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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 include descriptions of options whose names begin with
17 --enable 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 .
22 .SH "C++ SUPPORT"
23 .rs
24 .sp
25 By default, the \fBconfigure\fP script will search for a C++ compiler and C++
26 header files. If it finds them, it automatically builds the C++ wrapper library
27 for PCRE. You can disable this by adding
28 .sp
29 --disable-cpp
30 .sp
31 to the \fBconfigure\fP command.
32 .
34 .rs
35 .sp
36 To build PCRE with support for UTF-8 character strings, add
37 .sp
38 --enable-utf8
39 .sp
40 to the \fBconfigure\fP command. Of itself, this does not make PCRE treat
41 strings as UTF-8. As well as compiling PCRE with this option, you also have
42 have to set the PCRE_UTF8 option when you call the \fBpcre_compile()\fP
43 function.
44 .
46 .rs
47 .sp
48 UTF-8 support allows PCRE to process character values greater than 255 in the
49 strings that it handles. On its own, however, it does not provide any
50 facilities for accessing the properties of such characters. If you want to be
51 able to use the pattern escapes \eP, \ep, and \eX, which refer to Unicode
52 character properties, you must add
53 .sp
54 --enable-unicode-properties
55 .sp
56 to the \fBconfigure\fP command. This implies UTF-8 support, even if you have
57 not explicitly requested it.
58 .P
59 Including Unicode property support adds around 30K of tables to the PCRE
60 library. Only the general category properties such as \fILu\fP and \fINd\fP are
61 supported. Details are given in the
62 .\" HREF
63 \fBpcrepattern\fP
64 .\"
65 documentation.
66 .
68 .rs
69 .sp
70 By default, PCRE interprets character 10 (linefeed, LF) as indicating the end
71 of a line. This is the normal newline character on Unix-like systems. You can
72 compile PCRE to use character 13 (carriage return, CR) instead, by adding
73 .sp
74 --enable-newline-is-cr
75 .sp
76 to the \fBconfigure\fP command. There is also a --enable-newline-is-lf option,
77 which explicitly specifies linefeed as the newline character.
78 .sp
79 Alternatively, you can specify that line endings are to be indicated by the two
80 character sequence CRLF. If you want this, add
81 .sp
82 --enable-newline-is-crlf
83 .sp
84 to the \fBconfigure\fP command. There is a fourth option, specified by
85 .sp
86 --enable-newline-is-anycrlf
87 .sp
88 which causes PCRE to recognize any of the three sequences CR, LF, or CRLF as
89 indicating a line ending. Finally, a fifth option, specified by
90 .sp
91 --enable-newline-is-any
92 .sp
93 causes PCRE to recognize any Unicode newline sequence.
94 .P
95 Whatever line ending convention is selected when PCRE is built can be
96 overridden when the library functions are called. At build time it is
97 conventional to use the standard for your operating system.
98 .
100 .rs
101 .sp
102 The PCRE building process uses \fBlibtool\fP to build both shared and static
103 Unix libraries by default. You can suppress one of these by adding one of
104 .sp
105 --disable-shared
106 --disable-static
107 .sp
108 to the \fBconfigure\fP command, as required.
109 .
111 .rs
112 .sp
113 When PCRE is called through the POSIX interface (see the
114 .\" HREF
115 \fBpcreposix\fP
116 .\"
117 documentation), additional working storage is required for holding the pointers
118 to capturing substrings, because PCRE requires three integers per substring,
119 whereas the POSIX interface provides only two. If the number of expected
120 substrings is small, the wrapper function uses space on the stack, because this
121 is faster than using \fBmalloc()\fP for each call. The default threshold above
122 which the stack is no longer used is 10; it can be changed by adding a setting
123 such as
124 .sp
125 --with-posix-malloc-threshold=20
126 .sp
127 to the \fBconfigure\fP command.
128 .
130 .rs
131 .sp
132 Within a compiled pattern, offset values are used to point from one part to
133 another (for example, from an opening parenthesis to an alternation
134 metacharacter). By default, two-byte values are used for these offsets, leading
135 to a maximum size for a compiled pattern of around 64K. This is sufficient to
136 handle all but the most gigantic patterns. Nevertheless, some people do want to
137 process enormous patterns, so it is possible to compile PCRE to use three-byte
138 or four-byte offsets by adding a setting such as
139 .sp
140 --with-link-size=3
141 .sp
142 to the \fBconfigure\fP command. The value given must be 2, 3, or 4. Using
143 longer offsets slows down the operation of PCRE because it has to load
144 additional bytes when handling them.
145 .
147 .rs
148 .sp
149 When matching with the \fBpcre_exec()\fP function, PCRE implements backtracking
150 by making recursive calls to an internal function called \fBmatch()\fP. In
151 environments where the size of the stack is limited, this can severely limit
152 PCRE's operation. (The Unix environment does not usually suffer from this
153 problem, but it may sometimes be necessary to increase the maximum stack size.
154 There is a discussion in the
155 .\" HREF
156 \fBpcrestack\fP
157 .\"
158 documentation.) An alternative approach to recursion that uses memory from the
159 heap to remember data, instead of using recursive function calls, has been
160 implemented to work round the problem of limited stack size. If you want to
161 build a version of PCRE that works this way, add
162 .sp
163 --disable-stack-for-recursion
164 .sp
165 to the \fBconfigure\fP command. With this configuration, PCRE will use the
166 \fBpcre_stack_malloc\fP and \fBpcre_stack_free\fP variables to call memory
167 management functions. By default these point to \fBmalloc()\fP and
168 \fBfree()\fP, but you can replace the pointers so that your own functions are
169 used.
170 .P
171 Separate functions are provided rather than using \fBpcre_malloc\fP and
172 \fBpcre_free\fP because the usage is very predictable: the block sizes
173 requested are always the same, and the blocks are always freed in reverse
174 order. A calling program might be able to implement optimized functions that
175 perform better than \fBmalloc()\fP and \fBfree()\fP. PCRE runs noticeably more
176 slowly when built in this way. This option affects only the \fBpcre_exec()\fP
177 function; it is not relevant for the the \fBpcre_dfa_exec()\fP function.
178 .
180 .rs
181 .sp
182 Internally, PCRE has a function called \fBmatch()\fP, which it calls repeatedly
183 (sometimes recursively) when matching a pattern with the \fBpcre_exec()\fP
184 function. By controlling the maximum number of times this function may be
185 called during a single matching operation, a limit can be placed on the
186 resources used by a single call to \fBpcre_exec()\fP. The limit can be changed
187 at run time, as described in the
188 .\" HREF
189 \fBpcreapi\fP
190 .\"
191 documentation. The default is 10 million, but this can be changed by adding a
192 setting such as
193 .sp
194 --with-match-limit=500000
195 .sp
196 to the \fBconfigure\fP command. This setting has no effect on the
197 \fBpcre_dfa_exec()\fP matching function.
198 .P
199 In some environments it is desirable to limit the depth of recursive calls of
200 \fBmatch()\fP more strictly than the total number of calls, in order to
201 restrict the maximum amount of stack (or heap, if --disable-stack-for-recursion
202 is specified) that is used. A second limit controls this; it defaults to the
203 value that is set for --with-match-limit, which imposes no additional
204 constraints. However, you can set a lower limit by adding, for example,
205 .sp
206 --with-match-limit-recursion=10000
207 .sp
208 to the \fBconfigure\fP command. This value can also be overridden at run time.
209 .
211 .rs
212 .sp
213 PCRE uses fixed tables for processing characters whose code values are less
214 than 256. By default, PCRE is built with a set of tables that are distributed
215 in the file \fIpcre_chartables.c.dist\fP. These tables are for ASCII codes
216 only. If you add
217 .sp
218 --enable-rebuild-chartables
219 .sp
220 to the \fBconfigure\fP command, the distributed tables are no longer used.
221 Instead, a program called \fBdftables\fP is compiled and run. This outputs the
222 source for new set of tables, created in the default locale of your C runtime
223 system. (This method of replacing the tables does not work if you are cross
224 compiling, because \fBdftables\fP is run on the local host. If you need to
225 create alternative tables when cross compiling, you will have to do so "by
226 hand".)
227 .
229 .rs
230 .sp
231 PCRE assumes by default that it will run in an environment where the character
232 code is ASCII (or Unicode, which is a superset of ASCII). PCRE can, however, be
233 compiled to run in an EBCDIC environment by adding
234 .sp
235 --enable-ebcdic
236 .sp
237 to the \fBconfigure\fP command. This setting implies
238 --enable-rebuild-chartables.
239 .
240 .
241 .SH "SEE ALSO"
242 .rs
243 .sp
244 \fBpcreapi\fP(3), \fBpcre_config\fP(3).
245 .
246 .
248 .rs
249 .sp
250 .nf
251 Philip Hazel
252 University Computing Service
253 Cambridge CB2 3QH, England.
254 .fi
255 .
256 .
258 .rs
259 .sp
260 .nf
261 Last updated: 05 June 2007
262 Copyright (c) 1997-2007 University of Cambridge.
263 .fi


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