$re = '/<div class="main">(.*?)<\/div>/m';
$str = '<div class="main">Dieser Text soll ausgelesen werden</div>Dieser Text soll NICHT ausgelesen werden</div>';
preg_match_all($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