_INFO_STUDYSIZE. Remember to check that
85 \fBpcre[16]_study()\fP did return a non-NULL value before trying to save the
86 study data.
87 .
88 .
89 .SH "RE-USING A PRECOMPILED PATTERN"
90 .rs
91 .sp
92 Re-using a precompiled pattern is straightforward. Having reloaded it into main
93 memory, called \fBpcre[16]_pattern_to_host_byte_order()\fP if necessary,
94 you pass its pointer to \fBpcre[16]_exec()\fP or \fBpcre[16]_dfa_exec()\fP in
95 the usual way.
96 .P
97 However, if you passed a pointer to custom character tables when the pattern
98 was compiled (the \fItableptr\fP argument of \fBpcre[16]_compile()\fP), you
99 must now pass a similar pointer to \fBpcre[16]_exec()\fP or
100 \fBpcre[16]_dfa_exec()\fP, because the value saved with the compiled pattern
101 will obviously be nonsense. A field in a \fBpcre[16]_extra()\fP block is used
102 to pass this data, as described in the
103 .\" HTML <a href="pcreapi.html#extradata">
104 .\" </a>
105 section on matching a pattern
106 .\"
107 in the
108 .\" HREF
109 \fBpcreapi\fP
110 .\"
111 documentation.
112 .P
113 If you did not provide custom character tables when the pattern was compiled,
114 the pointer in the compiled pattern is NULL, which causes the matching
115 functions to use PCRE's internal tables. Thus, you do not need to take any
116 special action at run time in this case.
117 .P
118 If you saved study data with the compiled pattern, you need to create your own
119 \fBpcre[16]_extra\fP data block and set the \fIstudy_data\fP field to point to the
120 reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA bit in the
121 \fIflags\fP field to indicate that study data is present. Then pass the
122 \fBpcre[16]_extra\fP block to the matching function in the usual way. If the
123 pattern was studied for just-in-time optimization, that data cannot be saved,
124 and so is lost by a save/restore cycle.
125 .
126 .
127 .SH "COMPATIBILITY WITH DIFFERENT PCRE RELEASES"
128 .rs
129 .sp
130 In general, it is safest to recompile all saved patterns when you update to a
131 new PCRE release, though not all updates actually require this.
132 .
133 .
134 .
135 .SH AUTHOR
136 .rs
137 .sp
138 .nf
139 Philip Hazel
140 University Computing Service
141 Cambridge CB2 3QH, England.
142 .fi
143 .
144 .
145 .SH REVISION
146 .rs
147 .sp
148 .nf
149 Last updated: 10 January 2012
150 Copyright (c) 1997-2012 University of Cambridge.
151 .fi

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