Check a card number against the Luhn algorithm and see the arithmetic digit by digit. Detects the network from the IIN (Visa, Mastercard, AmEx, Discover, JCB, Diners Club) and checks the length against what that network issues. Digits, spaces or dashes all work.
Nothing leaves your browser — the check runs entirely on this page. Luhn only proves the digits are internally consistent; it says nothing about whether the card exists or has funds.
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