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');

##</code><code>##

#!/bin/bash
# This function inserts commas into numbers
# at every 3 digits and returns the result
# in a global variable called $STR.
#
function Commify
{
  # First, reverse the string
  STR=$(echo "$1" | rev)

  STR=$(echo "$STR" | sed -E 's/([0-9]{3})(?=[0-9])(?![0-9]*\.)/\1,/g')

  # Now reverse it back:
  STR=$(echo "$STR" | rev)
}
Commify 'Testing 1234 testing... -123456789.01234567800 testing test 4,500,000.00000'
echo $STR