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The excellent Simon Cozens tutorial at

http://www.perl.com/pub/2003/06/06/regexps.html

shows how to use regex's to isolate parenthesised text within a string with handling of nested/multiple parentheses. Cozens constructs a recursive Regex, and uses the ( ??{ $regex_variable } ) construct to defer interpolation of the variable until execution. It works for me!:

$ cat p2.pl #!/opt/perl5.16/bin/perl use strict; use warnings; our $paren = qr/ # Need declared variable with use stri +ct. \( ( [^()]+ # Not parens | (??{ our $paren }) # Another balanced group (not interpol +ated yet) )* \) /x; # 'x' means ignore whitespace, comment +s. my $stuff = "On the outside now then (we go( in( and in(&stop)(awhile) + ( further ))) but still (here) ) and now (for a while) we are out ag +ain."; $stuff =~ /($paren)/; print "----------\n"; print "$stuff\n"; print "----------\n"; print $1 . "\n"; print "----------\n";
$ ./p2.pl ---------- On the outside now then (we go( in( and in (&stop)(awhile) ( further ) +)) but still (here) ) and now (for a while) we are out again. ---------- (we go( in( and in (&stop)(awhile) ( further ))) but still (here) ) ---------- $

But if I try to use the regex again it does not work as I think it ought. In the above code when I replace the matching regex with /($paren)^()*($paren)/ I expect $2 to be "(for a while)", but that does not work:

$ cat p3.pl #!/opt/perl5.16/bin/perl use strict; use warnings; our $paren = qr/ # Need declared variable with use stri +ct. \( ( [^()]+ # Not parens | (??{ our $paren }) # Another balanced group (not interpol +ated yet) )* \) /x; # 'x' means ignore whitespace, comment +s. my $stuff = "On the outside now then (we go( in( and in (&stop)(awhile +) ( further ))) but still (here) ) and now (for a while) we are out a +gain."; $stuff =~ /($paren)[^()]*($paren)/; print "----------\n"; print "$stuff\n"; print "----------\n"; print $1 . "\n"; print "----------\n"; print $2 . "\n"; print "----------\n"; $ ./p3.pl ---------- On the outside now then (we go( in( and in (&stop)(awhile) ( further ) +)) but still (here) ) and now (for a while) we are out again. ---------- (we go( in( and in (&stop)(awhile) ( further ))) but still (here) ) ---------- ---------- $

$2 is empty. Does anyone know what is going on here? If I take out the ^()+ portion of the regex, then the program hangs when I run it.


In reply to Regex's, parentheses, and the mysterious ( ??{ } ) operator by Clovis_Sangrail

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