正则获取主域名

preg_match('/[w][w-]*.
(?:com
.cn|net.cn|gov.cn
|ac.cn|com|cn|co|net
|org|gov|info|xyz|name
|mobi|wang|top|xin|shop
|ltd|cc|club|vip|ink
|fun|online|tech|store
|red|pro|group|work
|ren|biz|site|tv|wiki
|design|live|pub)(/|$)/isU'
, "www.baidu.info/index.php", $domain);

$domain = !empty($domain[0]) ? $domain[0] : '';

$aa= str_replace(strstr($domain, '.'), '', $domain);


var_dump($aa);