3rd level domain with numbers at 2nd level = "invalid $domain syntax"

Due to regexp here https://github.com/regru/php-whois/blob/master/src/Phois/Whois/Whois.php#L23

* `example.com` -> passed
* `m.example3000.com` -> `invalid $domain syntax`

Here is online demonstration https://3v4l.org/KdAMn

Why there are no numbers allowed in 2nd level domain?