$re = '/\bHRB\b(?:[-\s]N(?:umme)?r)?[,.:\s]*(\d+)/m';
$str = '######## Most cases are likes this: ###############
HRB 21156
HRB, 1234
HRB: 99887
######## Some cases can be likes this: ###############
######## How to I catch the numbers as shown above? ##########
HRB-Nummer 21156
HRB-Nr. 12345
HRB-Nr: 21156
HRB Nr. 21156
HRB Nr: 21156
HRB Nr.: 21156
HRB Nummer 21156
';
preg_match_all($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