$re = '/(?:^|\G).*?([\w_]+\.(?:jpg|wmv))/s';
$str = 'output 1
abc.jpg
def.jpg
fgh.jpg
output 2
_01.wmv
_02.wmv
_03.wmv';
$subst = "\1";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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