How to validate a Credit Card Number?

Most credit card number can be validated using the Luhn algorithm, which is more or a less a glorified Modulo 10 formula!

The Luhn Formula:

• Drop the last digit from the number. The last digit is what we want to check against
• Reverse the numbers
• Multiply the digits in odd positions (1, 3, 5, etc.) by 2 and subtract 9 to all any result higher than 9
• Add all the numbers together
• The check digit (the last number of the card) is the amount that you would need to add to get a multiple of 10 (Modulo 10)

Luhn Example:

List of credit card number formats