From: Matthew Moss <matthew.moss.coder@...>
Date: 2007-04-28T02:58:46+09:00
Subject: Re: [QUIZ] Checking Credit Cards (#122)

>         +============+=============+===============+
>         | Card Type  | Begins With | Number Length |
>         +============+=============+===============+
>         | AMEX       | 34 or 37    | 15            |
>         +------------+-------------+---------------+
>         | Discover   | 6011        | 16            |
>         +------------+-------------+---------------+
>         | MasterCard | 51-55       | 16            |
>         +------------+-------------+---------------+
>         | Visa       | 4           | 13 or 16      |
>         +------------+-------------+---------------+

So is a card number like "4012 3456 7890" a valid Unknown, or an invalid Visa?
Similarly, for Unknown, should we accept any length, or go for the common 16?