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Contents of /code/tags/pcre-7.4/doc/pcreprecompile.3

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Tag 7.4
3 PCRE - Perl-compatible regular expressions
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.
16 .P
17 If you save compiled patterns to a file, you can copy them to a different host
18 and run them there. This works even if the new host has the opposite endianness
19 to the one on which the patterns were compiled. There may be a small
20 performance penalty, but it should be insignificant. However, compiling regular
21 expressions with one version of PCRE for use with a different version is not
22 guaranteed to work and may cause crashes.
23 .
24 .
26 .rs
27 .sh
28 The value returned by \fBpcre_compile()\fP points to a single block of memory
29 that holds the compiled pattern and associated data. You can find the length of
30 this block in bytes by calling \fBpcre_fullinfo()\fP with an argument of
31 PCRE_INFO_SIZE. You can then save the data in any appropriate manner. Here is
32 sample code that compiles a pattern and writes it to a file. It assumes that
33 the variable \fIfd\fP refers to a file that is open for output:
34 .sp
35 int erroroffset, rc, size;
36 char *error;
37 pcre *re;
38 .sp
39 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
40 if (re == NULL) { ... handle errors ... }
41 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
42 if (rc < 0) { ... handle errors ... }
43 rc = fwrite(re, 1, size, fd);
44 if (rc != size) { ... handle errors ... }
45 .sp
46 In this example, the bytes that comprise the compiled pattern are copied
47 exactly. Note that this is binary data that may contain any of the 256 possible
48 byte values. On systems that make a distinction between binary and non-binary
49 data, be sure that the file is opened for binary output.
50 .P
51 If you want to write more than one pattern to a file, you will have to devise a
52 way of separating them. For binary data, preceding each pattern with its length
53 is probably the most straightforward approach. Another possibility is to write
54 out the data in hexadecimal instead of binary, one pattern to a line.
55 .P
56 Saving compiled patterns in a file is only one possible way of storing them for
57 later use. They could equally well be saved in a database, or in the memory of
58 some daemon process that passes them via sockets to the processes that want
59 them.
60 .P
61 If the pattern has been studied, it is also possible to save the study data in
62 a similar way to the compiled pattern itself. When studying generates
63 additional information, \fBpcre_study()\fP returns a pointer to a
64 \fBpcre_extra\fP data block. Its format is defined in the
65 .\" HTML <a href="pcreapi.html#extradata">
66 .\" </a>
67 section on matching a pattern
68 .\"
69 in the
70 .\" HREF
71 \fBpcreapi\fP
72 .\"
73 documentation. The \fIstudy_data\fP field points to the binary study data, and
74 this is what you must save (not the \fBpcre_extra\fP block itself). The length
75 of the study data can be obtained by calling \fBpcre_fullinfo()\fP with an
76 argument of PCRE_INFO_STUDYSIZE. Remember to check that \fBpcre_study()\fP did
77 return a non-NULL value before trying to save the study data.
78 .
79 .
81 .rs
82 .sp
83 Re-using a precompiled pattern is straightforward. Having reloaded it into main
84 memory, you pass its pointer to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP in
85 the usual way. This should work even on another host, and even if that host has
86 the opposite endianness to the one where the pattern was compiled.
87 .P
88 However, if you passed a pointer to custom character tables when the pattern
89 was compiled (the \fItableptr\fP argument of \fBpcre_compile()\fP), you must
90 now pass a similar pointer to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP,
91 because the value saved with the compiled pattern will obviously be nonsense. A
92 field in a \fBpcre_extra()\fP block is used to pass this data, as described in
93 the
94 .\" HTML <a href="pcreapi.html#extradata">
95 .\" </a>
96 section on matching a pattern
97 .\"
98 in the
99 .\" HREF
100 \fBpcreapi\fP
101 .\"
102 documentation.
103 .P
104 If you did not provide custom character tables when the pattern was compiled,
105 the pointer in the compiled pattern is NULL, which causes \fBpcre_exec()\fP to
106 use PCRE's internal tables. Thus, you do not need to take any special action at
107 run time in this case.
108 .P
109 If you saved study data with the compiled pattern, you need to create your own
110 \fBpcre_extra\fP data block and set the \fIstudy_data\fP field to point to the
111 reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA bit in the
112 \fIflags\fP field to indicate that study data is present. Then pass the
113 \fBpcre_extra\fP block to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP in the
114 usual way.
115 .
116 .
118 .rs
119 .sp
120 In general, it is safest to recompile all saved patterns when you update to a
121 new PCRE release, though not all updates actually require this. Recompiling is
122 definitely needed for release 7.2.
123 .
124 .
125 .
127 .rs
128 .sp
129 .nf
130 Philip Hazel
131 University Computing Service
132 Cambridge CB2 3QH, England.
133 .fi
134 .
135 .
137 .rs
138 .sp
139 .nf
140 Last updated: 13 June 2007
141 Copyright (c) 1997-2007 University of Cambridge.
142 .fi


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