$re = '/(?m)(?<=cn=)[\w\s]+(?=\\\\?(?:ou=)?[\w\s]*$)/';
$str = 'ou=company\\ou=country\\ou=site\\cn=office\\cn=name\\ou=pet
ou=company\\ou=country\\ou=site\\cn=office\\cn=name\\';
preg_match_all($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