$re = '/\b[HPGX]\d{3}(?:\+[HPGX]\d{3})*\b/m';
$str = 'random text H326 H243+H546+H677 H234+H333 some other random texts
X123 X234+X567 X567+X890+X654';
preg_match_all($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