#!/usr/bin/perl

   use strict;

   my ($full, $part) = @ARGV;

   my ($left) = leftover($full, $part);

   if (defined($left)) { print "$full - $part = $left\n"; }
   else                { print "$full - $part = undef\n"; }

#-----------------------------------------------------------
sub leftover {

   my ($full, $part) = @_;
   my ($ch, $left);

   my ($regx) = join('+.*', (split('', $part)));

   if ($full =~ m{$regx}) {

      my (@fulls) = split('', $full);
      my (@parts) = split('', $part);

      while ($ch = shift(@fulls)) {
         ($ch eq $parts[0]) ? shift(@parts) : ($left .= $ch);
      }
   }
   else { $left = undef; };

   return $left;
}