JS 每隔多少位插入字符(mac地址格化)

功能：字符串转换小写，每隔2位插入字符：

 function sbuInsert(str, length) {
        let low = str.toLowerCase();
        let reg = new RegExp("\\w{1," + length + "}", "g");
        let ma = low.match(reg);
        return ma.join(":");
    }

 console.log(sbuInsert("XXXXXXXXXXXX", 2))