4 A @dfn{regular expression} (or @dfn{regexp}, or @dfn{pattern}) is a text
5 string that describes some (mathematical) set of strings. A regexp
6 @var{r} @dfn{matches} a string @var{s} if @var{s} is in the set of
7 strings described by @var{r}.
9 Using the Regex library, you can:
14 see if a string matches a specified pattern as a whole, and
17 search within a string for a substring matching a specified pattern.
21 Some regular expressions match only one string, i.e., the set they
22 describe has only one member. For example, the regular expression
23 @samp{foo} matches the string @samp{foo} and no others. Other regular
24 expressions match more than one string, i.e., the set they describe has
25 more than one member. For example, the regular expression @samp{f*}
26 matches the set of strings made up of any number (including zero) of
27 @samp{f}s. As you can see, some characters in regular expressions match
28 themselves (such as @samp{f}) and some don't (such as @samp{*}); the
29 ones that don't match themselves instead let you specify patterns that
30 describe many different strings.
32 To either match or search for a regular expression with the Regex
33 library functions, you must first compile it with a Regex pattern
34 compiling function. A @dfn{compiled pattern} is a regular expression
35 converted to the internal format used by the library functions. Once
36 you've compiled a pattern, you can use it for matching or searching any
39 The Regex library is used by including @file{regex.h}.
41 Regex provides three groups of functions with which you can operate on
42 regular expressions. One group---the @sc{gnu} group---is more powerful
43 but not completely compatible with the other two, namely the @sc{posix}
44 and Berkeley @sc{unix} groups; its interface was designed specifically
45 for @sc{gnu}. The other groups have the same interfaces as do the
46 regular expression functions in @sc{posix} and Berkeley
49 We wrote this chapter with programmers in mind, not users of
50 programs---such as Emacs---that use Regex. We describe the Regex
51 library in its entirety, not how to write regular expressions that a
52 particular program understands.
55 @node Regular Expression Syntax
56 @chapter Regular Expression Syntax
58 @cindex regular expressions, syntax of
59 @cindex syntax of regular expressions
61 @dfn{Characters} are things you can type. @dfn{Operators} are things in
62 a regular expression that match one or more characters. You compose
63 regular expressions from operators, which in turn you specify using one
66 Most characters represent what we call the match-self operator, i.e.,
67 they match themselves; we call these characters @dfn{ordinary}. Other
68 characters represent either all or parts of fancier operators; e.g.,
69 @samp{.} represents what we call the match-any-character operator
70 (which, no surprise, matches (almost) any character); we call these
71 characters @dfn{special}. Two different things determine what
72 characters represent what operators:
76 the regular expression syntax your program has told the Regex library to
80 the context of the character in the regular expression.
83 In the following sections, we describe these things in more detail.
87 * Predefined Syntaxes::
88 * Collating Elements vs. Characters::
89 * The Backslash Character::
98 In any particular syntax for regular expressions, some characters are
99 always special, others are sometimes special, and others are never
100 special. The particular syntax that Regex recognizes for a given
101 regular expression depends on the value in the @code{syntax} field of
102 the pattern buffer of that regular expression.
104 You get a pattern buffer by compiling a regular expression. @xref{GNU
105 Pattern Buffers}, and @ref{POSIX Pattern Buffers}, for more information
106 on pattern buffers. @xref{GNU Regular Expression Compiling}, @ref{POSIX
107 Regular Expression Compiling}, and @ref{BSD Regular Expression
108 Compiling}, for more information on compiling.
110 Regex considers the value of the @code{syntax} field to be a collection
111 of bits; we refer to these bits as @dfn{syntax bits}. In most cases,
112 they affect what characters represent what operators. We describe the
113 meanings of the operators to which we refer in @ref{Common Operators},
114 @ref{GNU Operators}, and @ref{GNU Emacs Operators}.
116 For reference, here is the complete list of syntax bits, in alphabetical
121 @cnindex RE_BACKSLASH_ESCAPE_IN_LIST
122 @item RE_BACKSLASH_ESCAPE_IN_LISTS
123 If this bit is set, then @samp{\} inside a list (@pxref{List Operators}
124 quotes (makes ordinary, if it's special) the following character; if
125 this bit isn't set, then @samp{\} is an ordinary character inside lists.
126 (@xref{The Backslash Character}, for what `\' does outside of lists.)
128 @cnindex RE_BK_PLUS_QM
130 If this bit is set, then @samp{\+} represents the match-one-or-more
131 operator and @samp{\?} represents the match-zero-or-more operator; if
132 this bit isn't set, then @samp{+} represents the match-one-or-more
133 operator and @samp{?} represents the match-zero-or-one operator. This
134 bit is irrelevant if @code{RE_LIMITED_OPS} is set.
136 @cnindex RE_CHAR_CLASSES
137 @item RE_CHAR_CLASSES
138 If this bit is set, then you can use character classes in lists; if this
139 bit isn't set, then you can't.
141 @cnindex RE_CONTEXT_INDEP_ANCHORS
142 @item RE_CONTEXT_INDEP_ANCHORS
143 If this bit is set, then @samp{^} and @samp{$} are special anywhere outside
144 a list; if this bit isn't set, then these characters are special only in
145 certain contexts. @xref{Match-beginning-of-line Operator}, and
146 @ref{Match-end-of-line Operator}.
148 @cnindex RE_CONTEXT_INDEP_OPS
149 @item RE_CONTEXT_INDEP_OPS
150 If this bit is set, then certain characters are special anywhere outside
151 a list; if this bit isn't set, then those characters are special only in
152 some contexts and are ordinary elsewhere. Specifically, if this bit
153 isn't set then @samp{*}, and (if the syntax bit @code{RE_LIMITED_OPS}
154 isn't set) @samp{+} and @samp{?} (or @samp{\+} and @samp{\?}, depending
155 on the syntax bit @code{RE_BK_PLUS_QM}) represent repetition operators
156 only if they're not first in a regular expression or just after an
157 open-group or alternation operator. The same holds for @samp{@{} (or
158 @samp{\@{}, depending on the syntax bit @code{RE_NO_BK_BRACES}) if
159 it is the beginning of a valid interval and the syntax bit
160 @code{RE_INTERVALS} is set.
162 @cnindex RE_CONTEXT_INVALID_OPS
163 @item RE_CONTEXT_INVALID_OPS
164 If this bit is set, then repetition and alternation operators can't be
165 in certain positions within a regular expression. Specifically, the
166 regular expression is invalid if it has:
171 a repetition operator first in the regular expression or just after a
172 match-beginning-of-line, open-group, or alternation operator; or
175 an alternation operator first or last in the regular expression, just
176 before a match-end-of-line operator, or just after an alternation or
181 If this bit isn't set, then you can put the characters representing the
182 repetition and alternation characters anywhere in a regular expression.
183 Whether or not they will in fact be operators in certain positions
184 depends on other syntax bits.
186 @cnindex RE_DOT_NEWLINE
188 If this bit is set, then the match-any-character operator matches
189 a newline; if this bit isn't set, then it doesn't.
191 @cnindex RE_DOT_NOT_NULL
192 @item RE_DOT_NOT_NULL
193 If this bit is set, then the match-any-character operator doesn't match
194 a null character; if this bit isn't set, then it does.
196 @cnindex RE_INTERVALS
198 If this bit is set, then Regex recognizes interval operators; if this bit
199 isn't set, then it doesn't.
201 @cnindex RE_LIMITED_OPS
203 If this bit is set, then Regex doesn't recognize the match-one-or-more,
204 match-zero-or-one or alternation operators; if this bit isn't set, then
207 @cnindex RE_NEWLINE_ALT
209 If this bit is set, then newline represents the alternation operator; if
210 this bit isn't set, then newline is ordinary.
212 @cnindex RE_NO_BK_BRACES
213 @item RE_NO_BK_BRACES
214 If this bit is set, then @samp{@{} represents the open-interval operator
215 and @samp{@}} represents the close-interval operator; if this bit isn't
216 set, then @samp{\@{} represents the open-interval operator and
217 @samp{\@}} represents the close-interval operator. This bit is relevant
218 only if @code{RE_INTERVALS} is set.
220 @cnindex RE_NO_BK_PARENS
221 @item RE_NO_BK_PARENS
222 If this bit is set, then @samp{(} represents the open-group operator and
223 @samp{)} represents the close-group operator; if this bit isn't set, then
224 @samp{\(} represents the open-group operator and @samp{\)} represents
225 the close-group operator.
227 @cnindex RE_NO_BK_REFS
229 If this bit is set, then Regex doesn't recognize @samp{\}@var{digit} as
230 the back reference operator; if this bit isn't set, then it does.
232 @cnindex RE_NO_BK_VBAR
234 If this bit is set, then @samp{|} represents the alternation operator;
235 if this bit isn't set, then @samp{\|} represents the alternation
236 operator. This bit is irrelevant if @code{RE_LIMITED_OPS} is set.
238 @cnindex RE_NO_EMPTY_RANGES
239 @item RE_NO_EMPTY_RANGES
240 If this bit is set, then a regular expression with a range whose ending
241 point collates lower than its starting point is invalid; if this bit
242 isn't set, then Regex considers such a range to be empty.
244 @cnindex RE_UNMATCHED_RIGHT_PAREN_ORD
245 @item RE_UNMATCHED_RIGHT_PAREN_ORD
246 If this bit is set and the regular expression has no matching open-group
247 operator, then Regex considers what would otherwise be a close-group
248 operator (based on how @code{RE_NO_BK_PARENS} is set) to match @samp{)}.
253 @node Predefined Syntaxes
254 @section Predefined Syntaxes
256 If you're programming with Regex, you can set a pattern buffer's
257 (@pxref{GNU Pattern Buffers}, and @ref{POSIX Pattern Buffers})
258 @code{syntax} field either to an arbitrary combination of syntax bits
259 (@pxref{Syntax Bits}) or else to the configurations defined by Regex.
260 These configurations define the syntaxes used by certain
261 programs---@sc{gnu} Emacs,
270 Egrep---in addition to syntaxes for @sc{posix} basic and extended
273 The predefined syntaxes---taken directly from @file{regex.h}---are:
276 #define RE_SYNTAX_EMACS 0
278 #define RE_SYNTAX_AWK \
279 (RE_BACKSLASH_ESCAPE_IN_LISTS | RE_DOT_NOT_NULL \
280 | RE_NO_BK_PARENS | RE_NO_BK_REFS \
281 | RE_NO_BK_VBAR | RE_NO_EMPTY_RANGES \
282 | RE_UNMATCHED_RIGHT_PAREN_ORD)
284 #define RE_SYNTAX_POSIX_AWK \
285 (RE_SYNTAX_POSIX_EXTENDED | RE_BACKSLASH_ESCAPE_IN_LISTS)
287 #define RE_SYNTAX_GREP \
288 (RE_BK_PLUS_QM | RE_CHAR_CLASSES \
289 | RE_HAT_LISTS_NOT_NEWLINE | RE_INTERVALS \
292 #define RE_SYNTAX_EGREP \
293 (RE_CHAR_CLASSES | RE_CONTEXT_INDEP_ANCHORS \
294 | RE_CONTEXT_INDEP_OPS | RE_HAT_LISTS_NOT_NEWLINE \
295 | RE_NEWLINE_ALT | RE_NO_BK_PARENS \
298 #define RE_SYNTAX_POSIX_EGREP \
299 (RE_SYNTAX_EGREP | RE_INTERVALS | RE_NO_BK_BRACES)
301 /* P1003.2/D11.2, section 4.20.7.1, lines 5078ff. */
302 #define RE_SYNTAX_ED RE_SYNTAX_POSIX_BASIC
304 #define RE_SYNTAX_SED RE_SYNTAX_POSIX_BASIC
306 /* Syntax bits common to both basic and extended POSIX regex syntax. */
307 #define _RE_SYNTAX_POSIX_COMMON \
308 (RE_CHAR_CLASSES | RE_DOT_NEWLINE | RE_DOT_NOT_NULL \
309 | RE_INTERVALS | RE_NO_EMPTY_RANGES)
311 #define RE_SYNTAX_POSIX_BASIC \
312 (_RE_SYNTAX_POSIX_COMMON | RE_BK_PLUS_QM)
314 /* Differs from ..._POSIX_BASIC only in that RE_BK_PLUS_QM becomes
315 RE_LIMITED_OPS, i.e., \? \+ \| are not recognized. Actually, this
316 isn't minimal, since other operators, such as \`, aren't disabled. */
317 #define RE_SYNTAX_POSIX_MINIMAL_BASIC \
318 (_RE_SYNTAX_POSIX_COMMON | RE_LIMITED_OPS)
320 #define RE_SYNTAX_POSIX_EXTENDED \
321 (_RE_SYNTAX_POSIX_COMMON | RE_CONTEXT_INDEP_ANCHORS \
322 | RE_CONTEXT_INDEP_OPS | RE_NO_BK_BRACES \
323 | RE_NO_BK_PARENS | RE_NO_BK_VBAR \
324 | RE_UNMATCHED_RIGHT_PAREN_ORD)
326 /* Differs from ..._POSIX_EXTENDED in that RE_CONTEXT_INVALID_OPS
327 replaces RE_CONTEXT_INDEP_OPS and RE_NO_BK_REFS is added. */
328 #define RE_SYNTAX_POSIX_MINIMAL_EXTENDED \
329 (_RE_SYNTAX_POSIX_COMMON | RE_CONTEXT_INDEP_ANCHORS \
330 | RE_CONTEXT_INVALID_OPS | RE_NO_BK_BRACES \
331 | RE_NO_BK_PARENS | RE_NO_BK_REFS \
332 | RE_NO_BK_VBAR | RE_UNMATCHED_RIGHT_PAREN_ORD)
335 @node Collating Elements vs. Characters
336 @section Collating Elements vs.@: Characters
338 @sc{posix} generalizes the notion of a character to that of a
339 collating element. It defines a @dfn{collating element} to be ``a
340 sequence of one or more bytes defined in the current collating sequence
341 as a unit of collation.''
343 This generalizes the notion of a character in
344 two ways. First, a single character can map into two or more collating
345 elements. For example, the German
352 collates as the collating element @samp{s} followed by another collating
353 element @samp{s}. Second, two or more characters can map into one
354 collating element. For example, the Spanish @samp{ll} collates after
355 @samp{l} and before @samp{m}.
357 Since @sc{posix}'s ``collating element'' preserves the essential idea of
358 a ``character,'' we use the latter, more familiar, term in this document.
360 @node The Backslash Character
361 @section The Backslash Character
364 The @samp{\} character has one of four different meanings, depending on
365 the context in which you use it and what syntax bits are set
366 (@pxref{Syntax Bits}). It can: 1) stand for itself, 2) quote the next
367 character, 3) introduce an operator, or 4) do nothing.
371 It stands for itself inside a list
372 (@pxref{List Operators}) if the syntax bit
373 @code{RE_BACKSLASH_ESCAPE_IN_LISTS} is not set. For example, @samp{[\]}
374 would match @samp{\}.
377 It quotes (makes ordinary, if it's special) the next character when you
382 outside a list,@footnote{Sometimes
383 you don't have to explicitly quote special characters to make
384 them ordinary. For instance, most characters lose any special meaning
385 inside a list (@pxref{List Operators}). In addition, if the syntax bits
386 @code{RE_CONTEXT_INVALID_OPS} and @code{RE_CONTEXT_INDEP_OPS}
387 aren't set, then (for historical reasons) the matcher considers special
388 characters ordinary if they are in contexts where the operations they
389 represent make no sense; for example, then the match-zero-or-more
390 operator (represented by @samp{*}) matches itself in the regular
391 expression @samp{*foo} because there is no preceding expression on which
392 it can operate. It is poor practice, however, to depend on this
393 behavior; if you want a special character to be ordinary outside a list,
394 it's better to always quote it, regardless.} or
397 inside a list and the syntax bit @code{RE_BACKSLASH_ESCAPE_IN_LISTS} is set.
402 It introduces an operator when followed by certain ordinary
403 characters---sometimes only when certain syntax bits are set. See the
404 cases @code{RE_BK_PLUS_QM}, @code{RE_NO_BK_BRACES}, @code{RE_NO_BK_VAR},
405 @code{RE_NO_BK_PARENS}, @code{RE_NO_BK_REF} in @ref{Syntax Bits}. Also:
409 @samp{\b} represents the match-word-boundary operator
410 (@pxref{Match-word-boundary Operator}).
413 @samp{\B} represents the match-within-word operator
414 (@pxref{Match-within-word Operator}).
417 @samp{\<} represents the match-beginning-of-word operator @*
418 (@pxref{Match-beginning-of-word Operator}).
421 @samp{\>} represents the match-end-of-word operator
422 (@pxref{Match-end-of-word Operator}).
425 @samp{\w} represents the match-word-constituent operator
426 (@pxref{Match-word-constituent Operator}).
429 @samp{\W} represents the match-non-word-constituent operator
430 (@pxref{Match-non-word-constituent Operator}).
433 @samp{\`} represents the match-beginning-of-buffer
434 operator and @samp{\'} represents the match-end-of-buffer operator
435 (@pxref{Buffer Operators}).
438 If Regex was compiled with the C preprocessor symbol @code{emacs}
439 defined, then @samp{\s@var{class}} represents the match-syntactic-class
440 operator and @samp{\S@var{class}} represents the
441 match-not-syntactic-class operator (@pxref{Syntactic Class Operators}).
446 In all other cases, Regex ignores @samp{\}. For example,
447 @samp{\n} matches @samp{n}.
451 @node Common Operators
452 @chapter Common Operators
454 You compose regular expressions from operators. In the following
455 sections, we describe the regular expression operators specified by
456 @sc{posix}; @sc{gnu} also uses these. Most operators have more than one
457 representation as characters. @xref{Regular Expression Syntax}, for
458 what characters represent what operators under what circumstances.
460 For most operators that can be represented in two ways, one
461 representation is a single character and the other is that character
462 preceded by @samp{\}. For example, either @samp{(} or @samp{\(}
463 represents the open-group operator. Which one does depends on the
464 setting of a syntax bit, in this case @code{RE_NO_BK_PARENS}. Why is
465 this so? Historical reasons dictate some of the varying
466 representations, while @sc{posix} dictates others.
468 Finally, almost all characters lose any special meaning inside a list
469 (@pxref{List Operators}).
472 * Match-self Operator:: Ordinary characters.
473 * Match-any-character Operator:: .
474 * Concatenation Operator:: Juxtaposition.
475 * Repetition Operators:: * + ? @{@}
476 * Alternation Operator:: |
477 * List Operators:: [...] [^...]
478 * Grouping Operators:: (...)
479 * Back-reference Operator:: \digit
480 * Anchoring Operators:: ^ $
483 @node Match-self Operator
484 @section The Match-self Operator (@var{ordinary character})
486 This operator matches the character itself. All ordinary characters
487 (@pxref{Regular Expression Syntax}) represent this operator. For
488 example, @samp{f} is always an ordinary character, so the regular
489 expression @samp{f} matches only the string @samp{f}. In
490 particular, it does @emph{not} match the string @samp{ff}.
492 @node Match-any-character Operator
493 @section The Match-any-character Operator (@code{.})
497 This operator matches any single printing or nonprinting character
498 except it won't match a:
502 if the syntax bit @code{RE_DOT_NEWLINE} isn't set.
505 if the syntax bit @code{RE_DOT_NOT_NULL} is set.
509 The @samp{.} (period) character represents this operator. For example,
510 @samp{a.b} matches any three-character string beginning with @samp{a}
511 and ending with @samp{b}.
513 @node Concatenation Operator
514 @section The Concatenation Operator
516 This operator concatenates two regular expressions @var{a} and @var{b}.
517 No character represents this operator; you simply put @var{b} after
518 @var{a}. The result is a regular expression that will match a string if
519 @var{a} matches its first part and @var{b} matches the rest. For
520 example, @samp{xy} (two match-self operators) matches @samp{xy}.
522 @node Repetition Operators
523 @section Repetition Operators
525 Repetition operators repeat the preceding regular expression a specified
529 * Match-zero-or-more Operator:: *
530 * Match-one-or-more Operator:: +
531 * Match-zero-or-one Operator:: ?
532 * Interval Operators:: @{@}
535 @node Match-zero-or-more Operator
536 @subsection The Match-zero-or-more Operator (@code{*})
540 This operator repeats the smallest possible preceding regular expression
541 as many times as necessary (including zero) to match the pattern.
542 @samp{*} represents this operator. For example, @samp{o*}
543 matches any string made up of zero or more @samp{o}s. Since this
544 operator operates on the smallest preceding regular expression,
545 @samp{fo*} has a repeating @samp{o}, not a repeating @samp{fo}. So,
546 @samp{fo*} matches @samp{f}, @samp{fo}, @samp{foo}, and so on.
548 Since the match-zero-or-more operator is a suffix operator, it may be
549 useless as such when no regular expression precedes it. This is the
554 is first in a regular expression, or
557 follows a match-beginning-of-line, open-group, or alternation
563 Three different things can happen in these cases:
567 If the syntax bit @code{RE_CONTEXT_INVALID_OPS} is set, then the
568 regular expression is invalid.
571 If @code{RE_CONTEXT_INVALID_OPS} isn't set, but
572 @code{RE_CONTEXT_INDEP_OPS} is, then @samp{*} represents the
573 match-zero-or-more operator (which then operates on the empty string).
576 Otherwise, @samp{*} is ordinary.
581 The matcher processes a match-zero-or-more operator by first matching as
582 many repetitions of the smallest preceding regular expression as it can.
583 Then it continues to match the rest of the pattern.
585 If it can't match the rest of the pattern, it backtracks (as many times
586 as necessary), each time discarding one of the matches until it can
587 either match the entire pattern or be certain that it cannot get a
588 match. For example, when matching @samp{ca*ar} against @samp{caaar},
589 the matcher first matches all three @samp{a}s of the string with the
590 @samp{a*} of the regular expression. However, it cannot then match the
591 final @samp{ar} of the regular expression against the final @samp{r} of
592 the string. So it backtracks, discarding the match of the last @samp{a}
593 in the string. It can then match the remaining @samp{ar}.
596 @node Match-one-or-more Operator
597 @subsection The Match-one-or-more Operator (@code{+} or @code{\+})
601 If the syntax bit @code{RE_LIMITED_OPS} is set, then Regex doesn't recognize
602 this operator. Otherwise, if the syntax bit @code{RE_BK_PLUS_QM} isn't
603 set, then @samp{+} represents this operator; if it is, then @samp{\+}
606 This operator is similar to the match-zero-or-more operator except that
607 it repeats the preceding regular expression at least once;
608 @pxref{Match-zero-or-more Operator}, for what it operates on, how some
609 syntax bits affect it, and how Regex backtracks to match it.
611 For example, supposing that @samp{+} represents the match-one-or-more
612 operator; then @samp{ca+r} matches, e.g., @samp{car} and
613 @samp{caaaar}, but not @samp{cr}.
615 @node Match-zero-or-one Operator
616 @subsection The Match-zero-or-one Operator (@code{?} or @code{\?})
619 If the syntax bit @code{RE_LIMITED_OPS} is set, then Regex doesn't
620 recognize this operator. Otherwise, if the syntax bit
621 @code{RE_BK_PLUS_QM} isn't set, then @samp{?} represents this operator;
622 if it is, then @samp{\?} does.
624 This operator is similar to the match-zero-or-more operator except that
625 it repeats the preceding regular expression once or not at all;
626 @pxref{Match-zero-or-more Operator}, to see what it operates on, how
627 some syntax bits affect it, and how Regex backtracks to match it.
629 For example, supposing that @samp{?} represents the match-zero-or-one
630 operator; then @samp{ca?r} matches both @samp{car} and @samp{cr}, but
633 @node Interval Operators
634 @subsection Interval Operators (@code{@{} @dots{} @code{@}} or @code{\@{} @dots{} @code{\@}})
636 @cindex interval expression
642 If the syntax bit @code{RE_INTERVALS} is set, then Regex recognizes
643 @dfn{interval expressions}. They repeat the smallest possible preceding
644 regular expression a specified number of times.
646 If the syntax bit @code{RE_NO_BK_BRACES} is set, @samp{@{} represents
647 the @dfn{open-interval operator} and @samp{@}} represents the
648 @dfn{close-interval operator} ; otherwise, @samp{\@{} and @samp{\@}} do.
650 Specifically, supposing that @samp{@{} and @samp{@}} represent the
651 open-interval and close-interval operators; then:
654 @item @{@var{count}@}
655 matches exactly @var{count} occurrences of the preceding regular
659 matches @var{min} or more occurrences of the preceding regular
662 @item @{@var{min}, @var{max}@}
663 matches at least @var{min} but no more than @var{max} occurrences of
664 the preceding regular expression.
668 The interval expression (but not necessarily the regular expression that
669 contains it) is invalid if:
673 @var{min} is greater than @var{max}, or
676 any of @var{count}, @var{min}, or @var{max} are outside the range
677 zero to @code{RE_DUP_MAX} (which symbol @file{regex.h}
682 If the interval expression is invalid and the syntax bit
683 @code{RE_NO_BK_BRACES} is set, then Regex considers all the
684 characters in the would-be interval to be ordinary. If that bit
685 isn't set, then the regular expression is invalid.
687 If the interval expression is valid but there is no preceding regular
688 expression on which to operate, then if the syntax bit
689 @code{RE_CONTEXT_INVALID_OPS} is set, the regular expression is invalid.
690 If that bit isn't set, then Regex considers all the characters---other
691 than backslashes, which it ignores---in the would-be interval to be
695 @node Alternation Operator
696 @section The Alternation Operator (@code{|} or @code{\|})
700 @cindex alternation operator
703 If the syntax bit @code{RE_LIMITED_OPS} is set, then Regex doesn't
704 recognize this operator. Otherwise, if the syntax bit
705 @code{RE_NO_BK_VBAR} is set, then @samp{|} represents this operator;
706 otherwise, @samp{\|} does.
708 Alternatives match one of a choice of regular expressions:
709 if you put the character(s) representing the alternation operator between
710 any two regular expressions @var{a} and @var{b}, the result matches
711 the union of the strings that @var{a} and @var{b} match. For
712 example, supposing that @samp{|} is the alternation operator, then
713 @samp{foo|bar|quux} would match any of @samp{foo}, @samp{bar} or
717 @c Nobody needs to disallow empty alternatives any more.
718 If the syntax bit @code{RE_NO_EMPTY_ALTS} is set, then if either of the regular
719 expressions @var{a} or @var{b} is empty, the
720 regular expression is invalid. More precisely, if this syntax bit is
721 set, then the alternation operator can't:
725 be first or last in a regular expression;
728 follow either another alternation operator or an open-group operator
729 (@pxref{Grouping Operators}); or
732 precede a close-group operator.
737 For example, supposing @samp{(} and @samp{)} represent the open and
738 close-group operators, then @samp{|foo}, @samp{foo|}, @samp{foo||bar},
739 @samp{foo(|bar)}, and @samp{(foo|)bar} would all be invalid.
742 The alternation operator operates on the @emph{largest} possible
743 surrounding regular expressions. (Put another way, it has the lowest
744 precedence of any regular expression operator.)
745 Thus, the only way you can
746 delimit its arguments is to use grouping. For example, if @samp{(} and
747 @samp{)} are the open and close-group operators, then @samp{fo(o|b)ar}
748 would match either @samp{fooar} or @samp{fobar}. (@samp{foo|bar} would
749 match @samp{foo} or @samp{bar}.)
752 The matcher usually tries all combinations of alternatives so as to
753 match the longest possible string. For example, when matching
754 @samp{(fooq|foo)*(qbarquux|bar)} against @samp{fooqbarquux}, it cannot
755 take, say, the first (``depth-first'') combination it could match, since
756 then it would be content to match just @samp{fooqbar}.
758 @comment xx something about leftmost-longest
762 @section List Operators (@code{[} @dots{} @code{]} and @code{[^} @dots{} @code{]})
764 @cindex matching list
771 @cindex nonmatching list
772 @cindex matching newline
773 @cindex bracket expression
775 @dfn{Lists}, also called @dfn{bracket expressions}, are a set of one or
776 more items. An @dfn{item} is a character,
778 (These get added when they get implemented.)
779 a collating symbol, an equivalence class expression,
781 a character class expression, or a range expression. The syntax bits
782 affect which kinds of items you can put in a list. We explain the last
783 two items in subsections below. Empty lists are invalid.
785 A @dfn{matching list} matches a single character represented by one of
786 the list items. You form a matching list by enclosing one or more items
787 within an @dfn{open-matching-list operator} (represented by @samp{[})
788 and a @dfn{close-list operator} (represented by @samp{]}).
790 For example, @samp{[ab]} matches either @samp{a} or @samp{b}.
791 @samp{[ad]*} matches the empty string and any string composed of just
792 @samp{a}s and @samp{d}s in any order. Regex considers invalid a regular
793 expression with a @samp{[} but no matching
796 @dfn{Nonmatching lists} are similar to matching lists except that they
797 match a single character @emph{not} represented by one of the list
798 items. You use an @dfn{open-nonmatching-list operator} (represented by
799 @samp{[^}@footnote{Regex therefore doesn't consider the @samp{^} to be
800 the first character in the list. If you put a @samp{^} character first
801 in (what you think is) a matching list, you'll turn it into a
802 nonmatching list.}) instead of an open-matching-list operator to start a
805 For example, @samp{[^ab]} matches any character except @samp{a} or
808 If the @code{posix_newline} field in the pattern buffer (@pxref{GNU
809 Pattern Buffers} is set, then nonmatching lists do not match a newline.
811 Most characters lose any special meaning inside a list. The special
812 characters inside a list follow.
816 ends the list if it's not the first list item. So, if you want to make
817 the @samp{]} character a list item, you must put it first.
820 quotes the next character if the syntax bit @code{RE_BACKSLASH_ESCAPE_IN_LISTS} is
824 Put these in if they get implemented.
827 represents the open-collating-symbol operator (@pxref{Collating Symbol
831 represents the close-collating-symbol operator.
834 represents the open-equivalence-class operator (@pxref{Equivalence Class
838 represents the close-equivalence-class operator.
843 represents the open-character-class operator (@pxref{Character Class
844 Operators}) if the syntax bit @code{RE_CHAR_CLASSES} is set and what
845 follows is a valid character class expression.
848 represents the close-character-class operator if the syntax bit
849 @code{RE_CHAR_CLASSES} is set and what precedes it is an
850 open-character-class operator followed by a valid character class name.
853 represents the range operator (@pxref{Range Operator}) if it's
854 not first or last in a list or the ending point of a range.
859 All other characters are ordinary. For example, @samp{[.*]} matches
860 @samp{.} and @samp{*}.
863 * Character Class Operators:: [:class:]
864 * Range Operator:: start-end
868 (If collating symbols and equivalence class expressions get implemented,
871 node Collating Symbol Operators
872 subsubsection Collating Symbol Operators (@code{[.} @dots{} @code{.]})
874 If the syntax bit @code{XX} is set, then you can represent
875 collating symbols inside lists. You form a @dfn{collating symbol} by
876 putting a collating element between an @dfn{open-collating-symbol
877 operator} and an @dfn{close-collating-symbol operator}. @samp{[.}
878 represents the open-collating-symbol operator and @samp{.]} represents
879 the close-collating-symbol operator. For example, if @samp{ll} is a
880 collating element, then @samp{[[.ll.]]} would match @samp{ll}.
882 node Equivalence Class Operators
883 subsubsection Equivalence Class Operators (@code{[=} @dots{} @code{=]})
884 @cindex equivalence class expression in regex
885 @cindex @samp{[=} in regex
886 @cindex @samp{=]} in regex
888 If the syntax bit @code{XX} is set, then Regex recognizes equivalence class
889 expressions inside lists. A @dfn{equivalence class expression} is a set
890 of collating elements which all belong to the same equivalence class.
891 You form an equivalence class expression by putting a collating
892 element between an @dfn{open-equivalence-class operator} and a
893 @dfn{close-equivalence-class operator}. @samp{[=} represents the
894 open-equivalence-class operator and @samp{=]} represents the
895 close-equivalence-class operator. For example, if @samp{a} and @samp{A}
896 were an equivalence class, then both @samp{[[=a=]]} and @samp{[[=A=]]}
897 would match both @samp{a} and @samp{A}. If the collating element in an
898 equivalence class expression isn't part of an equivalence class, then
899 the matcher considers the equivalence class expression to be a collating
904 @node Character Class Operators
905 @subsection Character Class Operators (@code{[:} @dots{} @code{:]})
907 @cindex character classes
908 @cindex @samp{[:} in regex
909 @cindex @samp{:]} in regex
911 If the syntax bit @code{RE_CHARACTER_CLASSES} is set, then Regex
912 recognizes character class expressions inside lists. A @dfn{character
913 class expression} matches one character from a given class. You form a
914 character class expression by putting a character class name between an
915 @dfn{open-character-class operator} (represented by @samp{[:}) and a
916 @dfn{close-character-class operator} (represented by @samp{:]}). The
917 character class names and their meanings are:
928 system-dependent; for @sc{gnu}, a space or tab
931 control characters (in the @sc{ascii} encoding, code 0177 and codes
938 same as @code{print} except omits space
944 printable characters (in the @sc{ascii} encoding, space
945 tilde---codes 040 through 0176)
948 neither control nor alphanumeric characters
951 space, carriage return, newline, vertical tab, and form feed
957 hexadecimal digits: @code{0}--@code{9}, @code{a}--@code{f}, @code{A}--@code{F}
962 These correspond to the definitions in the C library's @file{<ctype.h>}
963 facility. For example, @samp{[:alpha:]} corresponds to the standard
964 facility @code{isalpha}. Regex recognizes character class expressions
965 only inside of lists; so @samp{[[:alpha:]]} matches any letter, but
966 @samp{[:alpha:]} outside of a bracket expression and not followed by a
967 repetition operator matches just itself.
970 @subsection The Range Operator (@code{-})
972 Regex recognizes @dfn{range expressions} inside a list. They represent
974 that fall between two elements in the current collating sequence. You
975 form a range expression by putting a @dfn{range operator} between two
977 (If these get implemented, then substitute this for ``characters.'')
978 of any of the following: characters, collating elements, collating symbols,
979 and equivalence class expressions. The starting point of the range and
980 the ending point of the range don't have to be the same kind of item,
981 e.g., the starting point could be a collating element and the ending
982 point could be an equivalence class expression. If a range's ending
983 point is an equivalence class, then all the collating elements in that
984 class will be in the range.
986 characters.@footnote{You can't use a character class for the starting
987 or ending point of a range, since a character class is not a single
988 character.} @samp{-} represents the range operator. For example,
989 @samp{a-f} within a list represents all the characters from @samp{a}
993 If the syntax bit @code{RE_NO_EMPTY_RANGES} is set, then if the range's
994 ending point collates less than its starting point, the range (and the
995 regular expression containing it) is invalid. For example, the regular
996 expression @samp{[z-a]} would be invalid. If this bit isn't set, then
997 Regex considers such a range to be empty.
999 Since @samp{-} represents the range operator, if you want to make a
1000 @samp{-} character itself
1001 a list item, you must do one of the following:
1005 Put the @samp{-} either first or last in the list.
1008 Include a range whose starting point collates strictly lower than
1009 @samp{-} and whose ending point collates equal or higher. Unless a
1010 range is the first item in a list, a @samp{-} can't be its starting
1011 point, but @emph{can} be its ending point. That is because Regex
1012 considers @samp{-} to be the range operator unless it is preceded by
1013 another @samp{-}. For example, in the @sc{ascii} encoding, @samp{)},
1014 @samp{*}, @samp{+}, @samp{,}, @samp{-}, @samp{.}, and @samp{/} are
1015 contiguous characters in the collating sequence. You might think that
1016 @samp{[)-+--/]} has two ranges: @samp{)-+} and @samp{--/}. Rather, it
1017 has the ranges @samp{)-+} and @samp{+--}, plus the character @samp{/}, so
1018 it matches, e.g., @samp{,}, not @samp{.}.
1021 Put a range whose starting point is @samp{-} first in the list.
1025 For example, @samp{[-a-z]} matches a lowercase letter or a hyphen (in
1026 English, in @sc{ascii}).
1029 @node Grouping Operators
1030 @section Grouping Operators (@code{(} @dots{} @code{)} or @code{\(} @dots{} @code{\)})
1037 @cindex subexpressions
1038 @cindex parenthesizing
1040 A @dfn{group}, also known as a @dfn{subexpression}, consists of an
1041 @dfn{open-group operator}, any number of other operators, and a
1042 @dfn{close-group operator}. Regex treats this sequence as a unit, just
1043 as mathematics and programming languages treat a parenthesized
1044 expression as a unit.
1046 Therefore, using @dfn{groups}, you can:
1050 delimit the argument(s) to an alternation operator (@pxref{Alternation
1051 Operator}) or a repetition operator (@pxref{Repetition
1055 keep track of the indices of the substring that matched a given group.
1056 @xref{Using Registers}, for a precise explanation.
1061 use the back-reference operator (@pxref{Back-reference Operator}).
1064 use registers (@pxref{Using Registers}).
1070 If the syntax bit @code{RE_NO_BK_PARENS} is set, then @samp{(} represents
1071 the open-group operator and @samp{)} represents the
1072 close-group operator; otherwise, @samp{\(} and @samp{\)} do.
1074 If the syntax bit @code{RE_UNMATCHED_RIGHT_PAREN_ORD} is set and a
1075 close-group operator has no matching open-group operator, then Regex
1076 considers it to match @samp{)}.
1079 @node Back-reference Operator
1080 @section The Back-reference Operator (@dfn{\}@var{digit})
1082 @cindex back references
1084 If the syntax bit @code{RE_NO_BK_REF} isn't set, then Regex recognizes
1085 back references. A back reference matches a specified preceding group.
1086 The back reference operator is represented by @samp{\@var{digit}}
1087 anywhere after the end of a regular expression's @w{@var{digit}-th}
1088 group (@pxref{Grouping Operators}).
1090 @var{digit} must be between @samp{1} and @samp{9}. The matcher assigns
1091 numbers 1 through 9 to the first nine groups it encounters. By using
1092 one of @samp{\1} through @samp{\9} after the corresponding group's
1093 close-group operator, you can match a substring identical to the
1094 one that the group does.
1096 Back references match according to the following (in all examples below,
1097 @samp{(} represents the open-group, @samp{)} the close-group, @samp{@{}
1098 the open-interval and @samp{@}} the close-interval operator):
1102 If the group matches a substring, the back reference matches an
1103 identical substring. For example, @samp{(a)\1} matches @samp{aa} and
1104 @samp{(bana)na\1bo\1} matches @samp{bananabanabobana}. Likewise,
1105 @samp{(.*)\1} matches any (newline-free if the syntax bit
1106 @code{RE_DOT_NEWLINE} isn't set) string that is composed of two
1107 identical halves; the @samp{(.*)} matches the first half and the
1108 @samp{\1} matches the second half.
1111 If the group matches more than once (as it might if followed
1112 by, e.g., a repetition operator), then the back reference matches the
1113 substring the group @emph{last} matched. For example,
1114 @samp{((a*)b)*\1\2} matches @samp{aabababa}; first @w{group 1} (the
1115 outer one) matches @samp{aab} and @w{group 2} (the inner one) matches
1116 @samp{aa}. Then @w{group 1} matches @samp{ab} and @w{group 2} matches
1117 @samp{a}. So, @samp{\1} matches @samp{ab} and @samp{\2} matches
1121 If the group doesn't participate in a match, i.e., it is part of an
1122 alternative not taken or a repetition operator allows zero repetitions
1123 of it, then the back reference makes the whole match fail. For example,
1124 @samp{(one()|two())-and-(three\2|four\3)} matches @samp{one-and-three}
1125 and @samp{two-and-four}, but not @samp{one-and-four} or
1126 @samp{two-and-three}. For example, if the pattern matches
1127 @samp{one-and-}, then its @w{group 2} matches the empty string and its
1128 @w{group 3} doesn't participate in the match. So, if it then matches
1129 @samp{four}, then when it tries to back reference @w{group 3}---which it
1130 will attempt to do because @samp{\3} follows the @samp{four}---the match
1131 will fail because @w{group 3} didn't participate in the match.
1135 You can use a back reference as an argument to a repetition operator. For
1136 example, @samp{(a(b))\2*} matches @samp{a} followed by two or more
1137 @samp{b}s. Similarly, @samp{(a(b))\2@{3@}} matches @samp{abbbb}.
1139 If there is no preceding @w{@var{digit}-th} subexpression, the regular
1140 expression is invalid.
1143 @node Anchoring Operators
1144 @section Anchoring Operators
1147 @cindex regexp anchoring
1149 These operators can constrain a pattern to match only at the beginning or
1150 end of the entire string or at the beginning or end of a line.
1153 * Match-beginning-of-line Operator:: ^
1154 * Match-end-of-line Operator:: $
1158 @node Match-beginning-of-line Operator
1159 @subsection The Match-beginning-of-line Operator (@code{^})
1162 @cindex beginning-of-line operator
1165 This operator can match the empty string either at the beginning of the
1166 string or after a newline character. Thus, it is said to @dfn{anchor}
1167 the pattern to the beginning of a line.
1169 In the cases following, @samp{^} represents this operator. (Otherwise,
1170 @samp{^} is ordinary.)
1175 It (the @samp{^}) is first in the pattern, as in @samp{^foo}.
1177 @cnindex RE_CONTEXT_INDEP_ANCHORS @r{(and @samp{^})}
1179 The syntax bit @code{RE_CONTEXT_INDEP_ANCHORS} is set, and it is outside
1180 a bracket expression.
1182 @cindex open-group operator and @samp{^}
1183 @cindex alternation operator and @samp{^}
1185 It follows an open-group or alternation operator, as in @samp{a\(^b\)}
1186 and @samp{a\|^b}. @xref{Grouping Operators}, and @ref{Alternation
1191 These rules imply that some valid patterns containing @samp{^} cannot be
1192 matched; for example, @samp{foo^bar} if @code{RE_CONTEXT_INDEP_ANCHORS}
1195 @vindex not_bol @r{field in pattern buffer}
1196 If the @code{not_bol} field is set in the pattern buffer (@pxref{GNU
1197 Pattern Buffers}), then @samp{^} fails to match at the beginning of the
1198 string. @xref{POSIX Matching}, for when you might find this useful.
1200 @vindex newline_anchor @r{field in pattern buffer}
1201 If the @code{newline_anchor} field is set in the pattern buffer, then
1202 @samp{^} fails to match after a newline. This is useful when you do not
1203 regard the string to be matched as broken into lines.
1206 @node Match-end-of-line Operator
1207 @subsection The Match-end-of-line Operator (@code{$})
1210 @cindex end-of-line operator
1213 This operator can match the empty string either at the end of
1214 the string or before a newline character in the string. Thus, it is
1215 said to @dfn{anchor} the pattern to the end of a line.
1217 It is always represented by @samp{$}. For example, @samp{foo$} usually
1218 matches, e.g., @samp{foo} and, e.g., the first three characters of
1221 Its interaction with the syntax bits and pattern buffer fields is
1222 exactly the dual of @samp{^}'s; see the previous section. (That is,
1223 ``@samp{^}'' becomes ``@samp{$}'', ``beginning'' becomes ``end'',
1224 ``next'' becomes ``previous'', ``after'' becomes ``before'', and
1225 ``@code{not_bol}'' becomes ``@code{not_eol}''.)
1229 @chapter GNU Operators
1231 Following are operators that @sc{gnu} defines (and @sc{posix} doesn't).
1235 * Buffer Operators::
1238 @node Word Operators
1239 @section Word Operators
1241 The operators in this section require Regex to recognize parts of words.
1242 Regex uses a syntax table to determine whether or not a character is
1243 part of a word, i.e., whether or not it is @dfn{word-constituent}.
1246 * Non-Emacs Syntax Tables::
1247 * Match-word-boundary Operator:: \b
1248 * Match-within-word Operator:: \B
1249 * Match-beginning-of-word Operator:: \<
1250 * Match-end-of-word Operator:: \>
1251 * Match-word-constituent Operator:: \w
1252 * Match-non-word-constituent Operator:: \W
1255 @node Non-Emacs Syntax Tables
1256 @subsection Non-Emacs Syntax Tables
1258 A @dfn{syntax table} is an array indexed by the characters in your
1259 character set. In the @sc{ascii} encoding, therefore, a syntax table
1260 has 256 elements. Regex always uses a @code{char *} variable
1261 @code{re_syntax_table} as its syntax table. In some cases, it
1262 initializes this variable and in others it expects you to initialize it.
1266 If Regex is compiled with the preprocessor symbols @code{emacs} and
1267 @code{SYNTAX_TABLE} both undefined, then Regex allocates
1268 @code{re_syntax_table} and initializes an element @var{i} either to
1269 @code{Sword} (which it defines) if @var{i} is a letter, number, or
1270 @samp{_}, or to zero if it's not.
1273 If Regex is compiled with @code{emacs} undefined but @code{SYNTAX_TABLE}
1274 defined, then Regex expects you to define a @code{char *} variable
1275 @code{re_syntax_table} to be a valid syntax table.
1278 @xref{Emacs Syntax Tables}, for what happens when Regex is compiled with
1279 the preprocessor symbol @code{emacs} defined.
1283 @node Match-word-boundary Operator
1284 @subsection The Match-word-boundary Operator (@code{\b})
1287 @cindex word boundaries, matching
1289 This operator (represented by @samp{\b}) matches the empty string at
1290 either the beginning or the end of a word. For example, @samp{\brat\b}
1291 matches the separate word @samp{rat}.
1293 @node Match-within-word Operator
1294 @subsection The Match-within-word Operator (@code{\B})
1298 This operator (represented by @samp{\B}) matches the empty string within
1299 a word. For example, @samp{c\Brat\Be} matches @samp{crate}, but
1300 @samp{dirty \Brat} doesn't match @samp{dirty rat}.
1302 @node Match-beginning-of-word Operator
1303 @subsection The Match-beginning-of-word Operator (@code{\<})
1307 This operator (represented by @samp{\<}) matches the empty string at the
1308 beginning of a word.
1310 @node Match-end-of-word Operator
1311 @subsection The Match-end-of-word Operator (@code{\>})
1315 This operator (represented by @samp{\>}) matches the empty string at the
1318 @node Match-word-constituent Operator
1319 @subsection The Match-word-constituent Operator (@code{\w})
1323 This operator (represented by @samp{\w}) matches any word-constituent
1326 @node Match-non-word-constituent Operator
1327 @subsection The Match-non-word-constituent Operator (@code{\W})
1331 This operator (represented by @samp{\W}) matches any character that is
1332 not word-constituent.
1335 @node Buffer Operators
1336 @section Buffer Operators
1338 Following are operators which work on buffers. In Emacs, a @dfn{buffer}
1339 is, naturally, an Emacs buffer. For other programs, Regex considers the
1340 entire string to be matched as the buffer.
1343 * Match-beginning-of-buffer Operator:: \`
1344 * Match-end-of-buffer Operator:: \'
1348 @node Match-beginning-of-buffer Operator
1349 @subsection The Match-beginning-of-buffer Operator (@code{\`})
1353 This operator (represented by @samp{\`}) matches the empty string at the
1354 beginning of the buffer.
1356 @node Match-end-of-buffer Operator
1357 @subsection The Match-end-of-buffer Operator (@code{\'})
1361 This operator (represented by @samp{\'}) matches the empty string at the
1365 @node GNU Emacs Operators
1366 @chapter GNU Emacs Operators
1368 Following are operators that @sc{gnu} defines (and @sc{posix} doesn't)
1369 that you can use only when Regex is compiled with the preprocessor
1370 symbol @code{emacs} defined.
1373 * Syntactic Class Operators::
1377 @node Syntactic Class Operators
1378 @section Syntactic Class Operators
1380 The operators in this section require Regex to recognize the syntactic
1381 classes of characters. Regex uses a syntax table to determine this.
1384 * Emacs Syntax Tables::
1385 * Match-syntactic-class Operator:: \sCLASS
1386 * Match-not-syntactic-class Operator:: \SCLASS
1389 @node Emacs Syntax Tables
1390 @subsection Emacs Syntax Tables
1392 A @dfn{syntax table} is an array indexed by the characters in your
1393 character set. In the @sc{ascii} encoding, therefore, a syntax table
1396 If Regex is compiled with the preprocessor symbol @code{emacs} defined,
1397 then Regex expects you to define and initialize the variable
1398 @code{re_syntax_table} to be an Emacs syntax table. Emacs' syntax
1399 tables are more complicated than Regex's own (@pxref{Non-Emacs Syntax
1400 Tables}). @xref{Syntax, , Syntax, emacs, The GNU Emacs User's Manual},
1401 for a description of Emacs' syntax tables.
1403 @node Match-syntactic-class Operator
1404 @subsection The Match-syntactic-class Operator (@code{\s}@var{class})
1408 This operator matches any character whose syntactic class is represented
1409 by a specified character. @samp{\s@var{class}} represents this operator
1410 where @var{class} is the character representing the syntactic class you
1411 want. For example, @samp{w} represents the syntactic
1412 class of word-constituent characters, so @samp{\sw} matches any
1413 word-constituent character.
1415 @node Match-not-syntactic-class Operator
1416 @subsection The Match-not-syntactic-class Operator (@code{\S}@var{class})
1420 This operator is similar to the match-syntactic-class operator except
1421 that it matches any character whose syntactic class is @emph{not}
1422 represented by the specified character. @samp{\S@var{class}} represents
1423 this operator. For example, @samp{w} represents the syntactic class of
1424 word-constituent characters, so @samp{\Sw} matches any character that is
1425 not word-constituent.
1428 @node What Gets Matched?
1429 @chapter What Gets Matched?
1431 Regex usually matches strings according to the ``leftmost longest''
1432 rule; that is, it chooses the longest of the leftmost matches. This
1433 does not mean that for a regular expression containing subexpressions
1434 that it simply chooses the longest match for each subexpression, left to
1435 right; the overall match must also be the longest possible one.
1437 For example, @samp{(ac*)(c*d[ac]*)\1} matches @samp{acdacaaa}, not
1438 @samp{acdac}, as it would if it were to choose the longest match for the
1439 first subexpression.
1442 @node Programming with Regex
1443 @chapter Programming with Regex
1445 Here we describe how you use the Regex data structures and functions in
1446 C programs. Regex has three interfaces: one designed for @sc{gnu}, one
1447 compatible with @sc{posix} and one compatible with Berkeley @sc{unix}.
1450 * GNU Regex Functions::
1451 * POSIX Regex Functions::
1452 * BSD Regex Functions::
1456 @node GNU Regex Functions
1457 @section GNU Regex Functions
1459 If you're writing code that doesn't need to be compatible with either
1460 @sc{posix} or Berkeley @sc{unix}, you can use these functions. They
1461 provide more options than the other interfaces.
1464 * GNU Pattern Buffers:: The re_pattern_buffer type.
1465 * GNU Regular Expression Compiling:: re_compile_pattern ()
1466 * GNU Matching:: re_match ()
1467 * GNU Searching:: re_search ()
1468 * Matching/Searching with Split Data:: re_match_2 (), re_search_2 ()
1469 * Searching with Fastmaps:: re_compile_fastmap ()
1470 * GNU Translate Tables:: The `translate' field.
1471 * Using Registers:: The re_registers type and related fns.
1472 * Freeing GNU Pattern Buffers:: regfree ()
1476 @node GNU Pattern Buffers
1477 @subsection GNU Pattern Buffers
1479 @cindex pattern buffer, definition of
1480 @tindex re_pattern_buffer @r{definition}
1481 @tindex struct re_pattern_buffer @r{definition}
1483 To compile, match, or search for a given regular expression, you must
1484 supply a pattern buffer. A @dfn{pattern buffer} holds one compiled
1485 regular expression.@footnote{Regular expressions are also referred to as
1486 ``patterns,'' hence the name ``pattern buffer.''}
1488 You can have several different pattern buffers simultaneously, each
1489 holding a compiled pattern for a different regular expression.
1491 @file{regex.h} defines the pattern buffer @code{struct} as follows:
1494 /* Space that holds the compiled pattern. It is declared as
1495 `unsigned char *' because its elements are
1496 sometimes used as array indexes. */
1497 unsigned char *buffer;
1499 /* Number of bytes to which `buffer' points. */
1500 unsigned long allocated;
1502 /* Number of bytes actually used in `buffer'. */
1505 /* Syntax setting with which the pattern was compiled. */
1506 reg_syntax_t syntax;
1508 /* Pointer to a fastmap, if any, otherwise zero. re_search uses
1509 the fastmap, if there is one, to skip over impossible
1510 starting points for matches. */
1513 /* Either a translate table to apply to all characters before
1514 comparing them, or zero for no translation. The translation
1515 is applied to a pattern when it is compiled and to a string
1516 when it is matched. */
1519 /* Number of subexpressions found by the compiler. */
1522 /* Zero if this pattern cannot match the empty string, one else.
1523 Well, in truth it's used only in `re_search_2', to see
1524 whether or not we should use the fastmap, so we don't set
1525 this absolutely perfectly; see `re_compile_fastmap' (the
1526 `duplicate' case). */
1527 unsigned can_be_null : 1;
1529 /* If REGS_UNALLOCATED, allocate space in the `regs' structure
1530 for `max (RE_NREGS, re_nsub + 1)' groups.
1531 If REGS_REALLOCATE, reallocate space if necessary.
1532 If REGS_FIXED, use what's there. */
1533 #define REGS_UNALLOCATED 0
1534 #define REGS_REALLOCATE 1
1535 #define REGS_FIXED 2
1536 unsigned regs_allocated : 2;
1538 /* Set to zero when `regex_compile' compiles a pattern; set to one
1539 by `re_compile_fastmap' if it updates the fastmap. */
1540 unsigned fastmap_accurate : 1;
1542 /* If set, `re_match_2' does not return information about
1544 unsigned no_sub : 1;
1546 /* If set, a beginning-of-line anchor doesn't match at the
1547 beginning of the string. */
1548 unsigned not_bol : 1;
1550 /* Similarly for an end-of-line anchor. */
1551 unsigned not_eol : 1;
1553 /* If true, an anchor at a newline matches. */
1554 unsigned newline_anchor : 1;
1559 @node GNU Regular Expression Compiling
1560 @subsection GNU Regular Expression Compiling
1562 In @sc{gnu}, you can both match and search for a given regular
1563 expression. To do either, you must first compile it in a pattern buffer
1564 (@pxref{GNU Pattern Buffers}).
1566 @cindex syntax initialization
1567 @vindex re_syntax_options @r{initialization}
1568 Regular expressions match according to the syntax with which they were
1569 compiled; with @sc{gnu}, you indicate what syntax you want by setting
1570 the variable @code{re_syntax_options} (declared in @file{regex.h})
1571 before calling the compiling function, @code{re_compile_pattern} (see
1572 below). @xref{Syntax Bits}, and @ref{Predefined Syntaxes}.
1574 You can change the value of @code{re_syntax_options} at any time.
1575 Usually, however, you set its value once and then never change it.
1577 @cindex pattern buffer initialization
1578 @code{re_compile_pattern} takes a pattern buffer as an argument. You
1579 must initialize the following fields:
1583 @item translate @r{initialization}
1586 @vindex translate @r{initialization}
1587 Initialize this to point to a translate table if you want one, or to
1588 zero if you don't. We explain translate tables in @ref{GNU Translate
1592 @vindex fastmap @r{initialization}
1593 Initialize this to nonzero if you want a fastmap, or to zero if you
1598 @vindex buffer @r{initialization}
1599 @vindex allocated @r{initialization}
1601 If you want @code{re_compile_pattern} to allocate memory for the
1602 compiled pattern, set both of these to zero. If you have an existing
1603 block of memory (allocated with @code{malloc}) you want Regex to use,
1604 set @code{buffer} to its address and @code{allocated} to its size (in
1607 @code{re_compile_pattern} uses @code{realloc} to extend the space for
1608 the compiled pattern as necessary.
1612 To compile a pattern buffer, use:
1614 @findex re_compile_pattern
1617 re_compile_pattern (const char *@var{regex}, const int @var{regex_size},
1618 struct re_pattern_buffer *@var{pattern_buffer})
1622 @var{regex} is the regular expression's address, @var{regex_size} is its
1623 length, and @var{pattern_buffer} is the pattern buffer's address.
1625 If @code{re_compile_pattern} successfully compiles the regular
1626 expression, it returns zero and sets @code{*@var{pattern_buffer}} to the
1627 compiled pattern. It sets the pattern buffer's fields as follows:
1631 @vindex buffer @r{field, set by @code{re_compile_pattern}}
1632 to the compiled pattern.
1635 @vindex used @r{field, set by @code{re_compile_pattern}}
1636 to the number of bytes the compiled pattern in @code{buffer} occupies.
1639 @vindex syntax @r{field, set by @code{re_compile_pattern}}
1640 to the current value of @code{re_syntax_options}.
1643 @vindex re_nsub @r{field, set by @code{re_compile_pattern}}
1644 to the number of subexpressions in @var{regex}.
1646 @item fastmap_accurate
1647 @vindex fastmap_accurate @r{field, set by @code{re_compile_pattern}}
1648 to zero on the theory that the pattern you're compiling is different
1649 than the one previously compiled into @code{buffer}; in that case (since
1650 you can't make a fastmap without a compiled pattern),
1651 @code{fastmap} would either contain an incompatible fastmap, or nothing
1657 If @code{re_compile_pattern} can't compile @var{regex}, it returns an
1658 error string corresponding to one of the errors listed in @ref{POSIX
1659 Regular Expression Compiling}.
1663 @subsection GNU Matching
1665 @cindex matching with GNU functions
1667 Matching the @sc{gnu} way means trying to match as much of a string as
1668 possible starting at a position within it you specify. Once you've compiled
1669 a pattern into a pattern buffer (@pxref{GNU Regular Expression
1670 Compiling}), you can ask the matcher to match that pattern against a
1676 re_match (struct re_pattern_buffer *@var{pattern_buffer},
1677 const char *@var{string}, const int @var{size},
1678 const int @var{start}, struct re_registers *@var{regs})
1682 @var{pattern_buffer} is the address of a pattern buffer containing a
1683 compiled pattern. @var{string} is the string you want to match; it can
1684 contain newline and null characters. @var{size} is the length of that
1685 string. @var{start} is the string index at which you want to
1686 begin matching; the first character of @var{string} is at index zero.
1687 @xref{Using Registers}, for a explanation of @var{regs}; you can safely
1690 @code{re_match} matches the regular expression in @var{pattern_buffer}
1691 against the string @var{string} according to the syntax in
1692 @var{pattern_buffers}'s @code{syntax} field. (@xref{GNU Regular
1693 Expression Compiling}, for how to set it.) The function returns
1694 @math{-1} if the compiled pattern does not match any part of
1695 @var{string} and @math{-2} if an internal error happens; otherwise, it
1696 returns how many (possibly zero) characters of @var{string} the pattern
1699 An example: suppose @var{pattern_buffer} points to a pattern buffer
1700 containing the compiled pattern for @samp{a*}, and @var{string} points
1701 to @samp{aaaaab} (whereupon @var{size} should be 6). Then if @var{start}
1702 is 2, @code{re_match} returns 3, i.e., @samp{a*} would have matched the
1703 last three @samp{a}s in @var{string}. If @var{start} is 0,
1704 @code{re_match} returns 5, i.e., @samp{a*} would have matched all the
1705 @samp{a}s in @var{string}. If @var{start} is either 5 or 6, it returns
1708 If @var{start} is not between zero and @var{size}, then
1709 @code{re_match} returns @math{-1}.
1713 @subsection GNU Searching
1715 @cindex searching with GNU functions
1717 @dfn{Searching} means trying to match starting at successive positions
1718 within a string. The function @code{re_search} does this.
1720 Before calling @code{re_search}, you must compile your regular
1721 expression. @xref{GNU Regular Expression Compiling}.
1723 Here is the function declaration:
1728 re_search (struct re_pattern_buffer *@var{pattern_buffer},
1729 const char *@var{string}, const int @var{size},
1730 const int @var{start}, const int @var{range},
1731 struct re_registers *@var{regs})
1735 @vindex start @r{argument to @code{re_search}}
1736 @vindex range @r{argument to @code{re_search}}
1737 whose arguments are the same as those to @code{re_match} (@pxref{GNU
1738 Matching}) except that the two arguments @var{start} and @var{range}
1739 replace @code{re_match}'s argument @var{start}.
1741 If @var{range} is positive, then @code{re_search} attempts a match
1742 starting first at index @var{start}, then at @math{@var{start} + 1} if
1743 that fails, and so on, up to @math{@var{start} + @var{range}}; if
1744 @var{range} is negative, then it attempts a match starting first at
1745 index @var{start}, then at @math{@var{start} -1} if that fails, and so
1748 If @var{start} is not between zero and @var{size}, then @code{re_search}
1749 returns @math{-1}. When @var{range} is positive, @code{re_search}
1750 adjusts @var{range} so that @math{@var{start} + @var{range} - 1} is
1751 between zero and @var{size}, if necessary; that way it won't search
1752 outside of @var{string}. Similarly, when @var{range} is negative,
1753 @code{re_search} adjusts @var{range} so that @math{@var{start} +
1754 @var{range} + 1} is between zero and @var{size}, if necessary.
1756 If the @code{fastmap} field of @var{pattern_buffer} is zero,
1757 @code{re_search} matches starting at consecutive positions; otherwise,
1758 it uses @code{fastmap} to make the search more efficient.
1759 @xref{Searching with Fastmaps}.
1761 If no match is found, @code{re_search} returns @math{-1}. If
1762 a match is found, it returns the index where the match began. If an
1763 internal error happens, it returns @math{-2}.
1766 @node Matching/Searching with Split Data
1767 @subsection Matching and Searching with Split Data
1769 Using the functions @code{re_match_2} and @code{re_search_2}, you can
1770 match or search in data that is divided into two strings.
1777 re_match_2 (struct re_pattern_buffer *@var{buffer},
1778 const char *@var{string1}, const int @var{size1},
1779 const char *@var{string2}, const int @var{size2},
1780 const int @var{start},
1781 struct re_registers *@var{regs},
1782 const int @var{stop})
1786 is similar to @code{re_match} (@pxref{GNU Matching}) except that you
1787 pass @emph{two} data strings and sizes, and an index @var{stop} beyond
1788 which you don't want the matcher to try matching. As with
1789 @code{re_match}, if it succeeds, @code{re_match_2} returns how many
1790 characters of @var{string} it matched. Regard @var{string1} and
1791 @var{string2} as concatenated when you set the arguments @var{start} and
1792 @var{stop} and use the contents of @var{regs}; @code{re_match_2} never
1793 returns a value larger than @math{@var{size1} + @var{size2}}.
1800 re_search_2 (struct re_pattern_buffer *@var{buffer},
1801 const char *@var{string1}, const int @var{size1},
1802 const char *@var{string2}, const int @var{size2},
1803 const int @var{start}, const int @var{range},
1804 struct re_registers *@var{regs},
1805 const int @var{stop})
1809 is similarly related to @code{re_search}.
1812 @node Searching with Fastmaps
1813 @subsection Searching with Fastmaps
1816 If you're searching through a long string, you should use a fastmap.
1817 Without one, the searcher tries to match at consecutive positions in the
1818 string. Generally, most of the characters in the string could not start
1819 a match. It takes much longer to try matching at a given position in the
1820 string than it does to check in a table whether or not the character at
1821 that position could start a match. A @dfn{fastmap} is such a table.
1823 More specifically, a fastmap is an array indexed by the characters in
1824 your character set. Under the @sc{ascii} encoding, therefore, a fastmap
1825 has 256 elements. If you want the searcher to use a fastmap with a
1826 given pattern buffer, you must allocate the array and assign the array's
1827 address to the pattern buffer's @code{fastmap} field. You either can
1828 compile the fastmap yourself or have @code{re_search} do it for you;
1829 when @code{fastmap} is nonzero, it automatically compiles a fastmap the
1830 first time you search using a particular compiled pattern.
1832 To compile a fastmap yourself, use:
1834 @findex re_compile_fastmap
1837 re_compile_fastmap (struct re_pattern_buffer *@var{pattern_buffer})
1841 @var{pattern_buffer} is the address of a pattern buffer. If the
1842 character @var{c} could start a match for the pattern,
1843 @code{re_compile_fastmap} makes
1844 @code{@var{pattern_buffer}->fastmap[@var{c}]} nonzero. It returns
1845 @math{0} if it can compile a fastmap and @math{-2} if there is an
1846 internal error. For example, if @samp{|} is the alternation operator
1847 and @var{pattern_buffer} holds the compiled pattern for @samp{a|b}, then
1848 @code{re_compile_fastmap} sets @code{fastmap['a']} and
1849 @code{fastmap['b']} (and no others).
1851 @code{re_search} uses a fastmap as it moves along in the string: it
1852 checks the string's characters until it finds one that's in the fastmap.
1853 Then it tries matching at that character. If the match fails, it
1854 repeats the process. So, by using a fastmap, @code{re_search} doesn't
1855 waste time trying to match at positions in the string that couldn't
1858 If you don't want @code{re_search} to use a fastmap,
1859 store zero in the @code{fastmap} field of the pattern buffer before
1860 calling @code{re_search}.
1862 Once you've initialized a pattern buffer's @code{fastmap} field, you
1863 need never do so again---even if you compile a new pattern in
1864 it---provided the way the field is set still reflects whether or not you
1865 want a fastmap. @code{re_search} will still either do nothing if
1866 @code{fastmap} is null or, if it isn't, compile a new fastmap for the
1869 @node GNU Translate Tables
1870 @subsection GNU Translate Tables
1872 If you set the @code{translate} field of a pattern buffer to a translate
1873 table, then the @sc{gnu} Regex functions to which you've passed that
1874 pattern buffer use it to apply a simple transformation
1875 to all the regular expression and string characters at which they look.
1877 A @dfn{translate table} is an array indexed by the characters in your
1878 character set. Under the @sc{ascii} encoding, therefore, a translate
1879 table has 256 elements. The array's elements are also characters in
1880 your character set. When the Regex functions see a character @var{c},
1881 they use @code{translate[@var{c}]} in its place, with one exception: the
1882 character after a @samp{\} is not translated. (This ensures that, the
1883 operators, e.g., @samp{\B} and @samp{\b}, are always distinguishable.)
1885 For example, a table that maps all lowercase letters to the
1886 corresponding uppercase ones would cause the matcher to ignore
1887 differences in case.@footnote{A table that maps all uppercase letters to
1888 the corresponding lowercase ones would work just as well for this
1889 purpose.} Such a table would map all characters except lowercase letters
1890 to themselves, and lowercase letters to the corresponding uppercase
1891 ones. Under the @sc{ascii} encoding, here's how you could initialize
1892 such a table (we'll call it @code{case_fold}):
1895 for (i = 0; i < 256; i++)
1897 for (i = 'a'; i <= 'z'; i++)
1898 case_fold[i] = i - ('a' - 'A');
1901 You tell Regex to use a translate table on a given pattern buffer by
1902 assigning that table's address to the @code{translate} field of that
1903 buffer. If you don't want Regex to do any translation, put zero into
1904 this field. You'll get weird results if you change the table's contents
1905 anytime between compiling the pattern buffer, compiling its fastmap, and
1906 matching or searching with the pattern buffer.
1908 @node Using Registers
1909 @subsection Using Registers
1911 A group in a regular expression can match a (posssibly empty) substring
1912 of the string that regular expression as a whole matched. The matcher
1913 remembers the beginning and end of the substring matched by
1916 To find out what they matched, pass a nonzero @var{regs} argument to a
1917 @sc{gnu} matching or searching function (@pxref{GNU Matching} and
1918 @ref{GNU Searching}), i.e., the address of a structure of this type, as
1919 defined in @file{regex.h}:
1921 @c We don't bother to include this directly from regex.h,
1922 @c since it changes so rarely.
1924 @tindex re_registers
1925 @vindex num_regs @r{in @code{struct re_registers}}
1926 @vindex start @r{in @code{struct re_registers}}
1927 @vindex end @r{in @code{struct re_registers}}
1936 Except for (possibly) the @var{num_regs}'th element (see below), the
1937 @var{i}th element of the @code{start} and @code{end} arrays records
1938 information about the @var{i}th group in the pattern. (They're declared
1939 as C pointers, but this is only because not all C compilers accept
1940 zero-length arrays; conceptually, it is simplest to think of them as
1943 The @code{start} and @code{end} arrays are allocated in various ways,
1944 depending on the value of the @code{regs_allocated}
1945 @vindex regs_allocated
1946 field in the pattern buffer passed to the matcher.
1948 The simplest and perhaps most useful is to let the matcher (re)allocate
1949 enough space to record information for all the groups in the regular
1950 expression. If @code{regs_allocated} is @code{REGS_UNALLOCATED},
1951 @vindex REGS_UNALLOCATED
1952 the matcher allocates @math{1 + @var{re_nsub}} (another field in the
1953 pattern buffer; @pxref{GNU Pattern Buffers}). The extra element is set
1954 to @math{-1}, and sets @code{regs_allocated} to @code{REGS_REALLOCATE}.
1955 @vindex REGS_REALLOCATE
1956 Then on subsequent calls with the same pattern buffer and @var{regs}
1957 arguments, the matcher reallocates more space if necessary.
1959 It would perhaps be more logical to make the @code{regs_allocated} field
1960 part of the @code{re_registers} structure, instead of part of the
1961 pattern buffer. But in that case the caller would be forced to
1962 initialize the structure before passing it. Much existing code doesn't
1963 do this initialization, and it's arguably better to avoid it anyway.
1965 @code{re_compile_pattern} sets @code{regs_allocated} to
1966 @code{REGS_UNALLOCATED},
1967 so if you use the GNU regular expression
1968 functions, you get this behavior by default.
1970 xx document re_set_registers
1972 @sc{posix}, on the other hand, requires a different interface: the
1973 caller is supposed to pass in a fixed-length array which the matcher
1974 fills. Therefore, if @code{regs_allocated} is @code{REGS_FIXED}
1976 the matcher simply fills that array.
1978 The following examples illustrate the information recorded in the
1979 @code{re_registers} structure. (In all of them, @samp{(} represents the
1980 open-group and @samp{)} the close-group operator. The first character
1981 in the string @var{string} is at index 0.)
1983 @c xx i'm not sure this is all true anymore.
1988 If the regular expression has an @w{@var{i}-th}
1989 group not contained within another group that matches a
1990 substring of @var{string}, then the function sets
1991 @code{@w{@var{regs}->}start[@var{i}]} to the index in @var{string} where
1992 the substring matched by the @w{@var{i}-th} group begins, and
1993 @code{@w{@var{regs}->}end[@var{i}]} to the index just beyond that
1994 substring's end. The function sets @code{@w{@var{regs}->}start[0]} and
1995 @code{@w{@var{regs}->}end[0]} to analogous information about the entire
1998 For example, when you match @samp{((a)(b))} against @samp{ab}, you get:
2002 0 in @code{@w{@var{regs}->}start[0]} and 2 in @code{@w{@var{regs}->}end[0]}
2005 0 in @code{@w{@var{regs}->}start[1]} and 2 in @code{@w{@var{regs}->}end[1]}
2008 0 in @code{@w{@var{regs}->}start[2]} and 1 in @code{@w{@var{regs}->}end[2]}
2011 1 in @code{@w{@var{regs}->}start[3]} and 2 in @code{@w{@var{regs}->}end[3]}
2015 If a group matches more than once (as it might if followed by,
2016 e.g., a repetition operator), then the function reports the information
2017 about what the group @emph{last} matched.
2019 For example, when you match the pattern @samp{(a)*} against the string
2024 0 in @code{@w{@var{regs}->}start[0]} and 2 in @code{@w{@var{regs}->}end[0]}
2027 1 in @code{@w{@var{regs}->}start[1]} and 2 in @code{@w{@var{regs}->}end[1]}
2031 If the @w{@var{i}-th} group does not participate in a
2032 successful match, e.g., it is an alternative not taken or a
2033 repetition operator allows zero repetitions of it, then the function
2034 sets @code{@w{@var{regs}->}start[@var{i}]} and
2035 @code{@w{@var{regs}->}end[@var{i}]} to @math{-1}.
2037 For example, when you match the pattern @samp{(a)*b} against
2038 the string @samp{b}, you get:
2042 0 in @code{@w{@var{regs}->}start[0]} and 1 in @code{@w{@var{regs}->}end[0]}
2045 @math{-1} in @code{@w{@var{regs}->}start[1]} and @math{-1} in @code{@w{@var{regs}->}end[1]}
2049 If the @w{@var{i}-th} group matches a zero-length string, then the
2050 function sets @code{@w{@var{regs}->}start[@var{i}]} and
2051 @code{@w{@var{regs}->}end[@var{i}]} to the index just beyond that
2054 For example, when you match the pattern @samp{(a*)b} against the string
2059 0 in @code{@w{@var{regs}->}start[0]} and 1 in @code{@w{@var{regs}->}end[0]}
2062 0 in @code{@w{@var{regs}->}start[1]} and 0 in @code{@w{@var{regs}->}end[1]}
2066 The function sets @code{@w{@var{regs}->}start[0]} and
2067 @code{@w{@var{regs}->}end[0]} to analogous information about the entire
2070 For example, when you match the pattern @samp{(a*)} against the empty
2075 0 in @code{@w{@var{regs}->}start[0]} and 0 in @code{@w{@var{regs}->}end[0]}
2078 0 in @code{@w{@var{regs}->}start[1]} and 0 in @code{@w{@var{regs}->}end[1]}
2083 If an @w{@var{i}-th} group contains a @w{@var{j}-th} group
2084 in turn not contained within any other group within group @var{i} and
2085 the function reports a match of the @w{@var{i}-th} group, then it
2086 records in @code{@w{@var{regs}->}start[@var{j}]} and
2087 @code{@w{@var{regs}->}end[@var{j}]} the last match (if it matched) of
2088 the @w{@var{j}-th} group.
2090 For example, when you match the pattern @samp{((a*)b)*} against the
2091 string @samp{abb}, @w{group 2} last matches the empty string, so you
2092 get what it previously matched:
2096 0 in @code{@w{@var{regs}->}start[0]} and 3 in @code{@w{@var{regs}->}end[0]}
2099 2 in @code{@w{@var{regs}->}start[1]} and 3 in @code{@w{@var{regs}->}end[1]}
2102 2 in @code{@w{@var{regs}->}start[2]} and 2 in @code{@w{@var{regs}->}end[2]}
2105 When you match the pattern @samp{((a)*b)*} against the string
2106 @samp{abb}, @w{group 2} doesn't participate in the last match, so you
2111 0 in @code{@w{@var{regs}->}start[0]} and 3 in @code{@w{@var{regs}->}end[0]}
2114 2 in @code{@w{@var{regs}->}start[1]} and 3 in @code{@w{@var{regs}->}end[1]}
2117 0 in @code{@w{@var{regs}->}start[2]} and 1 in @code{@w{@var{regs}->}end[2]}
2121 If an @w{@var{i}-th} group contains a @w{@var{j}-th} group
2122 in turn not contained within any other group within group @var{i}
2123 and the function sets
2124 @code{@w{@var{regs}->}start[@var{i}]} and
2125 @code{@w{@var{regs}->}end[@var{i}]} to @math{-1}, then it also sets
2126 @code{@w{@var{regs}->}start[@var{j}]} and
2127 @code{@w{@var{regs}->}end[@var{j}]} to @math{-1}.
2129 For example, when you match the pattern @samp{((a)*b)*c} against the
2130 string @samp{c}, you get:
2134 0 in @code{@w{@var{regs}->}start[0]} and 1 in @code{@w{@var{regs}->}end[0]}
2137 @math{-1} in @code{@w{@var{regs}->}start[1]} and @math{-1} in @code{@w{@var{regs}->}end[1]}
2140 @math{-1} in @code{@w{@var{regs}->}start[2]} and @math{-1} in @code{@w{@var{regs}->}end[2]}
2145 @node Freeing GNU Pattern Buffers
2146 @subsection Freeing GNU Pattern Buffers
2148 To free any allocated fields of a pattern buffer, you can use the
2149 @sc{posix} function described in @ref{Freeing POSIX Pattern Buffers},
2150 since the type @code{regex_t}---the type for @sc{posix} pattern
2151 buffers---is equivalent to the type @code{re_pattern_buffer}. After
2152 freeing a pattern buffer, you need to again compile a regular expression
2153 in it (@pxref{GNU Regular Expression Compiling}) before passing it to
2154 a matching or searching function.
2157 @node POSIX Regex Functions
2158 @section POSIX Regex Functions
2160 If you're writing code that has to be @sc{posix} compatible, you'll need
2161 to use these functions. Their interfaces are as specified by @sc{posix},
2165 * POSIX Pattern Buffers:: The regex_t type.
2166 * POSIX Regular Expression Compiling:: regcomp ()
2167 * POSIX Matching:: regexec ()
2168 * Reporting Errors:: regerror ()
2169 * Using Byte Offsets:: The regmatch_t type.
2170 * Freeing POSIX Pattern Buffers:: regfree ()
2174 @node POSIX Pattern Buffers
2175 @subsection POSIX Pattern Buffers
2177 To compile or match a given regular expression the @sc{posix} way, you
2178 must supply a pattern buffer exactly the way you do for @sc{gnu}
2179 (@pxref{GNU Pattern Buffers}). @sc{posix} pattern buffers have type
2180 @code{regex_t}, which is equivalent to the @sc{gnu} pattern buffer
2181 type @code{re_pattern_buffer}.
2184 @node POSIX Regular Expression Compiling
2185 @subsection POSIX Regular Expression Compiling
2187 With @sc{posix}, you can only search for a given regular expression; you
2188 can't match it. To do this, you must first compile it in a
2189 pattern buffer, using @code{regcomp}.
2192 Before calling @code{regcomp}, you must initialize this pattern buffer
2193 as you do for @sc{gnu} (@pxref{GNU Regular Expression Compiling}). See
2194 below, however, for how to choose a syntax with which to compile.
2197 To compile a pattern buffer, use:
2202 regcomp (regex_t *@var{preg}, const char *@var{regex}, int @var{cflags})
2206 @var{preg} is the initialized pattern buffer's address, @var{regex} is
2207 the regular expression's address, and @var{cflags} is the compilation
2208 flags, which Regex considers as a collection of bits. Here are the
2209 valid bits, as defined in @file{regex.h}:
2214 @vindex REG_EXTENDED
2215 says to use @sc{posix} Extended Regular Expression syntax; if this isn't
2216 set, then says to use @sc{posix} Basic Regular Expression syntax.
2217 @code{regcomp} sets @var{preg}'s @code{syntax} field accordingly.
2221 @cindex ignoring case
2222 says to ignore case; @code{regcomp} sets @var{preg}'s @code{translate}
2223 field to a translate table which ignores case, replacing anything you've
2228 says to set @var{preg}'s @code{no_sub} field; @pxref{POSIX Matching},
2229 for what this means.
2238 match-any-character operator (@pxref{Match-any-character
2239 Operator}) doesn't match a newline.
2242 nonmatching list not containing a newline (@pxref{List
2243 Operators}) matches a newline.
2246 match-beginning-of-line operator (@pxref{Match-beginning-of-line
2247 Operator}) matches the empty string immediately after a newline,
2248 regardless of how @code{REG_NOTBOL} is set (@pxref{POSIX Matching}, for
2249 an explanation of @code{REG_NOTBOL}).
2252 match-end-of-line operator (@pxref{Match-beginning-of-line
2253 Operator}) matches the empty string immediately before a newline,
2254 regardless of how @code{REG_NOTEOL} is set (@pxref{POSIX Matching},
2255 for an explanation of @code{REG_NOTEOL}).
2261 If @code{regcomp} successfully compiles the regular expression, it
2262 returns zero and sets @code{*@var{pattern_buffer}} to the compiled
2263 pattern. Except for @code{syntax} (which it sets as explained above), it
2264 also sets the same fields the same way as does the @sc{gnu} compiling
2265 function (@pxref{GNU Regular Expression Compiling}).
2267 If @code{regcomp} can't compile the regular expression, it returns one
2268 of the error codes listed here. (Except when noted differently, the
2269 syntax of in all examples below is basic regular expression syntax.)
2273 @comment repetitions
2275 For example, the consecutive repetition operators @samp{**} in
2276 @samp{a**} are invalid. As another example, if the syntax is extended
2277 regular expression syntax, then the repetition operator @samp{*} with
2278 nothing on which to operate in @samp{*} is invalid.
2281 For example, the @var{count} @samp{-1} in @samp{a\@{-1} is invalid.
2284 For example, @samp{a\@{1} is missing a close-interval operator.
2288 For example, @samp{[a} is missing a close-list operator.
2291 For example, the range ending point @samp{z} that collates lower than
2292 does its starting point @samp{a} in @samp{[z-a]} is invalid. Also, the
2293 range with the character class @samp{[:alpha:]} as its starting point in
2294 @samp{[[:alpha:]-|]}.
2297 For example, the character class name @samp{foo} in @samp{[[:foo:]} is
2302 For example, @samp{a\)} is missing an open-group operator and @samp{\(a}
2303 is missing a close-group operator.
2306 For example, the back reference @samp{\2} that refers to a nonexistent
2307 subexpression in @samp{\(a\)\2} is invalid.
2309 @comment unfinished business
2312 Returned when a regular expression causes no other more specific error.
2315 For example, the trailing backslash @samp{\} in @samp{a\} is invalid, as is the
2318 @comment kitchen sink
2320 For example, in the extended regular expression syntax, the empty group
2321 @samp{()} in @samp{a()b} is invalid.
2325 Returned when a regular expression needs a pattern buffer larger than
2329 Returned when a regular expression makes Regex to run out of memory.
2334 @node POSIX Matching
2335 @subsection POSIX Matching
2337 Matching the @sc{posix} way means trying to match a null-terminated
2338 string starting at its first character. Once you've compiled a pattern
2339 into a pattern buffer (@pxref{POSIX Regular Expression Compiling}), you
2340 can ask the matcher to match that pattern against a string using:
2345 regexec (const regex_t *@var{preg}, const char *@var{string},
2346 size_t @var{nmatch}, regmatch_t @var{pmatch}[], int @var{eflags})
2350 @var{preg} is the address of a pattern buffer for a compiled pattern.
2351 @var{string} is the string you want to match.
2353 @xref{Using Byte Offsets}, for an explanation of @var{pmatch}. If you
2354 pass zero for @var{nmatch} or you compiled @var{preg} with the
2355 compilation flag @code{REG_NOSUB} set, then @code{regexec} will ignore
2356 @var{pmatch}; otherwise, you must allocate it to have at least
2357 @var{nmatch} elements. @code{regexec} will record @var{nmatch} byte
2358 offsets in @var{pmatch}, and set to @math{-1} any unused elements up to
2359 @math{@var{pmatch}@code{[@var{nmatch}]} - 1}.
2361 @var{eflags} specifies @dfn{execution flags}---namely, the two bits
2362 @code{REG_NOTBOL} and @code{REG_NOTEOL} (defined in @file{regex.h}). If
2363 you set @code{REG_NOTBOL}, then the match-beginning-of-line operator
2364 (@pxref{Match-beginning-of-line Operator}) always fails to match.
2365 This lets you match against pieces of a line, as you would need to if,
2366 say, searching for repeated instances of a given pattern in a line; it
2367 would work correctly for patterns both with and without
2368 match-beginning-of-line operators. @code{REG_NOTEOL} works analogously
2369 for the match-end-of-line operator (@pxref{Match-end-of-line
2370 Operator}); it exists for symmetry.
2372 @code{regexec} tries to find a match for @var{preg} in @var{string}
2373 according to the syntax in @var{preg}'s @code{syntax} field.
2374 (@xref{POSIX Regular Expression Compiling}, for how to set it.) The
2375 function returns zero if the compiled pattern matches @var{string} and
2376 @code{REG_NOMATCH} (defined in @file{regex.h}) if it doesn't.
2378 @node Reporting Errors
2379 @subsection Reporting Errors
2381 If either @code{regcomp} or @code{regexec} fail, they return a nonzero
2382 error code, the possibilities for which are defined in @file{regex.h}.
2383 @xref{POSIX Regular Expression Compiling}, and @ref{POSIX Matching}, for
2384 what these codes mean. To get an error string corresponding to these
2390 regerror (int @var{errcode},
2391 const regex_t *@var{preg},
2393 size_t @var{errbuf_size})
2397 @var{errcode} is an error code, @var{preg} is the address of the pattern
2398 buffer which provoked the error, @var{errbuf} is the error buffer, and
2399 @var{errbuf_size} is @var{errbuf}'s size.
2401 @code{regerror} returns the size in bytes of the error string
2402 corresponding to @var{errcode} (including its terminating null). If
2403 @var{errbuf} and @var{errbuf_size} are nonzero, it also returns in
2404 @var{errbuf} the first @math{@var{errbuf_size} - 1} characters of the
2405 error string, followed by a null.
2406 @var{errbuf_size} must be a nonnegative number less than or equal to the
2407 size in bytes of @var{errbuf}.
2409 You can call @code{regerror} with a null @var{errbuf} and a zero
2410 @var{errbuf_size} to determine how large @var{errbuf} need be to
2411 accommodate @code{regerror}'s error string.
2413 @node Using Byte Offsets
2414 @subsection Using Byte Offsets
2416 In @sc{posix}, variables of type @code{regmatch_t} hold analogous
2417 information, but are not identical to, @sc{gnu}'s registers (@pxref{Using
2418 Registers}). To get information about registers in @sc{posix}, pass to
2419 @code{regexec} a nonzero @var{pmatch} of type @code{regmatch_t}, i.e.,
2420 the address of a structure of this type, defined in
2432 When reading in @ref{Using Registers}, about how the matching function
2433 stores the information into the registers, substitute @var{pmatch} for
2434 @var{regs}, @code{@w{@var{pmatch}[@var{i}]->}rm_so} for
2435 @code{@w{@var{regs}->}start[@var{i}]} and
2436 @code{@w{@var{pmatch}[@var{i}]->}rm_eo} for
2437 @code{@w{@var{regs}->}end[@var{i}]}.
2439 @node Freeing POSIX Pattern Buffers
2440 @subsection Freeing POSIX Pattern Buffers
2442 To free any allocated fields of a pattern buffer, use:
2447 regfree (regex_t *@var{preg})
2451 @var{preg} is the pattern buffer whose allocated fields you want freed.
2452 @code{regfree} also sets @var{preg}'s @code{allocated} and @code{used}
2453 fields to zero. After freeing a pattern buffer, you need to again
2454 compile a regular expression in it (@pxref{POSIX Regular Expression
2455 Compiling}) before passing it to the matching function (@pxref{POSIX
2459 @node BSD Regex Functions
2460 @section BSD Regex Functions
2462 If you're writing code that has to be Berkeley @sc{unix} compatible,
2463 you'll need to use these functions whose interfaces are the same as those
2464 in Berkeley @sc{unix}.
2467 * BSD Regular Expression Compiling:: re_comp ()
2468 * BSD Searching:: re_exec ()
2471 @node BSD Regular Expression Compiling
2472 @subsection BSD Regular Expression Compiling
2474 With Berkeley @sc{unix}, you can only search for a given regular
2475 expression; you can't match one. To search for it, you must first
2476 compile it. Before you compile it, you must indicate the regular
2477 expression syntax you want it compiled according to by setting the
2478 variable @code{re_syntax_options} (declared in @file{regex.h} to some
2479 syntax (@pxref{Regular Expression Syntax}).
2481 To compile a regular expression use:
2486 re_comp (char *@var{regex})
2490 @var{regex} is the address of a null-terminated regular expression.
2491 @code{re_comp} uses an internal pattern buffer, so you can use only the
2492 most recently compiled pattern buffer. This means that if you want to
2493 use a given regular expression that you've already compiled---but it
2494 isn't the latest one you've compiled---you'll have to recompile it. If
2495 you call @code{re_comp} with the null string (@emph{not} the empty
2496 string) as the argument, it doesn't change the contents of the pattern
2499 If @code{re_comp} successfully compiles the regular expression, it
2500 returns zero. If it can't compile the regular expression, it returns
2501 an error string. @code{re_comp}'s error messages are identical to those
2502 of @code{re_compile_pattern} (@pxref{GNU Regular Expression
2506 @subsection BSD Searching
2508 Searching the Berkeley @sc{unix} way means searching in a string
2509 starting at its first character and trying successive positions within
2510 it to find a match. Once you've compiled a pattern using @code{re_comp}
2511 (@pxref{BSD Regular Expression Compiling}), you can ask Regex
2512 to search for that pattern in a string using:
2517 re_exec (char *@var{string})
2521 @var{string} is the address of the null-terminated string in which you
2524 @code{re_exec} returns either 1 for success or 0 for failure. It
2525 automatically uses a @sc{gnu} fastmap (@pxref{Searching with Fastmaps}).