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diff --git a/doc/re_syntax.n b/doc/re_syntax.n new file mode 100644 index 0000000..63eb76c --- /dev/null +++ b/doc/re_syntax.n @@ -0,0 +1,926 @@ +'\" +'\" Copyright (c) 1998 Sun Microsystems, Inc. +'\" Copyright (c) 1999 Scriptics Corporation +'\" +'\" See the file "license.terms" for information on usage and redistribution +'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. +'\" +'\" RCS: @(#) $Id: re_syntax.n,v 1.1 1999/06/24 21:15:13 jpeek Exp $ +'\" +.so man.macros +.TH re_syntax n "8.1" Tcl "Tcl Built-In Commands" +.BS +.SH NAME +re_syntax \- Syntax of Tcl regular expressions. +.BE + +.SH DESCRIPTION +.PP +A \fIregular expression\fR describes strings of characters. +It's a pattern that matches certain strings and doesn't match others. + +.SH "DIFFERENT FLAVORS OF REs" +.VS 8.1 +Regular expressions (``RE''s), as defined by POSIX, come in two +flavors: \fIextended\fR REs (``EREs'') and \fIbasic\fR REs (``BREs''). +EREs are roughly those of the traditional \fIegrep\fR, while BREs are +roughly those of the traditional \fIed\fR. This implementation adds +a third flavor, \fIadvanced\fR REs (``AREs''), basically EREs with +some significant extensions. +.PP +This manual page primarily describes AREs. BREs mostly exist for +backward compatibility in some old programs; they will be discussed at +the end. POSIX EREs are almost an exact subset of AREs. Features of +AREs that are not present in EREs will be indicated. + +.SH "REGULAR EXPRESSION SYNTAX" +.PP +Tcl regular expressions are implemented using the package written by +Henry Spencer, based on the 1003.2 spec and some (not quite all) of +the Perl5 extensions (thanks, Henry!). Much of the description of +regular expressions below is copied verbatim from his manual entry. +.PP +An ARE is one or more \fIbranches\fR, +separated by `\fB|\fR', +matching anything that matches any of the branches. +.PP +A branch is zero or more \fIconstraints\fR or \fIquantified atoms\fR, +concatenated. +It matches a match for the first, followed by a match for the second, etc; +an empty branch matches the empty string. +.PP +A quantified atom is an \fIatom\fR possibly followed +by a single \fIquantifier\fR. +Without a quantifier, it matches a match for the atom. +The quantifiers, +and what a so-quantified atom matches, are: +.RS 2 +.TP 6 +\fB*\fR +a sequence of 0 or more matches of the atom +.TP +\fB+\fR +a sequence of 1 or more matches of the atom +.TP +\fB?\fR +a sequence of 0 or 1 matches of the atom +.TP +\fB{\fIm\fB}\fR +a sequence of exactly \fIm\fR matches of the atom +.TP +\fB{\fIm\fB,}\fR +a sequence of \fIm\fR or more matches of the atom +.TP +\fB{\fIm\fB,\fIn\fB}\fR +a sequence of \fIm\fR through \fIn\fR (inclusive) matches of the atom; +\fIm\fR may not exceed \fIn\fR +.TP +\fB*? +? ?? {\fIm\fB}? {\fIm\fB,}? {\fIm\fB,\fIn\fB}?\fR +\fInon-greedy\fR quantifiers, +which match the same possibilities, +but prefer the smallest number rather than the largest number +of matches (see MATCHING) +.RE +.PP +The forms using +\fB{\fR and \fB}\fR +are known as \fIbound\fRs. +The numbers +\fIm\fR and \fIn\fR are unsigned decimal integers +with permissible values from 0 to 255 inclusive. +.PP +An atom is one of: +.RS 2 +.TP 6 +\fB(\fIre\fB)\fR +(where \fIre\fR is any regular expression) +matches a match for +\fIre\fR, with the match noted for possible reporting +.TP +\fB(?:\fIre\fB)\fR +as previous, +but does no reporting +(a ``non-capturing'' set of parentheses) +.TP +\fB()\fR +matches an empty string, +noted for possible reporting +.TP +\fB(?:)\fR +matches an empty string, +without reporting +.TP +\fB[\fIchars\fB]\fR +a \fIbracket expression\fR, +matching any one of the \fIchars\fR (see BRACKET EXPRESSIONS for more detail) +.TP + \fB.\fR +matches any single character +.TP +\fB\e\fIk\fR +(where \fIk\fR is a non-alphanumeric character) +matches that character taken as an ordinary character, +e.g. \e\e matches a backslash character +.TP +\fB\e\fIc\fR +where \fIc\fR is alphanumeric +(possibly followed by other characters), +an \fIescape\fR (AREs only), +see ESCAPES below +.TP +\fB{\fR +when followed by a character other than a digit, +matches the left-brace character `\fB{\fR'; +when followed by a digit, it is the beginning of a +\fIbound\fR (see above) +.TP +\fIx\fR +where \fIx\fR is +a single character with no other significance, matches that character. +.RE +.PP +A \fIconstraint\fR matches an empty string when specific conditions +are met. +A constraint may not be followed by a quantifier. +The simple constraints are as follows; some more constraints are +described later, under ESCAPES. +.RS 2 +.TP 8 +\fB^\fR +matches at the beginning of a line +.TP +\fB$\fR +matches at the end of a line +.TP +\fB(?=\fIre\fB)\fR +\fIpositive lookahead\fR (AREs only), matches at any point +where a substring matching \fIre\fR begins +.TP +\fB(?!\fIre\fB)\fR +\fInegative lookahead\fR (AREs only), matches at any point +where no substring matching \fIre\fR begins +.RE +.PP +The lookahead constraints may not contain back references (see later), +and all parentheses within them are considered non-capturing. +.PP +An RE may not end with `\fB\e\fR'. + +.SH "BRACKET EXPRESSIONS" +A \fIbracket expression\fR is a list of characters enclosed in `\fB[\|]\fR'. +It normally matches any single character from the list (but see below). +If the list begins with `\fB^\fR', +it matches any single character +(but see below) \fInot\fR from the rest of the list. +.PP +If two characters in the list are separated by `\fB\-\fR', +this is shorthand +for the full \fIrange\fR of characters between those two (inclusive) in the +collating sequence, +e.g. +\fB[0\-9]\fR +in ASCII matches any decimal digit. +Two ranges may not share an +endpoint, so e.g. +\fBa\-c\-e\fR +is illegal. +Ranges are very collating-sequence-dependent, +and portable programs should avoid relying on them. +.PP +To include a literal +\fB]\fR +or +\fB\-\fR +in the list, +the simplest method is to +enclose it in +\fB[.\fR and \fB.]\fR +to make it a collating element (see below). +Alternatively, +make it the first character +(following a possible `\fB^\fR'), +or (AREs only) precede it with `\fB\e\fR'. +Alternatively, for `\fB\-\fR', +make it the last character, +or the second endpoint of a range. +To use a literal +\fB\-\fR +as the first endpoint of a range, +make it a collating element +or (AREs only) precede it with `\fB\e\fR'. +With the exception of these, some combinations using +\fB[\fR +(see next +paragraphs), and escapes, +all other special characters lose their +special significance within a bracket expression. +.PP +Within a bracket expression, a collating element (a character, +a multi-character sequence that collates as if it were a single character, +or a collating-sequence name for either) +enclosed in +\fB[.\fR and \fB.]\fR +stands for the +sequence of characters of that collating element. +The sequence is a single element of the bracket expression's list. +A bracket expression in a locale which has +multi-character collating elements +can thus match more than one character. +Most insidiously, if +\fB^\fR +is used, +this can happen even if no multi-character collating +elements appear in the bracket expression! +If the collating sequence includes a +\fBch\fR +multi-character collating element, +then the RE +\fB[[.ch.]]*c\fR +matches the first five characters +of `\fBchchcc\fR', +and the RE +\fB[^c]b\fR +matches all of `\fBchb\fR'. +.PP +Within a bracket expression, a collating element enclosed in +\fB[=\fR +and +\fB=]\fR +is an equivalence class, standing for the sequences of characters +of all collating elements equivalent to that one, including itself. +(If there are no other equivalent collating elements, +the treatment is as if the enclosing delimiters were `\fB[.\fR'\& +and `\fB.]\fR'.) +For example, if +\fBo\fR +and +\fB\o'o^'\fR +are the members of an equivalence class, +then `\fB[[=o=]]\fR', `\fB[[=\o'o^'=]]\fR', +and `\fB[o\o'o^']\fR'\& +are all synonymous. +An equivalence class may not be an endpoint +of a range. +.PP +Within a bracket expression, the name of a \fIcharacter class\fR enclosed +in +\fB[:\fR +and +\fB:]\fR +stands for the list of all characters +(not all collating elements!) +belonging to that +class. +Standard character classes are (*** CHECK THESE! ***): +.PP +.RS +.ne 5 +.nf +.ta 3c +\fBalpha\fR A letter. +\fBupper\fR An upper-case letter. +\fBlower\fR A lower-case letter. +\fBdigit\fR A decimal digit. +\fBxdigit\fR A hexadecimal digit. +\fBalnum\fR An alphanumeric (letter or digit). +\fBspace\fR A character producing white space in displayed text. +\fBpunct\fR A punctuation character. +\fBprint\fR A printing character. +\fBgraph\fR A character with a visible representation. +\fBcntrl\fR A control character. +.fi +.RE +.PP +A locale may provide others. (*** NOT ANYMORE, TRUE? ***) +A character class may not be used as an endpoint of a range. +.PP +There are two special cases of bracket expressions: +the bracket expressions +\fB[[:<:]]\fR +and +\fB[[:>:]]\fR +are constraints, matching empty strings at +the beginning and end of a word respectively. +'\" note, discussion of escapes below references this definition of word +A word is defined as a sequence of +word characters +which is neither preceded nor followed by +word characters. +A word character is an +\fIalnum\fR +character (as defined by +\fIctype\fR(3)) +or an underscore +(\fB_\fR). +These special bracket expressions are deprecated; +users of AREs should use constraint escapes instead (see below). +.SH ESCAPES +Escapes (AREs only), which begin with a +\fB\e\fR +followed by an alphanumeric character, +come in several varieties: +character entry, class shorthands, constraint escapes, and back references. +A +\fB\e\fR +followed by an alphanumeric character but not constituting +a valid escape is illegal in AREs. +In EREs, there are no escapes: +outside a bracket expression, +a +\fB\e\fR +followed by an alphanumeric character merely stands for that +character as an ordinary character, +and inside a bracket expression, +\fB\e\fR +is an ordinary character. +(The latter is the one actual incompatibility between EREs and AREs.) +.PP +Character-entry escapes (AREs only) exist to make it easier to specify +non-printing and otherwise inconvenient characters in REs: +.RS 2 +.TP 5 +\fB\ea\fR +alert, aka bell, character, as in C +.TP +\fB\eb\fR +backspace, as in C +.TP +\fB\eB\fR +synonym for +\fB\e\fR +to help reduce backslash doubling in some +applications where there are multiple levels of backslash processing +.TP +\fB\ec\fIX\fR +(where X is any character) the character whose +low-order 5 bits are the same as those of +\fIX\fR, +and whose other bits are all zero +.TP +\fB\ee\fR +the character whose collating-sequence name +is `\fBESC\fR', +or failing that, the character with octal value 033 +.TP +\fB\ef\fR +formfeed, as in C +.TP +\fB\en\fR +newline, as in C +.TP +\fB\er\fR +carriage return, as in C +.TP +\fB\et\fR +horizontal tab, as in C +.TP +\fB\eu\fIwxyz\fR +(where +\fIwxyz\fR +is exactly four hexadecimal digits) +the Unicode character +\fBU+\fIwxyz\fR +in the local byte ordering +.TP +\fB\eU\fIstuvwxyz\fR +(where +\fIstuvwxyz\fR +is exactly eight hexadecimal digits) +reserved for a somewhat-hypothetical Unicode extension to 32 bits +.TP +\fB\ev\fR +vertical tab, as in C +are all available. +.TP +\fB\ex\fIhhh\fR +(where +\fIhhh\fR +is any sequence of hexadecimal digits) +the character whose hexadecimal value is +\fB0x\fIhhh\fR +(a single character no matter how many hexadecimal digits are used). +.TP +\fB\e0\fR +the character whose value is +\fB0\fR +.TP +\fB\e\fIxy\fR +(where +\fIxy\fR +is exactly two octal digits, +and is not a +\fIback reference\fR (see below)) +the character whose octal value is +\fB0\fIxy\fR +.TP +\fB\e\fIxyz\fR +(where +\fIxyz\fR +is exactly three octal digits, +and is not a +back reference (see below)) +the character whose octal value is +\fB0\fIxyz\fR +.RE +.PP +Hexadecimal digits are `\fB0\fR'-`\fB9\fR', `\fBa\fR'-`\fBf\fR', +and `\fBA\fR'-`\fBF\fR'. +Octal digits are `\fB0\fR'-`\fB7\fR'. +.PP +The character-entry escapes are always taken as ordinary characters. +For example, +\fB\e135\fR +is +\fB]\fR +in ASCII, +but +\fB\e135\fR +does not terminate a bracket expression. +Beware, however, that some applications (e.g., C compilers) interpret +such sequences themselves before the regular-expression package +gets to see them, which may require doubling (quadrupling, etc.) the `\fB\e\fR'. +.PP +Class-shorthand escapes (AREs only) provide shorthands for certain commonly-used +character classes: +.RS 2 +.TP 10 +\fB\ed\fR +\fB[[:digit:]]\fR +.TP +\fB\es\fR +\fB[[:space:]]\fR +.TP +\fB\ew\fR +\fB[[:alnum:]_]\fR +(note underscore) +.TP +\fB\eD\fR +\fB[^[:digit:]]\fR +.TP +\fB\eS\fR +\fB[^[:space:]]\fR +.TP +\fB\eW\fR +\fB[^[:alnum:]_]\fR +(note underscore) +.RE +.PP +Within bracket expressions, `\fB\ed\fR', `\fB\es\fR', +and `\fB\ew\fR'\& +lose their outer brackets, +and `\fB\eD\fR', `\fB\eS\fR', +and `\fB\eW\fR'\& +are illegal. +.PP +A constraint escape (AREs only) is a constraint, +matching the empty string if specific conditions are met, +written as an escape: +.RS 2 +.TP 6 +\fB\eA\fR +matches only at the beginning of the string +(see MATCHING, below, for how this differs from `\fB^\fR') +.TP +\fB\em\fR +matches only at the beginning of a word +.TP +\fB\eM\fR +matches only at the end of a word +.TP +\fB\ey\fR +matches only at the beginning or end of a word +.TP +\fB\eY\fR +matches only at a point which is not the beginning or end of a word +.TP +\fB\eZ\fR +matches only at the end of the string +(see MATCHING, below, for how this differs from `\fB$\fR') +.TP +\fB\e\fIm\fR +(where +\fIm\fR +is a nonzero digit) a \fIback reference\fR, see below +.TP +\fB\e\fImnn\fR +(where +\fIm\fR +is a nonzero digit, and +\fInn\fR +is some more digits, +and the decimal value +\fImnn\fR +is not greater than the number of closing capturing parentheses seen so far) +a \fIback reference\fR, see below +.RE +.PP +A word is defined as in the specification of +\fB[[:<:]]\fR +and +\fB[[:>:]]\fR +above. +Constraint escapes are illegal within bracket expressions. +.PP +A back reference (AREs only) matches the same string matched by the parenthesized +subexpression specified by the number, +so that (e.g.) +\fB([bc])\e1\fR +matches +\fBbb\fR +or +\fBcc\fR +but not `\fBbc\fR'. +The subexpression must entirely precede the back reference in the RE. +Subexpressions are numbered in the order of their leading parentheses. +Non-capturing parentheses do not define subexpressions. +.PP +There is an inherent historical ambiguity between octal character-entry +escapes and back references, which is resolved by heuristics, +as hinted at above. +A leading zero always indicates an octal escape. +A single non-zero digit, not followed by another digit, +is always taken as a back reference. +A multi-digit sequence not starting with a zero is taken as a back +reference if it comes after a suitable subexpression +(i.e. the number is in the legal range for a back reference), +and otherwise is taken as octal. +.SH "METASYNTAX" +In addition to the main syntax described above, there are some special +forms and miscellaneous syntactic facilities available. +.PP +Normally the flavor of RE being used is specified by +application-dependent means. +However, this can be overridden by a \fIdirector\fR. +If an RE of any flavor begins with `\fB***:\fR', +the rest of the RE is an ARE. +If an RE of any flavor begins with `\fB***=\fR', +the rest of the RE is taken to be a literal string, +with all characters considered ordinary characters. +.PP +An ARE may begin with \fIembedded options\fR: +a sequence +\fB(?\fIxyz\fB)\fR +(where +\fIxyz\fR +is one or more alphabetic characters) +specifies options affecting the rest of the RE. +These supplement, and can override, +any options specified by the application. +The available option letters are: +.RS 2 +.TP 3 +\fBb\fR +rest of RE is a BRE +.TP 3 +\fBc\fR +case-sensitive matching (usual default) +.TP 3 +\fBe\fR +rest of RE is an ERE +.TP 3 +\fBi\fR +case-insensitive matching (see MATCHING, below) +.TP 3 +\fBm\fR +historical synonym for +\fBn\fR +.TP 3 +\fBn\fR +newline-sensitive matching (see MATCHING, below) +.TP 3 +\fBp\fR +partial newline-sensitive matching (see MATCHING, below) +.TP 3 +\fBq\fR +rest of RE is a literal (``quoted'') string, all ordinary characters +.TP 3 +\fBs\fR +non-newline-sensitive matching (usual default) +.TP 3 +\fBt\fR +tight syntax (usual default; see below) +.TP 3 +\fBw\fR +inverse partial newline-sensitive (``weird'') matching (see MATCHING, below) +.TP 3 +\fBx\fR +expanded syntax (see below) +.RE +.PP +Embedded options take effect at the +\fB)\fR +terminating the sequence. +They are available only at the start of an ARE, +and may not be used later within it. +.PP +In addition to the usual (\fItight\fR) RE syntax, in which all characters are +significant, there is an \fIexpanded\fR syntax, +available in all flavors of RE +with the \fB-expanded\fR switch, or in AREs with the embedded x option. +In the expanded syntax, +white-space characters are ignored +and all characters between a +\fB#\fR +and the following newline (or the end of the RE) are ignored, +permitting paragraphing and commenting a complex RE. +There are three exceptions to that basic rule: +.RS 2 +.PP +a white-space character or `\fB#\fR' preceded by `\fB\e\fR' is retained +.PP +white space or `\fB#\fR' within a bracket expression is retained +.PP +white space and comments are illegal within multi-character symbols +like the ARE `\fB(?:\fR' or the BRE `\fB\e(\fR' +.RE +.PP +Expanded-syntax +white-space characters are blank, tab, newline, etc. (any character +defined as \fIspace\fR by +\fIctype\fR(3)). +Exactly how a multi-line expanded-syntax RE +can be entered interactively by a user, +if at all, is application-specific; +expanded syntax is primarily a scripting facility. +.PP +Finally, in an ARE, +outside bracket expressions, the sequence `\fB(?#\fIttt\fB)\fR' +(where +\fIttt\fR +is any text not containing a `\fB)\fR') +is a comment, +completely ignored. +Again, this is not allowed between the characters of +multi-character symbols like `\fB(?:\fR'. +Such comments are more a historical artifact than a useful facility, +and their use is deprecated; +use the expanded syntax instead. +.PP +\fINone\fR of these metasyntax extensions is available if the application +(or an initial +\fB***=\fR +director) +has specified that the user's input be treated as a literal string +rather than as an RE. +.SH MATCHING +In the event that an RE could match more than one substring of a given +string, +the RE matches the one starting earliest in the string. +If the RE could match more than one substring starting at that point, +its choice is determined by its \fIpreference\fR: +either the longest substring, or the shortest. +.PP +Most atoms, and all constraints, have no preference. +A parenthesized RE has the same preference (possibly none) as the RE. +A quantified atom with quantifier +\fB{\fIm\fB}\fR +or +\fB{\fIm\fB}?\fR +has the same preference (possibly none) as the atom itself. +A quantified atom with other normal quantifiers (including +\fB{\fIm\fB,\fIn\fB}\fR +with +\fIm\fR +equal to +\fIn\fR) +prefers longest match. +A quantified atom with other non-greedy quantifiers (including +\fB{\fIm\fB,\fIn\fB}?\fR +with +\fIm\fR +equal to +\fIn\fR) +prefers shortest match. +A branch has the same preference as the first quantified atom in it +which has a preference. +An RE consisting of two or more branches connected by the +\fB|\fR +operator prefers longest match. +.PP +Subject to the constraints imposed by the rules for matching the whole RE, +subexpressions also match the longest or shortest possible substrings, +based on their preferences, +with subexpressions starting earlier in the RE taking priority over +ones starting later. +Note that outer subexpressions thus take priority over +their component subexpressions. +.PP +Note that the quantifiers +\fB{1,1}\fR +and +\fB{1,1}?\fR +can be used to force longest and shortest preference, respectively, +on a subexpression or a whole RE. +.PP +Match lengths are measured in characters, not collating elements. +An empty string is considered longer than no match at all. +For example, +\fBbb*\fR +matches the three middle characters of `\fBabbbc\fR', +\fB(week|wee)(night|knights)\fR +matches all ten characters of `\fBweeknights\fR', +when +\fB(.*).*\fR +is matched against +\fBabc\fR +the parenthesized subexpression +matches all three characters, and +when +\fB(a*)*\fR +is matched against +\fBbc\fR +both the whole RE and the parenthesized +subexpression match an empty string. +.PP +If case-independent matching is specified, +the effect is much as if all case distinctions had vanished from the +alphabet. +When an alphabetic that exists in multiple cases appears as an +ordinary character outside a bracket expression, it is effectively +transformed into a bracket expression containing both cases, +so that +\fBx\fR +becomes `\fB[xX]\fR'. +When it appears inside a bracket expression, all case counterparts +of it are added to the bracket expression, so that +\fB[x]\fR +becomes +\fB[xX]\fR +and +\fB[^x]\fR +becomes `\fB[^xX]\fR'. +.PP +If newline-sensitive matching is specified, \fB.\fR +and bracket expressions using +\fB^\fR +will never match the newline character +(so that matches will never cross newlines unless the RE +explicitly arranges it) +and +\fB^\fR +and +\fB$\fR +will match the empty string after and before a newline +respectively, in addition to matching at beginning and end of string +respectively. +ARE +\fB\eA\fR +and +\fB\eZ\fR +continue to match beginning or end of string \fIonly\fR. +.PP +If partial newline-sensitive matching is specified, +this affects \fB.\fR +and bracket expressions +as with newline-sensitive matching, but not +\fB^\fR +and `\fB$\fR'. +.PP +If inverse partial newline-sensitive matching is specified, +this affects +\fB^\fR +and +\fB$\fR +as with +newline-sensitive matching, +but not \fB.\fR +and bracket expressions. +This isn't very useful but is provided for symmetry. +.SH "LIMITS AND COMPATIBILITY" +No particular limit is imposed on the length of REs. +Programs intended to be highly portable should not employ REs longer +than 256 bytes, +as a POSIX-compliant implementation can refuse to accept such REs. +.PP +The only feature of AREs that is actually incompatible with +POSIX EREs is that +\fB\e\fR +does not lose its special +significance inside bracket expressions. +All other ARE features use syntax which is illegal or has +undefined or unspecified effects in POSIX EREs; +the +\fB***\fR +syntax of directors likewise is outside the POSIX +syntax for both BREs and EREs. +.PP +Many of the ARE extensions are borrowed from Perl, but some have +been changed to clean them up, and a few Perl extensions are not present. +Incompatibilities of note include `\fB\eb\fR', `\fB\eB\fR', +the lack of special treatment for a trailing newline, +the addition of complemented bracket expressions to the things +affected by newline-sensitive matching, +the restrictions on parentheses and back references in lookahead constraints, +and the longest/shortest-match (rather than first-match) matching semantics. +.PP +The matching rules for REs containing both normal and non-greedy quantifiers +have changed since early beta-test versions of this package. +(The new rules are much simpler and cleaner, +but don't work as hard at guessing the user's real intentions.) +.PP +Henry Spencer's original 1986 \fIregexp\fR package, +still in widespread use (e.g., in pre-8.1 releases of Tcl), +implemented an early version of today's EREs. +There are four incompatibilities between \fIregexp\fR's near-EREs +(`RREs' for short) and AREs. +In roughly increasing order of significance: +.PP +.RS +In AREs, +\fB\e\fR +followed by an alphanumeric character is either an +escape or an error, +while in RREs, it was just another way of writing the +alphanumeric. +This should not be a problem because there was no reason to write +such a sequence in RREs. +.PP +\fB{\fR +followed by a digit in an ARE is the beginning of a bound, +while in RREs, +\fB{\fR +was always an ordinary character. +Such sequences should be rare, +and will often result in an error because following characters +will not look like a valid bound. +.PP +In AREs, +\fB\e\fR +remains a special character within `\fB[\|]\fR', +so a literal +\fB\e\fR +within +\fB[\|]\fR +must be written `\fB\e\e\fR'. +\fB\e\e\fR +also gives a literal +\fB\e\fR +within +\fB[\|]\fR +in RREs, +but only truly paranoid programmers routinely doubled the backslash. +.PP +AREs report the longest/shortest match for the RE, +rather than the first found in a specified search order. +This may affect some RREs which were written in the expectation that +the first match would be reported. +(The careful crafting of RREs to optimize the search order for fast +matching is obsolete (AREs examine all possible matches +in parallel, and their performance is largely insensitive to their +complexity) but cases where the search order was exploited to deliberately +find a match which was \fInot\fR the longest/shortest will need rewriting.) +.RE + +.SH "BASIC REGULAR EXPRESSIONS" +BREs differ from EREs in several respects. `\fB|\fR', `\fB+\fR', +and +\fB?\fR +are ordinary characters and there is no equivalent +for their functionality. +The delimiters for bounds are +\fB\e{\fR +and `\fB\e}\fR', +with +\fB{\fR +and +\fB}\fR +by themselves ordinary characters. +The parentheses for nested subexpressions are +\fB\e(\fR +and `\fB\e)\fR', +with +\fB(\fR +and +\fB)\fR +by themselves ordinary characters. +\fB^\fR +is an ordinary character except at the beginning of the +RE or the beginning of a parenthesized subexpression, +\fB$\fR +is an ordinary character except at the end of the +RE or the end of a parenthesized subexpression, +and +\fB*\fR +is an ordinary character if it appears at the beginning of the +RE or the beginning of a parenthesized subexpression +(after a possible leading `\fB^\fR'). +Finally, +single-digit back references are available, +and +\fB\e<\fR +and +\fB\e>\fR +are synonyms for +\fB[[:<:]]\fR +and +\fB[[:>:]]\fR +respectively; +no other escapes are available. + +.VE 8.1 + +.SH "SEE ALSO" +RegExp(3), regexp(n), regsub(n), lsearch(n), switch(n), text(n) + +.SH KEYWORDS +match, regular expression, string |