function CheckCC(CCNo)

Dim i, w, x, y
y = 0

'Ensure the proper format of the input
CCNo = Replace(Replace(Replace(CStr(CCNo), "-", ""), " ", ""), ".", "")

'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

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.)

Credit Card Validation (Luhn Algorithm)

validation

ASP