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Many companies sell products over the internet. When doing that, getting credit card input over the internet becomes important. And then validating those credit card numbers becomes important. Here is a combination of regular expression validation and standard JavaScript to check to see if a credit card number is a valid number. The code below will do that validation. However, note that this doesn't check to see if the specific credit card number has really been issued by the bank, or if the owner of the credit card has enough credit available, etc. You'll still need to go through your merchant account software to validate all that information, but this initial check assures that the credit card number itself is one that could have been issued.

This code handles the most popular types of credit card: Visa, Master Card, Discover, American Express, and Diner's Club. Each of these cards start out with a different prefix and card number length. This information is used for the regular expression validation. After the card number has the right prefix and is the correct length, a "mod 10" validation is performed on the credit card number. This code is a bit strange and needs some explanation. Cards with an even number of digits go through differently than cards with an odd number of digits (American Express). Every other digit, starting with the second digit (first digit for American Express) is added together. Then a second pass is made, again with every other digit, starting this time with the first digit (second digit for American Express). During this second pass, each digit is multipled by 2. If the digit multiplied by 2 is greater than or equal to 10, the total minus 9 is added to the original sum from the first pass. If the digit multiplied by 2 is less than 10, that total is added to the original sum from the first pass. After the two passes, the total should be evenly divisible by 10. This is the check that the JavaScript code goes through to validate the credit card.

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