#!/bin/perl -w
use strict;

while( my $string=<DATA>)
  { # this works
    while( $string=~ s{(a\d+)(b\d+)(.*)}{$1$3($2)}){}
    # this does not
    #$string=~ s{(?<=a\d+)(b\d+)(.*)}{$3($2)}g;

    print "string: $string\n"; 
  }


__DATA__
a1b1b2b3
a1b1a12b1b2a3b1b2b3db1b2