Determining credit/debit card type

I'm writing a POS application for the iPhone/iPod combination, using the same exact hardware used in Apple Stores:: EasyPay.

Ok, my hurdle is this: how do I identify which credit card type is being used as text is entered in the uitextfield?

I assume I've to use textField:shouldChangeCharactersInRange:replacementString:? However, I've got ideas floating around, but I simple cannot put them together to form code :/

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Any help is appreciated!

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Maybe I didn't uderstand your problem, but I think you can have a BIN's list (Bank Identification Number). For example, most credit card numbers beginning with '4' are Visa. If they begin with '5', they are probably Mastercard. This is only an example, you should have a complete list to identify each possible card. Unfortunately, a complete and updated list is paid, but you can have a good list searching for free information on the Web, like here.

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You can make a reasonable stab at guessing the card type from the first six digits of the card number, which is known as the Issuer Identification Number (IIN)

The trouble is, you'll struggle to get a complete list of IINs. Even the major acquirers struggle to keep an up to date list, as issuers frequently add or remove ranges.

Luckily, the card type shouldnt generally matter. You should be able to perform basic validation of the length, and luhn check digit, then submit the card to the processor for validation and authorization.

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You can contact Visa/MC and sign up for the BIN list updates. Visa is free but I believe MC has a fee. Once you sign up, you will get a cd mailed to you with the current BIN ranges monthly.

You can also go through a processor or ISO (independent sales org) if you are working with one. I assume since you have a payment app you have some sort of relationship with an ISO that sets ups merchant accounts for your customers. The ISO should be able to get the BIN list for you from the processor and probably for free.

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I think you are on the right track with textField:shouldChangeCharactersInRange:replacementString:, as this gets called each time the user changes the content in the text field. It sounds like you do not actually want to change the textField, but instead want to change another, related control that identifies the card type. You can use the algorithm described in Determine Credit Card Type By Number as a helper function to adjust credit card type accordingly when the user begins typing.

You may want to skip the full verification until the input string has the correct number of characters.

Update: an interesting response in the same thread mentions that actually letting the user pick credit card type is a good idea because it at least shows the user the list of credit cards you accept.