# Posted By

savageideas on 08/25/06

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# Credit Card Validation (Luhn Algorithm)

/ Published in: ASP  Uses the Luhn formula to quickly validate a credit card. Basically all the digits except for the last one are summed together and the output is a single digit (0 to 9). This digit is compared with the last digit ensure a proper credit card number is entered (Does not actually confirm that is is a real number, just that it is likely to be one. Example: Entering "4000-0000-0000-0002" will pass the check, but "4000-0000-0000-0003" will not pass.)

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`function CheckCC(CCNo) Dim i, w, x, yy = 0 'Ensure the proper format of the inputCCNo = Replace(Replace(Replace(CStr(CCNo), "-", ""), " ", ""), ".", "") 'Process digits from right to left, drop last digit if total length is evenw = 2 * (Len(CCNo) Mod 2)For i = Len(CCNo) - 1 To 1 Step -1x = Mid(CCNo, i, 1)if IsNumeric(x) ThenSelect Case (i Mod 2) + wCase 0, 3 'Even Digit - Odd where total length is odd (eg. Visa vs. Amx)y = y + CInt(x)Case 1, 2 'Odd Digit - Even where total length is odd (eg. Visa vs. Amx)x = CInt(x) * 2if x > 9 Then 'Break the digits (eg. 19 becomes 1 + 9)y = y + (x \ 10) + (x - 10)Elsey = y + xEnd ifEnd SelectEnd ifNext 'Return the 10's complement of the total    y = 10 - (y Mod 10)if y > 9 Then y = 0CheckCC = (CStr(y) = Right(CCNo, 1)) End function` Subscribe to comments