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36 .\" @(#)re_format.7 8.3 (Berkeley) 3/20/94
37 .\" $FreeBSD: src/lib/libc/regex/re_format.7,v 1.4.2.5 2002/01/22 12:40:10 ru Exp $
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45 .Nd POSIX 1003.2 regular expressions
52 modern REs (roughly those of
57 and obsolete REs (roughly those of
62 Obsolete REs mostly exist for backward compatibility in some old programs;
63 they will be discussed at the end.
65 leaves some aspects of RE syntax and semantics open;
66 `\(dd' marks decisions on these aspects that
67 may not be fully portable to other
71 A (modern) RE is one\(dd or more non-empty\(dd
75 It matches anything that matches one of the branches.
77 A branch is one\(dd or more
80 It matches a match for the first, followed by a match for the second, etc.
93 matches a sequence of 0 or more matches of the atom.
96 matches a sequence of 1 or more matches of the atom.
99 matches a sequence of 0 or 1 matches of the atom.
105 followed by an unsigned decimal integer,
108 possibly followed by another unsigned decimal integer,
111 The integers must lie between 0 and
114 and if there are two of them, the first may not exceed the second.
115 An atom followed by a bound containing one integer
118 a sequence of exactly
121 An atom followed by a bound
122 containing one integer
127 or more matches of the atom.
128 An atom followed by a bound
129 containing two integers
138 (inclusive) matches of the atom.
140 An atom is a regular expression enclosed in
142 (matching a match for the
146 (matching the null string)\(dd,
148 .Em bracket expression
151 (matching any single character),
153 (matching the null string at the beginning of a line),
155 (matching the null string at the end of a line), a
157 followed by one of the characters
159 (matching that character taken as an ordinary character),
162 followed by any other character\(dd
163 (matching that character taken as an ordinary character,
166 had not been present\(dd),
167 or a single character with no other significance (matching that character).
170 followed by a character other than a digit is an ordinary
171 character, not the beginning of a bound\(dd.
172 It is illegal to end an RE with
176 .Em bracket expression
177 is a list of characters enclosed in
179 It normally matches any single character from the list (but see below).
180 If the list begins with
182 it matches any single character
185 from the rest of the list.
186 If two characters in the list are separated by
191 of characters between those two (inclusive) in the
194 in ASCII matches any decimal digit.
195 It is illegal\(dd for two ranges to share an
198 Ranges are very collating-sequence-dependent,
199 and portable programs should avoid relying on them.
203 in the list, make it the first character
204 (following a possible
208 make it the first or last character,
209 or the second endpoint of a range.
212 as the first endpoint of a range,
217 to make it a collating element (see below).
218 With the exception of these and some combinations using
220 (see next paragraphs), all other special characters, including
222 lose their special significance within a bracket expression.
224 Within a bracket expression, a collating element (a character,
225 a multi-character sequence that collates as if it were a single character,
226 or a collating-sequence name for either)
232 sequence of characters of that collating element.
233 The sequence is a single element of the bracket expression's list.
234 A bracket expression containing a multi-character collating element
235 can thus match more than one character,
236 e.g. if the collating sequence includes a
241 matches the first five characters
245 Within a bracket expression, a collating element enclosed in
249 is an equivalence class, standing for the sequences of characters
250 of all collating elements equivalent to that one, including itself.
251 (If there are no other equivalent collating elements,
252 the treatment is as if the enclosing delimiters were
260 are the members of an equivalence class,
267 An equivalence class may not\(dd be an endpoint
270 Within a bracket expression, the name of a
276 stands for the list of all characters belonging to that
278 Standard character class names are:
280 .Bl -column "alnum" "digit" "xdigit" -offset indent
281 .It Em "alnum digit punct"
282 .It Em "alpha graph space"
283 .It Em "blank lower upper"
284 .It Em "cntrl print xdigit"
287 These stand for the character classes defined in
289 A locale may provide others.
290 A character class may not be used as an endpoint of a range.
292 There are two special cases\(dd of bracket expressions:
293 the bracket expressions
297 match the null string at the beginning and end of a word respectively.
298 A word is defined as a sequence of word characters
299 which is neither preceded nor followed by
301 A word character is an
303 character (as defined by
306 This is an extension,
307 compatible with but not specified by
309 and should be used with
310 caution in software intended to be portable to other systems.
312 In the event that an RE could match more than one substring of a given
314 the RE matches the one starting earliest in the string.
315 If the RE could match more than one substring starting at that point,
316 it matches the longest.
317 Subexpressions also match the longest possible substrings, subject to
318 the constraint that the whole match be as long as possible,
319 with subexpressions starting earlier in the RE taking priority over
321 Note that higher-level subexpressions thus take priority over
322 their lower-level component subexpressions.
324 Match lengths are measured in characters, not collating elements.
325 A null string is considered longer than no match at all.
328 matches the three middle characters of
330 .Ql (wee|week)(knights|nights)
331 matches all ten characters of
337 the parenthesized subexpression
338 matches all three characters, and
343 both the whole RE and the parenthesized
344 subexpression match the null string.
346 If case-independent matching is specified,
347 the effect is much as if all case distinctions had vanished from the
349 When an alphabetic that exists in multiple cases appears as an
350 ordinary character outside a bracket expression, it is effectively
351 transformed into a bracket expression containing both cases,
355 When it appears inside a bracket expression, all case counterparts
356 of it are added to the bracket expression, so that (e.g.)
365 No particular limit is imposed on the length of REs\(dd.
366 Programs intended to be portable should not employ REs longer
368 as an implementation can refuse to accept such REs and remain
373 regular expressions differ in several respects.
375 is an ordinary character and there is no equivalent
376 for its functionality.
380 are ordinary characters, and their functionality
381 can be expressed using bounds
388 in modern REs is equivalent to
390 The delimiters for bounds are
398 by themselves ordinary characters.
399 The parentheses for nested subexpressions are
407 by themselves ordinary characters.
409 is an ordinary character except at the beginning of the
410 RE or\(dd the beginning of a parenthesized subexpression,
412 is an ordinary character except at the end of the
413 RE or\(dd the end of a parenthesized subexpression,
416 is an ordinary character if it appears at the beginning of the
417 RE or the beginning of a parenthesized subexpression
418 (after a possible leading
420 Finally, there is one new type of atom, a
423 followed by a non-zero decimal digit
425 matches the same sequence of characters
428 parenthesized subexpression
429 (numbering subexpressions by the positions of their opening parentheses,
442 .%T Regular Expression Notation
448 Having two kinds of REs is a botch.
454 is an ordinary character in
455 the absence of an unmatched
457 this was an unintentional result of a wording error,
458 and change is likely.
461 Back references are a dreadful botch,
462 posing major problems for efficient implementations.
463 They are also somewhat vaguely defined
465 .Ql a\e(\e(b\e)*\e2\e)*d
471 specification of case-independent matching is vague.
473 .Dq one case implies all cases
474 definition given above
475 is current consensus among implementors as to the right interpretation.
477 The syntax for word boundaries is incredibly ugly.