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28 .\" @(#)regexp.3 8.1 (Berkeley) 6/4/93
39 .Nd regular expression handlers
45 .Fn regcomp "const char *exp"
47 .Fn regexec "const regexp *prog" "const char *string"
49 .Fn regsub "const regexp *prog" "const char *source" "char *dest"
52 This interface is made obsolete by
65 regular expressions and supporting facilities.
70 compiles a regular expression into a structure of type
72 and returns a pointer to it.
73 The space has been allocated using
75 and may be released by
82 .Dv NUL Ns -terminated
84 against the compiled regular expression
87 It returns 1 for success and 0 for failure, and adjusts the contents of
92 (see below) accordingly.
96 structure include at least the following (not necessarily in order):
97 .Bd -literal -offset indent
98 char *startp[NSUBEXP];
104 is defined (as 10) in the header file.
107 has been done using the
110 .Em startp Ns - Em endp
111 pair describes one substring
116 pointing to the first character of the substring and
119 pointing to the first character following the substring.
120 The 0th substring is the substring of
122 that matched the whole
124 The others are those substrings that matched parenthesized expressions
125 within the regular expression, with parenthesized expressions numbered
126 in left-to-right order of their opening parentheses.
135 making substitutions according to the
140 Each instance of `&' in
142 is replaced by the substring
151 is a digit, is replaced by
152 the substring indicated by
153 .Em startp Ns Bq Em n
155 .Em endp Ns Bq Em n .
156 To get a literal `&' or
161 to get a literal `\e' preceding `&' or
169 is called whenever an error is detected in
178 with a suitable indicator of origin,
186 can be replaced by the user if other actions are desirable.
187 .Sh REGULAR EXPRESSION SYNTAX
188 A regular expression is zero or more
191 It matches anything that matches one of the branches.
193 A branch is zero or more
196 It matches a match for the first, followed by a match for the second, etc.
200 possibly followed by `*', `+', or `?'.
201 An atom followed by `*' matches a sequence of 0 or more matches of the atom.
202 An atom followed by `+' matches a sequence of 1 or more matches of the atom.
203 An atom followed by `?' matches a match of the atom, or the null string.
205 An atom is a regular expression in parentheses (matching a match for the
206 regular expression), a
209 (matching any single character), `^' (matching the null string at the
210 beginning of the input string), `$' (matching the null string at the
211 end of the input string), a `\e' followed by a single character (matching
212 that character), or a single character with no other significance
213 (matching that character).
217 is a sequence of characters enclosed in `[]'.
218 It normally matches any single character from the sequence.
219 If the sequence begins with `^',
220 it matches any single character
222 from the rest of the sequence.
223 If two characters in the sequence are separated by `\-', this is shorthand
226 characters between them
227 (e.g.\& `[0-9]' matches any decimal digit).
228 To include a literal `]' in the sequence, make it the first character
229 (following a possible `^').
230 To include a literal `\-', make it the first or last character.
232 If a regular expression could match two different parts of the input string,
233 it will match the one which begins earliest.
234 If both begin in the same place but match different lengths, or match
235 the same length in different ways, life gets messier, as follows.
237 In general, the possibilities in a list of branches are considered in
238 left-to-right order, the possibilities for `*', `+', and `?' are
239 considered longest-first, nested constructs are considered from the
240 outermost in, and concatenated constructs are considered leftmost-first.
241 The match that will be chosen is the one that uses the earliest
242 possibility in the first choice that has to be made.
243 If there is more than one choice, the next will be made in the same manner
244 (earliest possibility) subject to the decision on the first choice.
250 `abc' in one of two ways.
251 The first choice is between `ab' and `a'; since `ab' is earlier, and does
252 lead to a successful overall match, it is chosen.
253 Since the `b' is already spoken for,
254 the `b*' must match its last possibility\(emthe empty string\(emsince
255 it must respect the earlier choice.
257 In the particular case where no `|'s are present and there is only one
258 `*', `+', or `?', the net effect is that the longest possible
259 match will be chosen.
262 presented with `xabbbby', will match `abbbb'.
265 is tried against `xabyabbbz', it
266 will match `ab' just after `x', due to the begins-earliest rule.
267 (In effect, the decision on where to start the match is the first choice
268 to be made, hence subsequent choices must respect it even if this leads them
269 to less-preferred alternatives.)
279 where failures are syntax errors, exceeding implementation limits,
280 or applying `+' or `*' to a possibly-null operand.
290 Both code and manual page for
296 were written at the University of Toronto
299 They are intended to be compatible with the Bell V8
301 but are not derived from Bell code.
303 Empty branches and empty regular expressions are not portable to V8.
305 The restriction against
306 applying `*' or `+' to a possibly-null operand is an artifact of the
307 simplistic implementation.
311 newline-separated branches;
317 compactness and simplicity,
318 it is not strikingly fast.
319 It does give special attention to handling simple cases quickly.