正则表达式-替换网址

var a = prompt("","www.youkuaiyun.com");
方法一
        // var reg = /^w{3}\.{1}[a-z]+\.{1}[a-z]*$/;
        // var str = "www.icketang.com";
        // console .log(a.replace(reg,str));
方法二
        var re = /^(w{3})(\.)([a-z]+)(\.[a-z]{2,3}){1,3}$/;
        var str =a.replace(re,function(match,$1,$2){
            return $1+$2+'icketang'+$2+"com";
        })
        console.log(str);