This and other RFCs are available on the web at http://dev.perl.org/rfc/ =head1 TITLE Brace-matching for Perl Regular Expressions =head1 VERSION Maintainer: Eric J. Roode <[EMAIL PROTECTED]> Date: 24 Aug 2000 Version: 1 Mailing List: [EMAIL PROTECTED] Number: 145 =head1 ABSTRACT It is quite difficult to match paired characters in Perl 5 regular expressions. A solution is proposed, using new \g (match opening grouping character) and \G (match closing grouping character) metacharacters. Two new special variables, @^g and @^G control which strings are considered grouping characters and what their complement is. =head1 DESCRIPTION A new regular expression metacharacter \g would match any of the following characters: ([{"'< in a regexp. A later \G metacharacter would match the corresponding closing pair character )]{"'> I<at the same nesting level> within the string being searched. For example, $string = "([b - (a + 1)] * 7)"; $string =~ /\g.*?\G/; The \g would match the first open parenthesis, the .*? would match the substring "[b - (a + 1)] * 7", and the \G would match the second close parenthesis. Used within a character class (square brackets in a regular expression), \g would match I<any> opening grouping character, and \G would match I<any> closing grouping character. Thus, [^\g\G]* would return a span of non- grouping characters. What exactly is matched by \g and \G is controlled by two new special variables, @^g and @^G, which are arrays of strings. The default values of these variables are: @^g = ('(', '{', '[', '"', "'", '<'); @^G = (')', '}', ']', '"', "'", '>'); One could restrict the arrays for various parsing situations: { local @^g = ( '(' ); local @^G = ( ')' ); ... } Or one could override the arrays completely: { local @^g = ( '/*', '"' ); local @^G = ( '*/', '"' ); $c_code_snippet =~ /\g(.*?)\G/; # Find string or comment } It is a run-time error to compile a regular expression that contains \g or \G while the @^g and @^G arrays do not contain the same number of elements. =head1 EXAMPLES # Recursive processing of nested groupings sub parse { my $string = shift; while ($string =~ /([^\g])*(\g)(.*?)(\G)([^\g\G]*)/g) { my ($pre, $quote, $mid, $endquote, $post) = ($1,$2,$3,$4,$5); process ($pre); parse ($mid); # Note recursion process ($post); } } =head1 IMPLEMENTATION Disclaimer: I know little about Perl internals, particularly the RE engine. When the RE engine encounters a \g, it should match if it finds an open grouping character. From that point forward, it should maintain an internal count of "like" open and close grouping characters, When it encounteres a \G metacharacter, it should match if it finds a closing grouping character of the same sort (ie, the complement to the specific string that was matched by the \g), I<and> if the nesting level of that pair is zero. So, in parsing the string "(abc[def](ghi)jkl)" with the RE /\g(.*?)\G/: First, \g matches "(". The engine remembers that it is looking for "(" and its complement ")". Next, as it processes .*?, scanning the string, it encounters the "[" between the "c" and the "d". It ignores it (it has no effect on the nesting-level count, since it is not "(" or ")"). As it continues scanning, it encounters the "]" between the "f" and the ")". The \G does not match this "]" character, because the \g must match a ")". Next, as it continues processing the .*?, it enounters the "(" between the "]" and the "g". This does match the current grouping set, so the engine increments the nesting level to 1. Next, it encounters the ")" between the "i" and the "j". The \G does not match this ")", because the nesting level is not zero. Having encountered the ")", however, the engine decrements the nesting level to 0. Finally, it encounters the ")" after the "l". This one does match the \G because the nesting level is 0. Nested \g\G pairs present a problem in that the engine must remember which pair of grouping characters it is looking for. Example: Parse the string "(abc[def](ghi)jkl)" with the RE /\g(.*?)\g(.*?)\G(.*?)\G/: \g matches "(". Engine remembers that this \g corresponds to "(", ")". .*? matches "abc". The second \g matches the "[". The engine remembers that I<this> \g corresponds to "[", "]". The second .*? matches "def". The \G matches "]", since the second \g was for square brackets, and the nesting level for square brackets is zero. The third .*? matches "(ghi)jkl". The ")" between the "i" and the "j" does not match the \G pattern because, as in the first example, the nesting level is not zero. The second \G matches ")". =head1 PROBLEMS How should a \G without a prior \g be interpreted in a regular expression? Probably the same as [\G] (ie, match any closing character), perhaps someone has a different idea. How should an expression like /\g?.*?\G/ be interpreted? Specifically, what should the meaning of the \G be if the optional \g does not match? =head1 REFERENCES perlre perldoc page for general discussion of existing regexps Mastering Regular Expressions book Blue Camel, chapter 2.
