$re = '/(\b\d{12}\b)(?:.*[\n]{0,3}.*DHL.*Retoure)/mi';
$str = 'Deutsche Post AG
~ Test Str. 176
12345 Entenhausen
81014617 0873 08.09.2021 15:50
Sendungs informa tionen:
123456789012
DHL Retoure
oe Re 8 oe 6 0 BE MERe ob pre eo Ste eo en ec ONO woes
Sendungsverfolgung unter:
wow. dhl. de/sendungsverfolgung
Vielen Dank fiir Ihren Besuch.
Ihre Deutsche Post AG
me x
';
preg_match_all($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