$re = '/(?:\sF\d+.*?\n\n)(\n)/';
$str = 'A N K U N F T 11.08.15
*** NEUBUCHUNG ***
11.08.15 xxx xxx X3 2830 14:25 17:50
18.08.15 xxx xxx X3 2831 18:40
F882129 dsdsaidsaia
F882129 xxxyxyagydaysd
A N K U N F T 18.08.15
*** NEUBUCHUNG ***
11.08.15 xxx xxx X3 2830 14:25 17:50
18.08.15 xxx xxx X3 2831 18:40
F881554 ZXCXZCXCXZCCXZ
F881554 xcvcxvcxvcvxc
F881554 xvcxvcxcvxxvccvxxcv
11.08.15 xxx xxx X3 2830 14:25 17:50
18.08.15 xxx xxx X3 2831 18:40
F881605 xczxcdfsfdsdfs
F881605 zxccxzxzdffdsfds
';
preg_match_all($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