$re = '/^(?:(?![0-9]{4}|[a-zA-Z]{4})[a-zA-Z0-9]{4})(?:(?:\b\,)(?![0-9]{4}|[a-zA-Z]{4})[a-zA-Z0-9]{4})*$/m';
$str = 'AAA1BBB2
AAA2,BBB3
AAA2,BBB3,BBB3,BBB3,BBB3,BBB3,BBB3,BBB3
1Q11,22V2
';
preg_match_all($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