Credit card luhn validator

A Luhn pass says the digits are self consistent and nothing more. It does not say the card is real, active, or open with any institution, and a matching brand pattern is a guess from public prefix ranges, not a BIN database. The working below doubles every second digit from the right, folds two digit products by subtracting nine, and passes when the total ends in 0; the check digit field runs the same arithmetic backwards to complete a partial number. Batch output shows masked numbers only, because pasted lists can carry real card numbers. These are format checks on numbers you paste, all computed in this tab.