Validating credit card checks
Go ahead, test drive the Luhn formula here, using fake digits from the bogus card below. In the unlikely event that you've given any thought at all to your credit card number, you probably view it as merely a series of random numerals: 16 digits on Visa, Master Card and Discover cards, 15 on American Express cards, and 14 on Diners Club/Carte Blanche cards.
Click the first button to enter the fake digits: Now, print out this Luhn formula PDF and test your own credit or debit card number. Want to know how, and more importantly why, it works? On a 16-digit card, the first six numerals identify the card issuer and the next nine numerals are the card account number (Am Ex uses an eight-digit account number).
@Noah: Because then you can notify the user early that they have made a mistake typing in their card number, making it easier for them to correct, rather than waiting for your server side code to check it.
Probably OP doesn't even follow this thread anymore but this may be helpful for someone else: checks the card type, validates its length and checks for mod 10 with Luhn algorithm.
ABA (American Banker's Association) routing numbers are used to identify financial institutions when making transactions.
This number is usually required when setting up a bank account for direct deposits, automated payments, etc.
Perhaps equally important at happy hour, every credit, debit and ATM card on earth still contains this mind-blowing bar trick worthy of the Beer Pong Hall of Fame.Rather than adding up the other digits and calculating the valid checksum value for comparison, it's included in the total to save some programming steps.In other words, if you had an eight-digit ABA number and wanted to find the correct checksum digit, you'd add those numbers as before.Credit card numbers are generated following this validity check, commonly known as the Luhn check or the Mod 10 check, which can be described as follows (for illustration, consider the card number I just started learning how to program two months ago so I am fairly new to this.The program doesn't compile and I don't know why and what I have to do to fix this.