JS将数字转换成三位逗号分隔的样式

function   formatNum(num)
{   
    if(!/^(\+|-)?(\d+)(\.\d+)?$/.test(num)){alert("wrong!");   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;   
}