$s = 'aaa.baidu.com www.baidu.com www.baidu.com.cn www.baidu.info';
preg_match_all('/(\w+)\.(?:com|info)/i', $s, $r);
print_r($r);
Array
(
[0] => Array
(
[0] => baidu.com
[1] => baidu.com
[2] => baidu.com
[3] => baidu.info
)
[1] => Array
(
[0] => baidu
[1] => baidu
[2] => baidu
[3] => baidu
)
)