$re = '/(P(?:[1-9]|1[012])\.S(?:[1-9]|\d{2}|10\d|11[012])\b)/m';
$str = 'DF62.1 P8.S12 P105.H10 RK121.2
DF57.1 P105.B4 P7.S5';
preg_match_all($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