$re = '/\b\d+%(?!.*\bDiscount\b)/ms';
$str = 'Lorem ipsum dolor 10% sit amet, consectetur adipiscing elit. Morbi et
feugiat Discount vitae 15% urna. Sed 20% et lorem in dapibus.
Mauris arcu dui, vestibulum eget eros eu, eleifend luctus risus.';
preg_match_all($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