$re = '/(?m)^N\s+(\d+)$/m';
$str = 'R.U.T .: 99.999.999-9
COMPANY
ELECTRONIC TICKET
Committed to you
N
54280631
COMPANY S.A. SALE
RUT: 99.999.999-9 Directory 111, City
R.U.T .: 99.999.999-9
COMPANY
ELECTRONIC TICKET
Committed to you
N 54280631
COMPANY S.A. SALE
RUT: 99.999.999-9 Directory 111, City';
preg_match_all($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