$re = '/(?si).*Invoice[\s_:.]+(?<anr>[^\s_]+).+/m';
$str = 'Payer:_xxxxxxx
Waybill Invoice_IN280625-3/4 Nimetus_EAN
Invoice issuer: John
Selle Invoice tasumisel palume m lisada Kesolev Invoice on poolte
vahel Invoice mittetähtaegsel tasumisel on müüjal õigus nõuda viivist 0,1% tasumata summalt päevas.
Kuni Invoice payment sumiseni k
';
preg_match_all($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