#!/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