Simple Test of Credit Card Number Validation Routines

Credit cards are somewhat self-verifying and self-identifying:

• Some of the digits can be examined to determine the card issuer (VISA/MC/AMEX).
• There is a consistent checksum.

Number of digits varies by Issuer with AMEX and Diner's being 15 digit and VISA, MC, Discover and BankCard being 16 (maybe). The test below requires that the number be entered without blanks or dashes. Accepting more free-form input is left as an exercise for the reader.