$re = '/(?i)\bInvoice[ _:.]+(?<anr>[A-Z/-]*[0-9][A-Z0-9/-]*)(?!\S)/';
$str = 'Payer:_xxxxxxx
Waybill Invoice_5IN280625-3/4 Nimetus_EAN
Invoice issuer: John
Selle Invoice tasumisel palume m lisada Kesolev Invoice on poolte
vahel Invoice mittetähtaegsel tasumisel on müüjal õigus nõuda viivist 0,1% tasumata summalt päevas.
Kuni Invoice Second280625-3/4 sumiseni k
';
preg_match($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