$re = '/vat[\s.]*[rn]/m';
$str = 'vat registration no.
vat number:
our vat number
vat
VAT amount
VAT rate
seller vat.reg.no. FI25891989
vat rate';
preg_match_all($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