It would go something like this:
^(((([0-9]{1,3},)([0-9]{3},)*[0-9]{3})|[0-9]{1,3})(\.[0-9]+)?)$
You'd have to modify that a bit if you want to exclude leading zeros. If you do want to do that, I recommend you take Sandeep's suggestion and look into constructing it yourself using some of the useful tools freely available to you.