$re = '/(?i)^(?!,)((?:,|^)(?:[a-z]{4}[0-9]{2}|[0-9]{2}[a-z]{4}))+$/m';
$str = 'JEAN01,JEAN02,03JEAN,JEAN04';
preg_match_all($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