sample code

 javascript


 ([
    "http://www.baidu.com/",
    "https://360.com/",
    "taobao.com",
    "xxx.com.cn",
    "xxxx.baidu.com.cn"
]).forEach(function(s) {
    var m = s.match(/(?:http(?:s)?:\/\/)?(?:www\.)?(.*?)\./);
    console.log(m ? m[1] : "<no match>");
});

output

 baidu
360
taobao
xxx
xxxx