$re = '/^[A-Za-z0-9][A-Za-z0-9-.]*\.\D{2,4}$/m';
$str = 'ec2-11-111-222-333.cd-blahblah-1.compute.amazonaws.com
domaine.com
subdomain.domain.com
12533d5.dkkkd.com
2dotsextension.co
1dotextension.c
ekkej_dhh.com
12552.2225
112.25.25
12345.com
12345.123.com
domaine.123
whatever
9999-ee.99
email@domain.com
.jjdj.kkd
-subdomain.domain.com
@subdomain.domain.com
112.25.25
';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php