$re = '/^(\d{2}-\d{2}-\d{4}\h+\d{2}:\d{2}:\d{2})\h+-\h*(.*(?:\R(?!\d{2}-\d{2}-\d{4}\h+\d{2}:\d{2}:\d).*)*)/m';
$str = '28-03-2020 23:00:00 - This
is
line
1
28-03-2021 14:23:15 - This
is
line
2
28-03-2020 23:00:00 - This
is
12
line
1';
preg_match_all($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