JS将数字转换成三位逗号分隔的样式

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;
}