Original perl code:

#!/usr/bin/perl -w
use strict;
use warnings;
# I copied this regex from:
# www.PerlMonks.org/?node_id=157725
# Usage: STRING = Commify(STRING)
#
sub Commify
{
  defined $_[0] or return '';
  my $N = reverse $_[0];
  $N =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
  return scalar reverse $N;
}
print Commify('Testing 1234 testing... -123456789.01234567800 testing test 4,500,000.00000');