Credit Card Validation (Luhn Formula)

Posted: 2002-06-01
By: ArchiveBot
Viewed: 77

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ASP/ HTML

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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.)

Original Author: BrewGuru99

Code

Function CheckCC(CCNo)
Dim i, w, x, y

y = 0
CCNo = Replace(Replace(Replace(CStr(CCNo), "-", ""), " ", ""), ".", "") 'Ensure proper format of the input
'Process digits from right to left, drop last digit if total length is even
w = 2 * (Len(CCNo) Mod 2)
For i = Len(CCNo) - 1 To 1 Step -1
  x = Mid(CCNo, i, 1)
  If IsNumeric(x) Then
   Select Case (i Mod 2) + w
    Case 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) * 2
     If x > 9 Then
      'Break the digits (eg. 19 becomes 1 + 9)
      y = y + (x 10) + (x - 10)
     Else
      y = y + x
     End If
   End Select
  End If
Next
'Return the 10's complement of the total
y = 10 - (y Mod 10)
If y > 9 Then y = 0
CheckCC = (CStr(y) = Right(CCNo, 1))
End Function

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