2 '\" Copyright (c) 1998 Sun Microsystems, Inc.
3 '\" Copyright (c) 1999 Scriptics Corporation
5 '\" See the file "license.terms" for information on usage and redistribution
6 '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
8 .\" The -*- nroff -*- definitions below are for supplemental macros used
9 .\" in Tcl/Tk manual entries.
11 .\" .AP type name in/out ?indent?
12 .\" Start paragraph describing an argument to a library procedure.
13 .\" type is type of argument (int, etc.), in/out is either "in", "out",
14 .\" or "in/out" to describe whether procedure reads or modifies arg,
15 .\" and indent is equivalent to second arg of .IP (shouldn't ever be
16 .\" needed; use .AS below instead)
19 .\" Give maximum sizes of arguments for setting tab stops. Type and
20 .\" name are examples of largest possible arguments that will be passed
21 .\" to .AP later. If args are omitted, default tab stops are used.
24 .\" Start box enclosure. From here until next .BE, everything will be
25 .\" enclosed in one large box.
28 .\" End of box enclosure.
31 .\" Begin code excerpt.
36 .\" .VS ?version? ?br?
37 .\" Begin vertical sidebar, for use in marking newly-changed parts
38 .\" of man pages. The first argument is ignored and used for recording
39 .\" the version when the .VS was added, so that the sidebars can be
40 .\" found and removed when they reach a certain age. If another argument
41 .\" is present, then a line break is forced before starting the sidebar.
44 .\" End of vertical sidebar.
47 .\" Begin an indented unfilled display.
50 .\" End of indented unfilled display.
53 .\" Start of list of standard options for a Tk widget. The manpage
54 .\" argument defines where to look up the standard options; if
55 .\" omitted, defaults to "options". The options follow on successive
56 .\" lines, in three columns separated by tabs.
59 .\" End of list of standard options for a Tk widget.
61 .\" .OP cmdName dbName dbClass
62 .\" Start of description of a specific option. cmdName gives the
63 .\" option's name as specified in the class command, dbName gives
64 .\" the option's name in the option database, and dbClass gives
65 .\" the option's class in the option database.
68 .\" Print arg1 underlined, then print arg2 normally.
71 .\" Print arg1 in quotes, then arg2 normally (for trailing punctuation).
74 .\" Print an open parenthesis, arg1 in quotes, then arg2 normally
75 .\" (for trailing punctuation) and then a closing parenthesis.
77 .\" # Set up traps and other miscellaneous stuff for Tcl/Tk man pages.
81 .\" # Start an argument description
85 . ie !"\\$2"" .TP \\n()Cu
90 \&\\$1 \\fI\\$2\\fP (\\$3)
103 .\" # define tabbing values for .AP
106 .if !"\\$1"" .nr )A \\w'\\$1'u+3n
109 .if !"\\$2"" .nr )B \\w'\\$2'u+\\n()Au+3n
110 .nr )C \\n()Bu+\\w'(in/out)'u+2n
112 .AS Tcl_Interp Tcl_CreateInterp in/out
113 .\" # BS - start boxed text
114 .\" # ^y = starting y location
122 .if n \l'\\n(.lu\(ul'
125 .\" # BE - end boxed text (draw box now)
130 .ie n \l'\\n(^lu\(ul'
132 .\" Draw four-sided box normally, but don't draw top of
133 .\" box if the box started on an earlier page.
135 \h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
138 \h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
145 .\" # VS - start vertical sidebar
146 .\" # ^Y = starting y location
147 .\" # ^v = 1 (for troff; for nroff this doesn't matter)
151 .ie n 'mc \s12\(br\s0
154 .\" # VE - end of vertical sidebar
162 \h'|\\n(^lu+3n'\L'|\\n(^Yu-1v\(bv'\v'\\n(^tu+1v-\\n(^Yu'\h'-|\\n(^lu+3n'
169 .\" # Special macro to handle page bottom: finish off current
170 .\" # box/sidebar if in box/sidebar mode, then invoked standard
171 .\" # page bottom macro.
178 .\" Draw three-sided box if this is the box's first page,
179 .\" draw two sides but no top otherwise.
180 .ie !\\n(^b-1 \h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c
181 .el \h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c
184 .nr ^x \\n(^tu+1v-\\n(^Yu
185 \kx\h'-\\nxu'\h'|\\n(^lu+3n'\ky\L'-\\n(^xu'\v'\\n(^xu'\h'|0u'\c
198 .\" # DS - begin display
204 .\" # DE - end display
210 .\" # SO - start of list of standard options
212 'ie '\\$1'' .ds So \\fBoptions\\fR
213 'el .ds So \\fB\\$1\\fR
214 .SH "STANDARD OPTIONS"
220 .\" # SE - end of list of standard options
225 See the \\*(So manual entry for details on the standard options.
227 .\" # OP - start of full description for a single option
232 Command-Line Name: \\fB\\$1\\fR
233 Database Name: \\fB\\$2\\fR
234 Database Class: \\fB\\$3\\fR
238 .\" # CS - begin code excerpt
244 .\" # CE - end code excerpt
249 .\" # UL - underline word
253 .\" # QW - apply quotation marks to word
255 .ie '\\*(lq'"' ``\\$1''\\$2
256 .\"" fix emacs highlighting
257 .el \\*(lq\\$1\\*(rq\\$2
259 .\" # PQ - apply parens and quotation marks to word
261 .ie '\\*(lq'"' (``\\$1''\\$2)\\$3
262 .\"" fix emacs highlighting
263 .el (\\*(lq\\$1\\*(rq\\$2)\\$3
265 .\" # QR - quoted range
267 .ie '\\*(lq'"' ``\\$1''\\-``\\$2''\\$3
268 .\"" fix emacs highlighting
269 .el \\*(lq\\$1\\*(rq\\-\\*(lq\\$2\\*(rq\\$3
271 .\" # MT - "empty" string
275 .ie '\w'o''\w'\C'^o''' .ds qo \C'^o'
277 .TH re_syntax n "8.1" Tcl "Tcl Built-In Commands"
280 re_syntax \- Syntax of Tcl regular expressions
284 A \fIregular expression\fR describes strings of characters.
285 It's a pattern that matches certain strings and does not match others.
286 .SH "DIFFERENT FLAVORS OF REs"
289 as defined by POSIX, come in two flavors: \fIextended\fR REs
293 EREs are roughly those of the traditional \fIegrep\fR, while BREs are
294 roughly those of the traditional \fIed\fR. This implementation adds
295 a third flavor, \fIadvanced\fR REs
297 basically EREs with some significant extensions.
299 This manual page primarily describes AREs. BREs mostly exist for
300 backward compatibility in some old programs; they will be discussed at
301 the end. POSIX EREs are almost an exact subset of AREs. Features of
302 AREs that are not present in EREs will be indicated.
303 .SH "REGULAR EXPRESSION SYNTAX"
305 Tcl regular expressions are implemented using the package written by
306 Henry Spencer, based on the 1003.2 spec and some (not quite all) of
307 the Perl5 extensions (thanks, Henry!). Much of the description of
308 regular expressions below is copied verbatim from his manual entry.
310 An ARE is one or more \fIbranches\fR,
313 matching anything that matches any of the branches.
315 A branch is zero or more \fIconstraints\fR or \fIquantified atoms\fR,
317 It matches a match for the first, followed by a match for the second, etc;
318 an empty branch matches the empty string.
320 A quantified atom is an \fIatom\fR possibly followed
321 by a single \fIquantifier\fR.
322 Without a quantifier, it matches a single match for the atom.
324 and what a so-quantified atom matches, are:
329 a sequence of 0 or more matches of the atom
333 a sequence of 1 or more matches of the atom
337 a sequence of 0 or 1 matches of the atom
341 a sequence of exactly \fIm\fR matches of the atom
345 a sequence of \fIm\fR or more matches of the atom
347 \fB{\fIm\fB,\fIn\fB}\fR
349 a sequence of \fIm\fR through \fIn\fR (inclusive) matches of the atom;
350 \fIm\fR may not exceed \fIn\fR
352 \fB*? +? ?? {\fIm\fB}? {\fIm\fB,}? {\fIm\fB,\fIn\fB}?\fR
354 \fInon-greedy\fR quantifiers, which match the same possibilities,
355 but prefer the smallest number rather than the largest number
356 of matches (see \fBMATCHING\fR)
359 The forms using \fB{\fR and \fB}\fR are known as \fIbound\fRs. The
360 numbers \fIm\fR and \fIn\fR are unsigned decimal integers with
361 permissible values from 0 to 255 inclusive.
365 .IP \fB(\fIre\fB)\fR 6
366 matches a match for \fIre\fR (\fIre\fR is any regular expression) with
367 the match noted for possible reporting
368 .IP \fB(?:\fIre\fB)\fR
369 as previous, but does no reporting (a
373 matches an empty string, noted for possible reporting
375 matches an empty string, without reporting
376 .IP \fB[\fIchars\fB]\fR
377 a \fIbracket expression\fR, matching any one of the \fIchars\fR (see
378 \fBBRACKET EXPRESSIONS\fR for more detail)
380 matches any single character
382 matches the non-alphanumeric character \fIk\fR
383 taken as an ordinary character, e.g. \fB\e\e\fR matches a backslash
386 where \fIc\fR is alphanumeric (possibly followed by other characters),
387 an \fIescape\fR (AREs only), see \fBESCAPES\fR below
389 when followed by a character other than a digit, matches the
392 when followed by a digit, it is the beginning of a \fIbound\fR (see above)
394 where \fIx\fR is a single character with no other significance,
395 matches that character.
398 A \fIconstraint\fR matches an empty string when specific conditions
399 are met. A constraint may not be followed by a quantifier. The
400 simple constraints are as follows; some more constraints are described
401 later, under \fBESCAPES\fR.
406 matches at the beginning of a line
410 matches at the end of a line
414 \fIpositive lookahead\fR (AREs only), matches at any point where a
415 substring matching \fIre\fR begins
419 \fInegative lookahead\fR (AREs only), matches at any point where no
420 substring matching \fIre\fR begins
423 The lookahead constraints may not contain back references (see later),
424 and all parentheses within them are considered non-capturing.
426 An RE may not end with
428 .SH "BRACKET EXPRESSIONS"
429 A \fIbracket expression\fR is a list of characters enclosed in
431 It normally matches any single character from the list
432 (but see below). If the list begins with
434 it matches any single character (but see below) \fInot\fR from the
437 If two characters in the list are separated by
439 this is shorthand for the full \fIrange\fR of characters between those two
440 (inclusive) in the collating sequence, e.g.
442 in Unicode matches any conventional decimal digit. Two ranges may not share an
445 is illegal. Ranges in Tcl always use the
446 Unicode collating sequence, but other programs may use other collating
447 sequences and this can be a source of incompatibility between programs.
449 To include a literal \fB]\fR or \fB\-\fR in the list, the simplest
450 method is to enclose it in \fB[.\fR and \fB.]\fR to make it a
451 collating element (see below). Alternatively, make it the first
452 character (following a possible
454 or (AREs only) precede it with
458 make it the last character, or the second endpoint of a range. To use
459 a literal \fB\-\fR as the first endpoint of a range, make it a
460 collating element or (AREs only) precede it with
462 With the exception of
463 these, some combinations using \fB[\fR (see next paragraphs), and
464 escapes, all other special characters lose their special significance
465 within a bracket expression.
466 .SS "CHARACTER CLASSES"
467 Within a bracket expression, the name of a \fIcharacter class\fR
468 enclosed in \fB[:\fR and \fB:]\fR stands for the list of all
469 characters (not all collating elements!) belonging to that class.
470 Standard character classes are:
474 An upper-case letter.
482 An alphanumeric (letter or digit).
484 A "printable" (same as graph, except also including space).
486 A space or tab character.
488 A character producing white space in displayed text.
490 A punctuation character.
492 A character with a visible representation (includes both \fBalnum\fR
497 A locale may provide others. A character class may not be used as an endpoint
501 (\fINote:\fR the current Tcl implementation has only one locale, the Unicode
502 locale, which supports exactly the above classes.)
504 .SS "BRACKETED CONSTRAINTS"
505 There are two special cases of bracket expressions: the bracket
510 are constraints, matching empty strings at the beginning and end of a word
512 .\" note, discussion of escapes below references this definition of word
513 A word is defined as a sequence of word characters that is neither preceded
514 nor followed by word characters. A word character is an \fIalnum\fR character
517 These special bracket expressions are deprecated; users of AREs should use
518 constraint escapes instead (see below).
519 .SS "COLLATING ELEMENTS"
520 Within a bracket expression, a collating element (a character, a
521 multi-character sequence that collates as if it were a single
522 character, or a collating-sequence name for either) enclosed in
523 \fB[.\fR and \fB.]\fR stands for the sequence of characters of that
524 collating element. The sequence is a single element of the bracket
525 expression's list. A bracket expression in a locale that has
526 multi-character collating elements can thus match more than one
527 character. So (insidiously), a bracket expression that starts with
528 \fB^\fR can match multi-character collating elements even if none of
529 them appear in the bracket expression!
532 (\fINote:\fR Tcl has no multi-character collating elements. This information
533 is only for illustration.)
536 For example, assume the collating sequence includes a \fBch\fR multi-character
537 collating element. Then the RE
543 matches the first five characters of
551 matches the multi-character
553 .SS "EQUIVALENCE CLASSES"
554 Within a bracket expression, a collating element enclosed in \fB[=\fR
555 and \fB=]\fR is an equivalence class, standing for the sequences of
556 characters of all collating elements equivalent to that one, including
557 itself. (If there are no other equivalent collating elements, the
558 treatment is as if the enclosing delimiters were
562 For example, if \fBo\fR and \fB\*(qo\fR are the members of an
563 equivalence class, then
565 .QW \fB[[=\*(qo=]]\fR ,
567 .QW \fB[o\*(qo]\fR \&
568 are all synonymous. An equivalence class may not be an endpoint of a range.
571 (\fINote:\fR Tcl implements only the Unicode locale. It does not define any
572 equivalence classes. The examples above are just illustrations.)
575 Escapes (AREs only), which begin with a \fB\e\fR followed by an
576 alphanumeric character, come in several varieties: character entry,
577 class shorthands, constraint escapes, and back references. A \fB\e\fR
578 followed by an alphanumeric character but not constituting a valid
579 escape is illegal in AREs. In EREs, there are no escapes: outside a
580 bracket expression, a \fB\e\fR followed by an alphanumeric character
581 merely stands for that character as an ordinary character, and inside
582 a bracket expression, \fB\e\fR is an ordinary character. (The latter
583 is the one actual incompatibility between EREs and AREs.)
584 .SS "CHARACTER-ENTRY ESCAPES"
585 Character-entry escapes (AREs only) exist to make it easier to specify
586 non-printing and otherwise inconvenient characters in REs:
591 alert (bell) character, as in C
599 synonym for \fB\e\fR to help reduce backslash doubling in some
600 applications where there are multiple levels of backslash processing
604 (where \fIX\fR is any character) the character whose low-order 5 bits
605 are the same as those of \fIX\fR, and whose other bits are all zero
609 the character whose collating-sequence name is
611 or failing that, the character with octal value 033
623 carriage return, as in C
627 horizontal tab, as in C
631 (where \fIwxyz\fR is one up to four hexadecimal digits) the Unicode
632 character \fBU+\fIwxyz\fR in the local byte ordering
636 (where \fIstuvwxyz\fR is one up to eight hexadecimal digits) reserved
637 for a Unicode extension up to 21 bits. The digits are parsed until the
638 first non-hexadecimal character is encountered, the maximun of eight
639 hexadecimal digits are reached, or an overflow would occur in the maximum
640 value of \fBU+\fI10ffff\fR.
644 vertical tab, as in C are all available.
648 (where \fIhh\fR is one or two hexadecimal digits) the character
649 whose hexadecimal value is \fB0x\fIhh\fR.
653 the character whose value is \fB0\fR
657 (where \fIxyz\fR is exactly three octal digits, and is not a \fIback
658 reference\fR (see below)) the character whose octal value is
659 \fB0\fIxyz\fR. The first digit must be in the range 0-3, otherwise
660 the two-digit form is assumed.
664 (where \fIxy\fR is exactly two octal digits, and is not a \fIback
665 reference\fR (see below)) the character whose octal value is
669 Hexadecimal digits are
670 .QR \fB0\fR \fB9\fR ,
671 .QR \fBa\fR \fBf\fR ,
673 .QR \fBA\fR \fBF\fR .
675 .QR \fB0\fR \fB7\fR .
677 The character-entry escapes are always taken as ordinary characters.
678 For example, \fB\e135\fR is \fB]\fR in Unicode, but \fB\e135\fR does
679 not terminate a bracket expression. Beware, however, that some
680 applications (e.g., C compilers and the Tcl interpreter if the regular
681 expression is not quoted with braces) interpret such sequences
682 themselves before the regular-expression package gets to see them,
683 which may require doubling (quadrupling, etc.) the
685 .SS "CLASS-SHORTHAND ESCAPES"
686 Class-shorthand escapes (AREs only) provide shorthands for certain
687 commonly-used character classes:
700 \fB[[:alnum:]_]\fR (note underscore)
712 \fB[^[:alnum:]_]\fR (note underscore)
715 Within bracket expressions,
720 lose their outer brackets, and
725 are illegal. (So, for example,
728 .QW \fB[a-c[:digit:]]\fR .
731 which is equivalent to
732 .QW \fB[a-c^[:digit:]]\fR ,
734 .SS "CONSTRAINT ESCAPES"
735 A constraint escape (AREs only) is a constraint, matching the empty
736 string if specific conditions are met, written as an escape:
741 matches only at the beginning of the string (see \fBMATCHING\fR,
742 below, for how this differs from
747 matches only at the beginning of a word
751 matches only at the end of a word
755 matches only at the beginning or end of a word
759 matches only at a point that is not the beginning or end of a word
763 matches only at the end of the string (see \fBMATCHING\fR, below, for
764 how this differs from
769 (where \fIm\fR is a nonzero digit) a \fIback reference\fR, see below
773 (where \fIm\fR is a nonzero digit, and \fInn\fR is some more digits,
774 and the decimal value \fImnn\fR is not greater than the number of
775 closing capturing parentheses seen so far) a \fIback reference\fR, see
779 A word is defined as in the specification of
783 above. Constraint escapes are illegal within bracket expressions.
784 .SS "BACK REFERENCES"
785 A back reference (AREs only) matches the same string matched by the
786 parenthesized subexpression specified by the number, so that (e.g.)
794 The subexpression must entirely precede the back reference in the RE.
795 Subexpressions are numbered in the order of their leading parentheses.
796 Non-capturing parentheses do not define subexpressions.
798 There is an inherent historical ambiguity between octal
799 character-entry escapes and back references, which is resolved by
800 heuristics, as hinted at above. A leading zero always indicates an
801 octal escape. A single non-zero digit, not followed by another digit,
802 is always taken as a back reference. A multi-digit sequence not
803 starting with a zero is taken as a back reference if it comes after a
804 suitable subexpression (i.e. the number is in the legal range for a
805 back reference), and otherwise is taken as octal.
807 In addition to the main syntax described above, there are some special
808 forms and miscellaneous syntactic facilities available.
810 Normally the flavor of RE being used is specified by
811 application-dependent means. However, this can be overridden by a
812 \fIdirector\fR. If an RE of any flavor begins with
814 the rest of the RE is an ARE. If an RE of any flavor begins with
816 the rest of the RE is taken to be a literal string, with
817 all characters considered ordinary characters.
819 An ARE may begin with \fIembedded options\fR: a sequence
820 \fB(?\fIxyz\fB)\fR (where \fIxyz\fR is one or more alphabetic
821 characters) specifies options affecting the rest of the RE. These
822 supplement, and can override, any options specified by the
823 application. The available option letters are:
832 case-sensitive matching (usual default)
840 case-insensitive matching (see \fBMATCHING\fR, below)
844 historical synonym for \fBn\fR
848 newline-sensitive matching (see \fBMATCHING\fR, below)
852 partial newline-sensitive matching (see \fBMATCHING\fR, below)
856 rest of RE is a literal
858 string, all ordinary characters
862 non-newline-sensitive matching (usual default)
866 tight syntax (usual default; see below)
870 inverse partial newline-sensitive
872 matching (see \fBMATCHING\fR, below)
876 expanded syntax (see below)
879 Embedded options take effect at the \fB)\fR terminating the sequence.
880 They are available only at the start of an ARE, and may not be used
883 In addition to the usual (\fItight\fR) RE syntax, in which all
884 characters are significant, there is an \fIexpanded\fR syntax,
885 available in all flavors of RE with the \fB\-expanded\fR switch, or in
886 AREs with the embedded x option. In the expanded syntax, white-space
887 characters are ignored and all characters between a \fB#\fR and the
888 following newline (or the end of the RE) are ignored, permitting
889 paragraphing and commenting a complex RE. There are three exceptions
892 a white-space character or
900 within a bracket expression is retained
902 white space and comments are illegal within multi-character symbols
908 Expanded-syntax white-space characters are blank, tab, newline, and
909 any character that belongs to the \fIspace\fR character class.
911 Finally, in an ARE, outside bracket expressions, the sequence
912 .QW \fB(?#\fIttt\fB)\fR
913 (where \fIttt\fR is any text not containing a
915 is a comment, completely ignored. Again, this is not
916 allowed between the characters of multi-character symbols like
918 Such comments are more a historical artifact than a useful facility,
919 and their use is deprecated; use the expanded syntax instead.
921 \fINone\fR of these metasyntax extensions is available if the
922 application (or an initial
924 director) has specified that the
925 user's input be treated as a literal string rather than as an RE.
927 In the event that an RE could match more than one substring of a given
928 string, the RE matches the one starting earliest in the string. If
929 the RE could match more than one substring starting at that point, its
930 choice is determined by its \fIpreference\fR: either the longest
931 substring, or the shortest.
933 Most atoms, and all constraints, have no preference. A parenthesized
934 RE has the same preference (possibly none) as the RE. A quantified
935 atom with quantifier \fB{\fIm\fB}\fR or \fB{\fIm\fB}?\fR has the same
936 preference (possibly none) as the atom itself. A quantified atom with
937 other normal quantifiers (including \fB{\fIm\fB,\fIn\fB}\fR with
938 \fIm\fR equal to \fIn\fR) prefers longest match. A quantified atom
939 with other non-greedy quantifiers (including \fB{\fIm\fB,\fIn\fB}?\fR
940 with \fIm\fR equal to \fIn\fR) prefers shortest match. A branch has
941 the same preference as the first quantified atom in it which has a
942 preference. An RE consisting of two or more branches connected by the
943 \fB|\fR operator prefers longest match.
945 Subject to the constraints imposed by the rules for matching the whole
946 RE, subexpressions also match the longest or shortest possible
947 substrings, based on their preferences, with subexpressions starting
948 earlier in the RE taking priority over ones starting later. Note that
949 outer subexpressions thus take priority over their component
952 Note that the quantifiers \fB{1,1}\fR and \fB{1,1}?\fR can be used to
953 force longest and shortest preference, respectively, on a
954 subexpression or a whole RE.
956 Match lengths are measured in characters, not collating elements. An
957 empty string is considered longer than no match at all. For example,
959 matches the three middle characters of
961 .QW \fB(week|wee)(night|knights)\fR
962 matches all ten characters of
963 .QW \fBweeknights\fR ,
968 the parenthesized subexpression matches all three characters, and when
972 both the whole RE and the parenthesized subexpression match an empty string.
974 If case-independent matching is specified, the effect is much as if
975 all case distinctions had vanished from the alphabet. When an
976 alphabetic that exists in multiple cases appears as an ordinary
977 character outside a bracket expression, it is effectively transformed
978 into a bracket expression containing both cases, so that \fBx\fR
981 When it appears inside a bracket expression,
982 all case counterparts of it are added to the bracket expression, so
992 If newline-sensitive matching is specified, \fB.\fR and bracket
993 expressions using \fB^\fR will never match the newline character (so
994 that matches will never cross newlines unless the RE explicitly
995 arranges it) and \fB^\fR and \fB$\fR will match the empty string after
996 and before a newline respectively, in addition to matching at
997 beginning and end of string respectively. ARE \fB\eA\fR and \fB\eZ\fR
998 continue to match beginning or end of string \fIonly\fR.
1000 If partial newline-sensitive matching is specified, this affects
1001 \fB.\fR and bracket expressions as with newline-sensitive matching,
1002 but not \fB^\fR and \fB$\fR.
1004 If inverse partial newline-sensitive matching is specified, this
1005 affects \fB^\fR and \fB$\fR as with newline-sensitive matching, but
1006 not \fB.\fR and bracket expressions. This is not very useful but is
1007 provided for symmetry.
1008 .SH "LIMITS AND COMPATIBILITY"
1009 No particular limit is imposed on the length of REs. Programs
1010 intended to be highly portable should not employ REs longer than 256
1011 bytes, as a POSIX-compliant implementation can refuse to accept such
1014 The only feature of AREs that is actually incompatible with POSIX EREs
1015 is that \fB\e\fR does not lose its special significance inside bracket
1016 expressions. All other ARE features use syntax which is illegal or
1017 has undefined or unspecified effects in POSIX EREs; the \fB***\fR
1018 syntax of directors likewise is outside the POSIX syntax for both BREs
1021 Many of the ARE extensions are borrowed from Perl, but some have been
1022 changed to clean them up, and a few Perl extensions are not present.
1023 Incompatibilities of note include
1026 the lack of special treatment for a trailing newline, the addition of
1027 complemented bracket expressions to the things affected by
1028 newline-sensitive matching, the restrictions on parentheses and back
1029 references in lookahead constraints, and the longest/shortest-match
1030 (rather than first-match) matching semantics.
1032 The matching rules for REs containing both normal and non-greedy
1033 quantifiers have changed since early beta-test versions of this
1034 package. (The new rules are much simpler and cleaner, but do not work
1035 as hard at guessing the user's real intentions.)
1037 Henry Spencer's original 1986 \fIregexp\fR package, still in
1038 widespread use (e.g., in pre-8.1 releases of Tcl), implemented an
1039 early version of today's EREs. There are four incompatibilities
1040 between \fIregexp\fR's near-EREs
1041 .PQ RREs " for short"
1042 and AREs. In roughly increasing order of significance:
1044 In AREs, \fB\e\fR followed by an alphanumeric character is either an
1045 escape or an error, while in RREs, it was just another way of writing
1046 the alphanumeric. This should not be a problem because there was no
1047 reason to write such a sequence in RREs.
1049 \fB{\fR followed by a digit in an ARE is the beginning of a bound,
1050 while in RREs, \fB{\fR was always an ordinary character. Such
1051 sequences should be rare, and will often result in an error because
1052 following characters will not look like a valid bound.
1054 In AREs, \fB\e\fR remains a special character within
1056 so a literal \fB\e\fR within \fB[\|]\fR must be written
1058 \fB\e\e\fR also gives a literal \fB\e\fR within \fB[\|]\fR in RREs,
1059 but only truly paranoid programmers routinely doubled the backslash.
1061 AREs report the longest/shortest match for the RE, rather than the
1062 first found in a specified search order. This may affect some RREs
1063 which were written in the expectation that the first match would be
1064 reported. (The careful crafting of RREs to optimize the search order
1065 for fast matching is obsolete (AREs examine all possible matches in
1066 parallel, and their performance is largely insensitive to their
1067 complexity) but cases where the search order was exploited to
1068 deliberately find a match which was \fInot\fR the longest/shortest
1069 will need rewriting.)
1070 .SH "BASIC REGULAR EXPRESSIONS"
1071 BREs differ from EREs in several respects.
1074 and \fB?\fR are ordinary characters and there is no equivalent for their
1075 functionality. The delimiters for bounds are \fB\e{\fR and
1077 with \fB{\fR and \fB}\fR by themselves ordinary characters. The
1078 parentheses for nested subexpressions are \fB\e(\fR and
1080 with \fB(\fR and \fB)\fR by themselves ordinary
1081 characters. \fB^\fR is an ordinary character except at the beginning
1082 of the RE or the beginning of a parenthesized subexpression, \fB$\fR
1083 is an ordinary character except at the end of the RE or the end of a
1084 parenthesized subexpression, and \fB*\fR is an ordinary character if
1085 it appears at the beginning of the RE or the beginning of a
1086 parenthesized subexpression (after a possible leading
1088 Finally, single-digit back references are available, and \fB\e<\fR and
1089 \fB\e>\fR are synonyms for
1093 respectively; no other escapes are available.
1095 RegExp(3), regexp(n), regsub(n), lsearch(n), switch(n), text(n)
1097 match, regular expression, string
1098 .\" Local Variables:
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