Js numeric conversion to 3 bit comma separated sample code

Example code:

function formatNum(strNum) {
if (strNum.length <= 3) {
return strNum;
}
if (!/^(+|-)?(d+)(.d+)?$/.test(strNum)) {
return strNum;
}
var a = RegExp.$1, b = RegExp.$2, c = RegExp.$3;
var re = new RegExp();
re.compile("(\d)(\d{3})(,|$)");
while (re.test(b)) {
b = b.replace(re, "$1,$2$3");
}
return a + "" + b + "" + c;
}