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32 .\" @(#)regexp.3 8.1 (Berkeley) 6/4/93
33 .\" $FreeBSD: src/lib/libcompat/regexp/regexp.3,v 1.6.2.2 2001/12/17 10:08:29 ru Exp $
43 .Nd regular expression handlers
49 .Fn regcomp "const char *exp"
51 .Fn regexec "const regexp *prog" "const char *string"
53 .Fn regsub "const regexp *prog" "const char *source" "char *dest"
56 This interface is made obsolete by
69 regular expressions and supporting facilities.
74 compiles a regular expression into a structure of type
76 and returns a pointer to it.
77 The space has been allocated using
79 and may be released by
86 .Dv NUL Ns -terminated
88 against the compiled regular expression
91 It returns 1 for success and 0 for failure, and adjusts the contents of
96 (see below) accordingly.
100 structure include at least the following (not necessarily in order):
101 .Bd -literal -offset indent
102 char *startp[NSUBEXP];
108 is defined (as 10) in the header file.
111 has been done using the
114 .Em startp Ns - Em endp
115 pair describes one substring
120 pointing to the first character of the substring and
123 pointing to the first character following the substring.
124 The 0th substring is the substring of
126 that matched the whole
128 The others are those substrings that matched parenthesized expressions
129 within the regular expression, with parenthesized expressions numbered
130 in left-to-right order of their opening parentheses.
139 making substitutions according to the
144 Each instance of `&' in
146 is replaced by the substring
155 is a digit, is replaced by
156 the substring indicated by
157 .Em startp Ns Bq Em n
159 .Em endp Ns Bq Em n .
160 To get a literal `&' or
165 to get a literal `\e' preceding `&' or
173 is called whenever an error is detected in
182 with a suitable indicator of origin,
190 can be replaced by the user if other actions are desirable.
191 .Sh REGULAR EXPRESSION SYNTAX
192 A regular expression is zero or more
195 It matches anything that matches one of the branches.
197 A branch is zero or more
200 It matches a match for the first, followed by a match for the second, etc.
204 possibly followed by `*', `+', or `?'.
205 An atom followed by `*' matches a sequence of 0 or more matches of the atom.
206 An atom followed by `+' matches a sequence of 1 or more matches of the atom.
207 An atom followed by `?' matches a match of the atom, or the null string.
209 An atom is a regular expression in parentheses (matching a match for the
210 regular expression), a
213 (matching any single character), `^' (matching the null string at the
214 beginning of the input string), `$' (matching the null string at the
215 end of the input string), a `\e' followed by a single character (matching
216 that character), or a single character with no other significance
217 (matching that character).
221 is a sequence of characters enclosed in `[]'.
222 It normally matches any single character from the sequence.
223 If the sequence begins with `^',
224 it matches any single character
226 from the rest of the sequence.
227 If two characters in the sequence are separated by `\-', this is shorthand
230 characters between them
231 (e.g. `[0-9]' matches any decimal digit).
232 To include a literal `]' in the sequence, make it the first character
233 (following a possible `^').
234 To include a literal `\-', make it the first or last character.
236 If a regular expression could match two different parts of the input string,
237 it will match the one which begins earliest.
238 If both begin in the same place but match different lengths, or match
239 the same length in different ways, life gets messier, as follows.
241 In general, the possibilities in a list of branches are considered in
242 left-to-right order, the possibilities for `*', `+', and `?' are
243 considered longest-first, nested constructs are considered from the
244 outermost in, and concatenated constructs are considered leftmost-first.
245 The match that will be chosen is the one that uses the earliest
246 possibility in the first choice that has to be made.
247 If there is more than one choice, the next will be made in the same manner
248 (earliest possibility) subject to the decision on the first choice.
254 `abc' in one of two ways.
255 The first choice is between `ab' and `a'; since `ab' is earlier, and does
256 lead to a successful overall match, it is chosen.
257 Since the `b' is already spoken for,
258 the `b*' must match its last possibility\(emthe empty string\(emsince
259 it must respect the earlier choice.
261 In the particular case where no `|'s are present and there is only one
262 `*', `+', or `?', the net effect is that the longest possible
263 match will be chosen.
266 presented with `xabbbby', will match `abbbb'.
269 is tried against `xabyabbbz', it
270 will match `ab' just after `x', due to the begins-earliest rule.
271 (In effect, the decision on where to start the match is the first choice
272 to be made, hence subsequent choices must respect it even if this leads them
273 to less-preferred alternatives.)
283 where failures are syntax errors, exceeding implementation limits,
284 or applying `+' or `*' to a possibly-null operand.
294 Both code and manual page for
300 were written at the University of Toronto
303 They are intended to be compatible with the Bell V8
305 but are not derived from Bell code.
307 Empty branches and empty regular expressions are not portable to V8.
309 The restriction against
310 applying `*' or `+' to a possibly-null operand is an artifact of the
311 simplistic implementation.
315 newline-separated branches;
321 compactness and simplicity,
322 it's not strikingly fast.
323 It does give special attention to handling simple cases quickly.