$re = '/((?<key>order\s*number).*)?(?<!x)(?<value>\d{9}|\d{3}-\d{6}|\d{3}-\d{3}-\d{3})/im';
$str = 'x123456789
X123456789
x123-456-789
X123-456-789
123-456-789
ordernumber123-456-789
order number123-456789
order number 123456789
123-456789
123456789
ordernumber-x123456787
ordernumber-123456787
ordernumber - 123456789
ordernumber #123456789
ordernumber *anysplcharacter*123456789
ordernumber !@#$%^&123456789
ordernumber !@#$%^&x123456789
';
preg_match_all($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