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my $str = "eeeeaaaabbbeeee";
$str =~ s/((a+)(??{'b'x length$2}))/'c' x (length($1) * .5) . 'd' x (l
+ength($1) * .5)/e;

#!/usr/bin/perl
use strict;
...
};
my $parser = Parse::RecDescent->new($grammar);
$parser->match('sing aaaaaabbb song');