$re = '/\b\d+x\d+(?!(?:\+\d+\+)?\d)/m';
$str = 'Mon0 connected 1920x1200+0+0 0mm x 0 mm
1920x1200_c 59.95*
Mon0 connected 1920x1200+0+0 0mm x 0 mm
1920x1200_c 59.95*
Mon0 connected primary 1920x1200+0+1200 0mm x 0 mm
1920x1200_c 59.95*
Mon0 connected 1029x1200+1920+1200 0mm x 0 mm
1920x1200_c 59.95*';
preg_match_all($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