#!/usr/bin/perl -ln

use strict;
use warnings;

sub expand;
sub doit;

print for expand $_;

sub expand {
    doit split /\[(.*?)\]/, shift, -1;
}

sub doit {
    return @_ if @_ == 1;
    my ($pre,$pat,$post)=splice @_, 0, 3;
    map { doit $pre . $_ . $post, @_ }
      map { /(\w+):(\w+)/ ? $1..$2 : $_ }
      split /,/, $pat;
    }

__END__

##</code><code>##

echo pre_[aa,01:03,bb]_[x,y]_post | ./xstr.pl
pre_aa_x_post
pre_aa_y_post
pre_01_x_post
pre_01_y_post
pre_02_x_post
pre_02_y_post
pre_03_x_post
pre_03_y_post
pre_bb_x_post
pre_bb_y_post

##</code><code>##

sub expand {
    doit split 
      / (?<!\\)\[ 
      (.*?)
      (?<!\\)\]/x, shift, -1;
}

##</code><code>##

sub expand {
    doit split /<SOME_REGEX>/, shift, -1;
}