$re = '/\<elem\skey=\"fqdn\"\>(?<fullDomainName>[^\<]+)<\/elem\>/';
$str = '<elem key="date">2016-11-27T13:01:38+01:00</elem>
<elem key="fqdn">XXXXXXXXX.mydomain.domain.com</elem>
<elem key="domain_dns">mydomain.domain.com</elem>
<elem key="forest_dns">domain.com</elem>
<elem key="workgroup">XXX\\x00</elem>
';
preg_match_all($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