1 .\" Copyright (c) 1992, 1993, 1994 Henry Spencer.
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32 .\" @(#)regex.3 8.4 (Berkeley) 3/20/94
33 .\" $FreeBSD: src/lib/libc/regex/regex.3,v 1.21 2007/01/09 00:28:04 imp Exp $
43 .Nd regular-expression library
50 .Fa "regex_t * restrict preg" "const char * restrict pattern" "int cflags"
54 .Fa "const regex_t * restrict preg" "const char * restrict string"
55 .Fa "size_t nmatch" "regmatch_t pmatch[restrict]" "int eflags"
59 .Fa "int errcode" "const regex_t * restrict preg"
60 .Fa "char * restrict errbuf" "size_t errbuf_size"
63 .Fn regfree "regex_t *preg"
65 These routines implement
74 compiles an RE written as a string into an internal form,
76 matches that internal form against a string and reports results,
78 transforms error codes from either into human-readable messages,
81 frees any dynamically-allocated storage used by the internal form
86 declares two structure types,
90 the former for compiled internal forms and the latter for match reporting.
91 It also declares the four functions,
94 and a number of constants with names starting with
100 compiles the regular expression contained in the
103 subject to the flags in
105 and places the results in the
107 structure pointed to by
112 is the bitwise OR of zero or more of the following flags:
113 .Bl -tag -width REG_EXTENDED
118 rather than the obsolete
123 This is a synonym for 0,
124 provided as a counterpart to
126 to improve readability.
128 Compile with recognition of all special characters turned off.
129 All characters are thus considered ordinary,
133 This is an extension,
134 compatible with but not specified by
136 and should be used with
137 caution in software intended to be portable to other systems.
145 Compile for matching that ignores upper/lower case distinctions.
149 Compile for matching that need only report success or failure,
150 not what was matched.
152 Compile for newline-sensitive matching.
153 By default, newline is a completely ordinary character with no special
154 meaning in either REs or strings.
157 bracket expressions and
162 anchor matches the null string after any newline in the string
163 in addition to its normal function,
166 anchor matches the null string before any newline in the
167 string in addition to its normal function.
169 The regular expression ends,
170 not at the first NUL,
171 but just before the character pointed to by the
173 member of the structure pointed to by
179 This flag permits inclusion of NULs in the RE;
180 they are considered ordinary characters.
181 This is an extension,
182 compatible with but not specified by
184 and should be used with
185 caution in software intended to be portable to other systems.
190 returns 0 and fills in the structure pointed to by
192 One member of that structure
199 contains the number of parenthesized subexpressions within the RE
200 (except that the value of this member is undefined if the
205 fails, it returns a non-zero error code;
212 matches the compiled RE pointed to by
216 subject to the flags in
218 and reports results using
221 and the returned value.
222 The RE must have been compiled by a previous invocation of
224 The compiled form is not altered during execution of
226 so a single compiled RE can be used simultaneously by multiple threads.
229 the NUL-terminated string pointed to by
231 is considered to be the text of an entire line, minus any terminating
235 argument is the bitwise OR of zero or more of the following flags:
236 .Bl -tag -width REG_STARTEND
238 The first character of
240 is not the beginning of a line, so the
242 anchor should not match before it.
243 This does not affect the behavior of newlines under
248 does not end a line, so the
250 anchor should not match before it.
251 This does not affect the behavior of newlines under
254 The string is considered to start at
257 .Fa pmatch Ns [0]. Ns Va rm_so
258 and to have a terminating NUL located at
261 .Fa pmatch Ns [0]. Ns Va rm_eo
262 (there need not actually be a NUL at that location),
263 regardless of the value of
265 See below for the definition of
269 This is an extension,
270 compatible with but not specified by
272 and should be used with
273 caution in software intended to be portable to other systems.
279 affects only the location of the string,
280 not how it is matched.
285 for a discussion of what is matched in situations where an RE or a
286 portion thereof could match any of several substrings of
291 returns 0 for success and the non-zero code
294 Other non-zero error codes may be returned in exceptional situations;
300 was specified in the compilation of the RE,
307 argument (but see below for the case where
312 points to an array of
316 Such a structure has at least the members
322 (a signed arithmetic type at least as large as an
326 containing respectively the offset of the first character of a substring
327 and the offset of the first character after the end of the substring.
328 Offsets are measured from the beginning of the
332 An empty substring is denoted by equal offsets,
333 both indicating the character following the empty substring.
335 The 0th member of the
337 array is filled in to indicate what substring of
339 was matched by the entire RE.
340 Remaining members report what substring was matched by parenthesized
341 subexpressions within the RE;
344 reports subexpression
346 with subexpressions counted (starting at 1) by the order of their opening
347 parentheses in the RE, left to right.
348 Unused entries in the array (corresponding either to subexpressions that
349 did not participate in the match at all, or to subexpressions that do not
350 exist in the RE (that is,
353 .Fa preg Ns -> Ns Va re_nsub ) )
359 If a subexpression participated in the match several times,
360 the reported substring is the last one it matched.
361 (Note, as an example in particular, that when the RE
365 the parenthesized subexpression matches each of the three
368 an infinite number of empty strings following the last
370 so the reported substring is one of the empties.)
376 must point to at least one
383 to hold the input offsets for
385 Use for output is still entirely controlled by
394 will not be changed by a successful
406 to a human-readable, printable message.
410 .No non\- Ns Dv NULL ,
411 the error code should have arisen from use of
416 and if the error code came from
418 it should have been the result from the most recent
424 may be able to supply a more detailed message using information
430 places the NUL-terminated message into the buffer pointed to by
432 limiting the length (including the NUL) to at most
435 If the whole message will not fit,
436 as much of it as will fit before the terminating NUL is supplied.
438 the returned value is the size of buffer needed to hold the whole
439 message (including terminating NUL).
444 is ignored but the return value is still correct.
454 that results is the printable name of the error code,
457 rather than an explanation thereof.
468 member of the structure it points to
469 must point to the printable name of an error code;
470 in this case, the result in
472 is the decimal digits of
473 the numeric value of the error code
474 (0 if the name is not recognized).
478 are intended primarily as debugging facilities;
480 compatible with but not specified by
482 and should be used with
483 caution in software intended to be portable to other systems.
484 Be warned also that they are considered experimental and changes are possible.
489 frees any dynamically-allocated storage associated with the compiled RE
494 is no longer a valid compiled RE
495 and the effect of supplying it to
501 None of these functions references global variables except for tables
503 all are safe for use from multiple threads if the arguments are safe.
504 .Sh IMPLEMENTATION CHOICES
505 There are a number of decisions that
507 leaves up to the implementor,
508 either by explicitly saying
510 or by virtue of them being
511 forbidden by the RE grammar.
512 This implementation treats them as follows.
516 for a discussion of the definition of case-independent matching.
518 There is no particular limit on the length of REs,
519 except insofar as memory is limited.
520 Memory usage is approximately linear in RE size, and largely insensitive
521 to RE complexity, except for bounded repetitions.
524 for one short RE using them
525 that will run almost any system out of memory.
527 A backslashed character other than one specifically given a magic meaning
530 (such magic meanings occur only in obsolete
533 is taken as an ordinary character.
541 Equivalence classes cannot begin or end bracket-expression ranges.
542 The endpoint of one range cannot begin another.
545 the limit on repetition counts in bounded repetitions, is 255.
547 A repetition operator
552 cannot follow another
554 A repetition operator cannot begin an expression or subexpression
561 cannot appear first or last in a (sub)expression or after another
565 cannot be an empty subexpression.
566 An empty parenthesized subexpression,
568 is legal and matches an
570 An empty string is not a legal RE.
574 followed by a digit is considered the beginning of bounds for a
575 bounded repetition, which must then follow the syntax for bounds.
579 followed by a digit is considered an ordinary character.
584 beginning and ending subexpressions in obsolete
586 REs are anchors, not ordinary characters.
588 Non-zero error codes from
592 include the following:
594 .Bl -tag -width REG_ECOLLATE -compact
601 invalid regular expression
603 invalid collating element
605 invalid character class
608 applied to unescapable character
610 invalid backreference number
624 invalid repetition count(s) in
627 invalid character range in
638 empty (sub)expression
640 cannot happen - you found a bug
642 invalid argument, e.g.\& negative-length string
644 illegal byte sequence (bad multibyte character)
651 sections 2.8 (Regular Expression Notation)
653 B.5 (C Binding for Regular Expression Matching).
655 Originally written by
657 Altered for inclusion in the
661 This is an alpha release with known defects.
662 Please report problems.
664 The back-reference code is subtle and doubts linger about its correctness
671 This will improve with later releases.
675 exceeding 0 is expensive;
677 exceeding 1 is worse.
681 is largely insensitive to RE complexity
684 references are massively expensive.
685 RE length does matter; in particular, there is a strong speed bonus
686 for keeping RE length under about 30 characters,
687 with most special characters counting roughly double.
692 implements bounded repetitions by macro expansion,
693 which is costly in time and space if counts are large
694 or bounded repetitions are nested.
696 .Ql "((((a{1,100}){1,100}){1,100}){1,100}){1,100}"
697 will (eventually) run almost any existing machine out of swap space.
699 There are suspected problems with response to obscure error conditions.
701 certain kinds of internal overflow,
702 produced only by truly enormous REs or by multiply nested bounded repetitions,
703 are probably not handled well.
709 are legal REs because
712 a special character only in the presence of a previous unmatched
714 This cannot be fixed until the spec is fixed.
716 The standard's definition of back references is vague.
718 .Ql "a\e(\e(b\e)*\e2\e)*d"
721 Until the standard is clarified,
722 behavior in such cases should not be relied on.
724 The implementation of word-boundary matching is a bit of a kludge,
725 and bugs may lurk in combinations of word-boundary matching and anchoring.
727 Word-boundary matching does not work properly in multibyte locales.
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