$re = '/(?m)^{{Oberbegriffe}}(?:\n:\[\d+].*)*/m';
$str = '
{{Verkleinerungsformen}}
:[1] [[Äpfelchen]], [[Äpfelein]], [[Äpflein]]
{{Oberbegriffe}}
:[1] [[Kernobst]], [[Obst]]; [[Frucht]]
:[4] [[Kot]]
:[7] [[Gut]]
{{Unterbegriffe}}
:[1] [[Augustapfel]], [[Bohnapfel]], [[Bratapfel]], [[Essapfel]], [[Fallapfel]],
';
preg_match_all($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