$re = '/\bMarkerword:[^.]+\./m';
$str = 'We got an example here which covers it to 99% all and this sentence is already part of it. Because this is the first Markerword: with a lot of other things like commata, special characters ä, ü, ß or % and more in it. This proper Markerword: contains more of it multiple times and in caotic characters. If the Markerword: contains this, then cut the whole sequence out - until the first dot after that markerword. And if there are other sentences or even just words inbetween or at the end or before we ignore them all.';
preg_match_all($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