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diff --git a/doc/regexp.n b/doc/regexp.n index ed61c8d..0d08dcf 100644 --- a/doc/regexp.n +++ b/doc/regexp.n @@ -1,18 +1,18 @@ '\" -'\" Copyright (c) 1993 The Regents of the University of California. -'\" Copyright (c) 1994-1996 Sun Microsystems, Inc. +'\" Copyright (c) 1998 Sun Microsystems, Inc. '\" '\" See the file "license.terms" for information on usage and redistribution '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. '\" -'\" RCS: @(#) $Id: regexp.n,v 1.2 1998/09/14 18:39:54 stanton Exp $ +'\" RCS: @(#) $Id: regexp.n,v 1.3 1999/04/16 00:46:35 stanton Exp $ '\" .so man.macros -.TH regexp n "" Tcl "Tcl Built-In Commands" +.TH regexp n 8.1 Tcl "Tcl Built-In Commands" .BS '\" Note: do not modify the .SH NAME line immediately below! .SH NAME regexp \- Match a regular expression against a string + .SH SYNOPSIS \fBregexp \fR?\fIswitches\fR? \fIexp string \fR?\fImatchVar\fR? ?\fIsubMatchVar subMatchVar ...\fR? .BE @@ -31,27 +31,61 @@ the characters in \fIstring\fR that matched the leftmost parenthesized subexpression within \fIexp\fR, the next \fIsubMatchVar\fR will contain the characters that matched the next parenthesized subexpression to the right in \fIexp\fR, and so on. -.LP +.PP If the initial arguments to \fBregexp\fR start with \fB\-\fR then they are treated as switches. The following switches are currently supported: -.TP 10 +.TP 15 \fB\-nocase\fR Causes upper-case characters in \fIstring\fR to be treated as lower case during the matching process. -.TP 10 +.TP 15 \fB\-indices\fR Changes what is stored in the \fIsubMatchVar\fRs. -Instead of storing the matching characters from \fBstring\fR, +Instead of storing the matching characters from \fIstring\fR, each variable will contain a list of two decimal strings giving the indices in \fIstring\fR of the first and last characters in the matching range of characters. -.TP 10 +.VS 8.1 +.TP 15 +\fB\-expanded\fR +Enables use of the expanded regular expression syntax where +whitespace and comments are ignored. This is the same as specifying +the \fB(?x)\fR embedded option (see METASYNTAX, below). +.TP 15 +\fB\-line\fR +Enables newline-sensitive matching. By default, newline is a +completely ordinary character with no special meaning. With this +flag, `[^' bracket expressions and `.' never match newline, `^' +matches an empty string after any newline in addition to its normal +function, and `$' matches an empty string before any newline in +addition to its normal function. This flag is equivalent to +specifying both \fB\-linestop\fR and \fB\-lineanchor\fR, or the +\fB(?n)\fR embedded option (see METASYNTAX, below). +.TP 15 +\fB\-linestop\fR +Changes the behavior of `[^' bracket expressions and `.' so that they +stop at newlines. This is the same as specifying the \fB(?p)\fR +embedded option (see METASYNTAX, below). +.TP 15 +\fB\-lineanchor\fR +Changes the behavior of `^' and `$' (the ``anchors'') so they match the +beginning and end of a line respectively. This is the same as +specifying the \fB(?w)\fR embedded option (see METASYNTAX, below). +.TP 15 +\fB\-about\fR +Instead of attempting to match the regular expression, returns a list +containing information about the regular expression. The first +element of the list is a subexpression count. The second element is a +list of property names that describe various attributes of the regular +expression. This switch is primarily intended for debugging purposes. +.VE 8.1 +.TP 15 \fB\-\|\-\fR Marks the end of switches. The argument following this one will be treated as \fIexp\fR even if it starts with a \fB\-\fR. -.LP +.PP If there are more \fIsubMatchVar\fR's than parenthesized subexpressions within \fIexp\fR, or if a particular subexpression in \fIexp\fR doesn't match the string (e.g. because it was in a @@ -59,87 +93,956 @@ portion of the expression that wasn't matched), then the corresponding \fIsubMatchVar\fR will be set to ``\fB\-1 \-1\fR'' if \fB\-indices\fR has been specified or to an empty string otherwise. -.SH "REGULAR EXPRESSIONS" -.PP -Regular expressions are implemented using Henry Spencer's package -(thanks, Henry!), -and much of the description of regular expressions below is copied verbatim -from his manual entry. -.PP -A regular expression is zero or more \fIbranches\fR, separated by ``|''. -It matches anything that matches one of the branches. -.PP -A branch is zero or more \fIpieces\fR, concatenated. -It matches a match for the first, followed by a match for the second, etc. -.PP -A piece is an \fIatom\fR possibly followed by ``*'', ``+'', or ``?''. -An atom followed by ``*'' matches a sequence of 0 or more matches of the atom. -An atom followed by ``+'' matches a sequence of 1 or more matches of the atom. -An atom followed by ``?'' matches a match of the atom, or the null string. -.PP -An atom is a regular expression in parentheses (matching a match for the -regular expression), a \fIrange\fR (see below), ``.'' -(matching any single character), ``^'' (matching the null string at the -beginning of the input string), ``$'' (matching the null string at the -end of the input string), a ``\e'' followed by a single character (matching -that character), or a single character with no other significance -(matching that character). -.PP -A \fIrange\fR is a sequence of characters enclosed in ``[]''. -It normally matches any single character from the sequence. -If the sequence begins with ``^'', -it matches any single character \fInot\fR from the rest of the sequence. -If two characters in the sequence are separated by ``\-'', this is shorthand -for the full list of ASCII characters between them -(e.g. ``[0-9]'' matches any decimal digit). -To include a literal ``]'' in the sequence, make it the first character -(following a possible ``^''). -To include a literal ``\-'', make it the first or last character. +.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 "CHOOSING AMONG ALTERNATIVE MATCHES" -.PP -In general there may be more than one way to match a regular expression -to an input string. For example, consider the command -.CS -\fBregexp (a*)b* aabaaabb x y\fR -.CE -Considering only the rules given so far, \fBx\fR and \fBy\fR could -end up with the values \fBaabb\fR and \fBaa\fR, \fBaaab\fR and \fBaaa\fR, -\fBab\fR and \fBa\fR, or any of several other combinations. -To resolve this potential ambiguity \fBregexp\fR chooses among -alternatives using the rule ``first then longest''. -In other words, it considers the possible matches in order working -from left to right across the input string and the pattern, and it -attempts to match longer pieces of the input string before shorter -ones. More specifically, the following rules apply in decreasing -order of priority: -.IP [1] -If a regular expression could match two different parts of an input string -then it will match the one that begins earliest. -.IP [2] -If a regular expression contains \fB|\fR operators then the leftmost -matching sub-expression is chosen. -.IP [3] -In \fB*\fR, \fB+\fR, and \fB?\fR constructs, longer matches are chosen -in preference to shorter ones. -.IP [4] -In sequences of expression components the components are considered -from left to right. -.LP -In the example from above, \fB(a*)b*\fR matches \fBaab\fR: the \fB(a*)\fR -portion of the pattern is matched first and it consumes the leading -\fBaa\fR; then the \fBb*\fR portion of the pattern consumes the -next \fBb\fR. Or, consider the following example: -.CS -\fBregexp (ab|a)(b*)c abc x y z\fR -.CE -After this command \fBx\fR will be \fBabc\fR, \fBy\fR will be -\fBab\fR, and \fBz\fR will be an empty string. -Rule 4 specifies that \fB(ab|a)\fR gets first shot at the input -string and Rule 2 specifies that the \fBab\fR sub-expression -is checked before the \fBa\fR sub-expression. -Thus the \fBb\fR has already been claimed before the \fB(b*)\fR -component is checked and \fB(b*)\fR must match an empty string. +.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 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 class names are: +.PP +.RS +.ne 5 +.nf +.ta 3c 6c 9c +\fBalnum digit punct +alpha graph space +blank lower upper +cntrl print xdigit\fR +.fi +.RE +.PP +These stand for the character classes defined in +\fIctype\fR(3). +A locale may provide others. +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 KEYWORDS match, regular expression, string |