Reg Expression For Currency

I need a regular expression to format U.S. currency as 1,215.25 (commas and decimals)...if it is not already in that format. It should also allow a '-' sign as the first character for negative numbers. I have looked in the FAQ and through this group, plus the net and cannot seem to find the right example.

Any pointers or solutions appreciated. Thanks. I found the belowposted in this group:

<script type="text/javascript">
var RgX = /^$(d{1,3}(,d{3})*|(d+))(.d{2})?$/

function NewComma(Q) { Q = String(Q)
return (Q == (Q=Q.replace(RgX, "$1$2,$3"))) ? Q : NewComma(Q)
}

function setFormat(elm){
var frmCurr= NewComma(elm);
alert(frmCurr);
}

setFormat(121525);
</script>