$re = '/!(?!(.|\n|\r)*FastEthernet\d\d?\/\d\d?\.\d{1,3})/';
$str = 'interface FastEthernet0/0
shutdown
!
interface FastEthernet0/0.100
ip address 192.168.1.100
!
interface FastEthernet0/1
shutdown
!';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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