$re = '/\b(17|18|19|20|21)\d{8}\b/m';
$str = 'String1: /TYPE/000/R SEPA TR//XXXX/XX//XXXX/2000000000//XXXX/NOTPROVIDED//XXXX/NAME/XXXXXXX XXXX//INFO/XXXXXXXX//XXX/XXXXXXXX/
String 2: /TYPE/000/X XXXX XX//CODE/XX//BORI/Dossiernummer: 2111111111 Invoice: 2233333333 (betaling week 00)//XXXX/XXXXXXXXXX//XXXX/NAME/XXXXXXX//INFO/XX00XXXX0000000000//ABC/XXXXXX0XXXX/
String 3: /TYPE/000/R SEPA TR//XXXX/XX//XXXX/Dossiernummer 1700000000//EREF/NOTPROVIDED//XXXX/NAME/XX XXXXX XX XXX//INFO/XX000000X0000//XXX/XXXXXXXXXX/
String 4: /TYPE/000/R SEPA TR//XXXX/XX//XXXX/Dossiernummer 17000000001//EREF/NOTPROVIDED//XXXX/NAME/XX XXXXX XX XXX//INFO/XX000000X0000//XXX/XXXXXXXXXX/
String 5: /TYPE/000/R SEPA TR//XXXX/XX//XXXX/Dossiernummer 180000000a//EREF/NOTPROVIDED//XXXX/NAME/XX XXXXX XX XXX//INFO/XX000000X0000//XXX/XXXXXXXXXX/';
preg_match_all($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