tomazos,
(This is the "classic example" of something that can't be done with regular expressions, and needs a context-free grammar. This is because within the regexp /(a*)(b*)/ there is no way to assert that length($1) == length ($2). Anyhoo...)

While this is probably strictly true, Perl is all about letting you get the job done: 

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

[download]

Anyhoo...I have just recently started learning Parse::RecDescent. Update: I am not sure if this is what you had in mind, but the following accomplishes what you want without using sneaky experimental regex features. 

#!/usr/bin/perl
use strict;
use warnings;
use Parse::RecDescent;

$Parse::RecDescent::skip = '';

my $grammar = q{
    match  : PREFIX TOKEN SUFFIX
             {print join '', @item[1..3]}
    PREFIX : /.*?(?=a+b+)/
    TOKEN  : /a+b+/
             {
                 my $str   = $item[1];
                 my $a_cnt = $str =~ tr/a//;
                 my $b_cnt = $str =~ tr/b//;
                 if ($a_cnt == $b_cnt) {
                     $return = ('c' x $a_cnt) . ('d' x $b_cnt);
                 }
                 elsif ($a_cnt > $b_cnt) {
                     $return = ('a' x ($a_cnt - $b_cnt)) . ('c' x $b_c
+nt) . ('d' x $b_cnt);
                 }
                 else {
                     $return = ('c' x $a_cnt) . ('d' x $a_cnt) . ('b' 
+x ($b_cnt - $a_cnt));
                 }
             }
    SUFFIX : /.*$/
};
my $parser = Parse::RecDescent->new($grammar);
$parser->match('sing aaaaaabbb song');

[download]
A lot of this code could be simplified and improved. I am neither a regex nor Parse::RecDescent guru. I did show how either could work.

Once you have a string of a's followed by one or more b's ($item[1]), you only needed to calculate the desired string and assign it to $return. An explicit assignment to $return is not necessary as you could just let the last expression be returned as with Perl's subroutines.

Cheers - L~R

In reply to Re: Parse::RecDescent for simple syntax-directed translation by Limbic~Region
in thread Parse::RecDescent for simple syntax-directed translation by tomazos

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