#!/usr/bin/perl

use strict;
use warnings;
use Test::More;

my %output_for
    = ( '215000007801' => '21507801',
        '300000324002' => '30324002',
        '890000457651' => '89457651',
        '210004563401' => '214563401',
        '201045139158' => '20145139158', );

plan tests => scalar keys %output_for;

while ( my ($input, $correct_output) = each %output_for ) {
    my $orig_input = $input;
    $input = &compress_numstring( $input );
    is( $input, $correct_output, "Solved '$orig_input'" );
}

sub compress_numstring {
  my $starting_num = shift;
  return $starting_num
   unless(defined $starting_num && $starting_num =~ /^(\d{2})(\d+)/);
  my( $keep, $modify ) = ($1,$2);
  ( $modify =~ s/^0+// ) ||
  ( $modify =~ s/0{2,}/0/ ) ||
  ( $modify =~ s/(?<!0)0([1-9]|$)/$1/ );
  return $keep . $modify;
}