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3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 This document describes the optional features of PCRE that can be selected when
8 the library is compiled. They are all selected, or deselected, by providing
9 options to the \fBconfigure\fP script that is run before the \fBmake\fP
10 command. The complete list of options for \fBconfigure\fP (which includes the
11 standard ones such as the selection of the installation directory) can be
12 obtained by running
13 .sp
14 ./configure --help
15 .sp
16 The following sections describe certain options whose names begin with --enable
17 or --disable. These settings specify changes to the defaults for the
18 \fBconfigure\fP command. Because of the way that \fBconfigure\fP works,
19 --enable and --disable always come in pairs, so the complementary option always
20 exists as well, but as it specifies the default, it is not described.
21 .
23 .rs
24 .sp
25 To build PCRE with support for UTF-8 character strings, add
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27 --enable-utf8
28 .sp
29 to the \fBconfigure\fP command. Of itself, this does not make PCRE treat
30 strings as UTF-8. As well as compiling PCRE with this option, you also have
31 have to set the PCRE_UTF8 option when you call the \fBpcre_compile()\fP
32 function.
33 .
35 .rs
36 .sp
37 UTF-8 support allows PCRE to process character values greater than 255 in the
38 strings that it handles. On its own, however, it does not provide any
39 facilities for accessing the properties of such characters. If you want to be
40 able to use the pattern escapes \eP, \ep, and \eX, which refer to Unicode
41 character properties, you must add
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43 --enable-unicode-properties
44 .sp
45 to the \fBconfigure\fP command. This implies UTF-8 support, even if you have
46 not explicitly requested it.
47 .P
48 Including Unicode property support adds around 90K of tables to the PCRE
49 library, approximately doubling its size. Only the general category properties
50 such as \fILu\fP and \fINd\fP are supported. Details are given in the
51 .\" HREF
52 \fBpcrepattern\fP
53 .\"
54 documentation.
55 .
57 .rs
58 .sp
59 By default, PCRE treats character 10 (linefeed) as the newline character. This
60 is the normal newline character on Unix-like systems. You can compile PCRE to
61 use character 13 (carriage return) instead by adding
62 .sp
63 --enable-newline-is-cr
64 .sp
65 to the \fBconfigure\fP command. For completeness there is also a
66 --enable-newline-is-lf option, which explicitly specifies linefeed as the
67 newline character.
68 .
70 .rs
71 .sp
72 The PCRE building process uses \fBlibtool\fP to build both shared and static
73 Unix libraries by default. You can suppress one of these by adding one of
74 .sp
75 --disable-shared
76 --disable-static
77 .sp
78 to the \fBconfigure\fP command, as required.
79 .
81 .rs
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83 When PCRE is called through the POSIX interface (see the
84 .\" HREF
85 \fBpcreposix\fP
86 .\"
87 documentation), additional working storage is required for holding the pointers
88 to capturing substrings, because PCRE requires three integers per substring,
89 whereas the POSIX interface provides only two. If the number of expected
90 substrings is small, the wrapper function uses space on the stack, because this
91 is faster than using \fBmalloc()\fP for each call. The default threshold above
92 which the stack is no longer used is 10; it can be changed by adding a setting
93 such as
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95 --with-posix-malloc-threshold=20
96 .sp
97 to the \fBconfigure\fP command.
98 .
100 .rs
101 .sp
102 Internally, PCRE has a function called \fBmatch()\fP, which it calls repeatedly
103 (possibly recursively) when matching a pattern with the \fBpcre_exec()\fP
104 function. By controlling the maximum number of times this function may be
105 called during a single matching operation, a limit can be placed on the
106 resources used by a single call to \fBpcre_exec()\fP. The limit can be changed
107 at run time, as described in the
108 .\" HREF
109 \fBpcreapi\fP
110 .\"
111 documentation. The default is 10 million, but this can be changed by adding a
112 setting such as
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114 --with-match-limit=500000
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116 to the \fBconfigure\fP command. This setting has no effect on the
117 \fBpcre_dfa_exec()\fP matching function.
118 .
120 .rs
121 .sp
122 Within a compiled pattern, offset values are used to point from one part to
123 another (for example, from an opening parenthesis to an alternation
124 metacharacter). By default, two-byte values are used for these offsets, leading
125 to a maximum size for a compiled pattern of around 64K. This is sufficient to
126 handle all but the most gigantic patterns. Nevertheless, some people do want to
127 process enormous patterns, so it is possible to compile PCRE to use three-byte
128 or four-byte offsets by adding a setting such as
129 .sp
130 --with-link-size=3
131 .sp
132 to the \fBconfigure\fP command. The value given must be 2, 3, or 4. Using
133 longer offsets slows down the operation of PCRE because it has to load
134 additional bytes when handling them.
135 .P
136 If you build PCRE with an increased link size, test 2 (and test 5 if you are
137 using UTF-8) will fail. Part of the output of these tests is a representation
138 of the compiled pattern, and this changes with the link size.
139 .
141 .rs
142 .sp
143 When matching with the \fBpcre_exec()\fP function, PCRE implements backtracking
144 by making recursive calls to an internal function called \fBmatch()\fP. In
145 environments where the size of the stack is limited, this can severely limit
146 PCRE's operation. (The Unix environment does not usually suffer from this
147 problem.) An alternative approach that uses memory from the heap to remember
148 data, instead of using recursive function calls, has been implemented to work
149 round this problem. If you want to build a version of PCRE that works this way,
150 add
151 .sp
152 --disable-stack-for-recursion
153 .sp
154 to the \fBconfigure\fP command. With this configuration, PCRE will use the
155 \fBpcre_stack_malloc\fP and \fBpcre_stack_free\fP variables to call memory
156 management functions. Separate functions are provided because the usage is very
157 predictable: the block sizes requested are always the same, and the blocks are
158 always freed in reverse order. A calling program might be able to implement
159 optimized functions that perform better than the standard \fBmalloc()\fP and
160 \fBfree()\fP functions. PCRE runs noticeably more slowly when built in this
161 way. This option affects only the \fBpcre_exec()\fP function; it is not
162 relevant for the the \fBpcre_dfa_exec()\fP function.
163 .
165 .rs
166 .sp
167 PCRE assumes by default that it will run in an environment where the character
168 code is ASCII (or Unicode, which is a superset of ASCII). PCRE can, however, be
169 compiled to run in an EBCDIC environment by adding
170 .sp
171 --enable-ebcdic
172 .sp
173 to the \fBconfigure\fP command.
174 .P
175 .in 0
176 Last updated: 28 February 2005
177 .br
178 Copyright (c) 1997-2005 University of Cambridge.

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