js为数字添加千位分隔符“,”

<script language="JavaScript" type="text/javascript">
function formatNumber(num){
 if(!/^(\+|-)?(\d+)(\.\d+)?$/.test(num)){
  return num;
 }
 var a = RegExp.$1,b = RegExp.$2,c = RegExp.$3;
 var re = new RegExp().compile("(\\d)(\\d{3})(,|$)");
 while(re.test(b)){
  b = b.replace(re,"$1,$2$3");
 }
 return a +""+ b +""+ c;
}

var num=1234567/3;
alert("num="+num+"，四舍五入："+Math.round(num)+"，两位有效数字："+num.toFixed(2)+"，添加千位分隔符："+formatNumber(num));
</script>