$re = '/^.*?Thing #\d+(?:(?!hidden=(?:true|false))[\S\s])*?hidden=false/';
$str = 'Thing #1
.........
hidden=true
.........
Thing #2
.........
hidden=false
.........';
preg_match_all($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