$re = '/(.*)(week.*)/';
$str = '_9978967868768763ghjgj7598983678hjva_week1-6.pdf
_78637832bdsjh275637bkkaww786b_week12-32.pdf
8786767867868978713bkwkhjhkjqw_week1-622.pdf
867813567bdjsbgbadsyu6712353125561235_week100-6.pdf';
preg_match_all($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