$re = '/^((?:\d{6})(?:(?:,[^\n]\d{6}){1,})?)$/m';
$str = '123456
123456, 456789, 987654
1
,
123456, 123456, 987654,
123456, 123456
123456, 456789, 987654, 123456, 1234567
123456, 456789, 987654, 123456, 123456, 345678
1,
,
,
123,';
preg_match_all($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