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Determine if a credit card number is valid using VB6

If you support a Web site that accepts online payments, the first step in any process that accepts credit card numbers should be verifying the number. This tip shows how to do this using VB6.

Credit card numbers are not assigned at random. Each number, usually 16 digits long, must adhere to certain mathematical conditions to be valid. This is called the Luhn Check, and it is used by almost all major cards. If you support a Web site that accepts online payments, the first step in any process that accepts credit card numbers should be verifying the number. This tip shows how you can do this using VB6.

Using VB6, the credit card number is passed, as a string, to the function CCNumberValid. The string must contain only digits—no embedded spaces or dashes. The function returns True if the number is valid and False if not.

Public Function CCNumberValid(ByValCCNumber As String) As Boolean

    Dim Result As Long
    Dim Total As Long
    Dim idx As Integer
    Dim i As Integer
    Dim j As Integer
              
    j = 1
    For i = Len(CCNumber) To 1 Step -1
        Result = (CInt(Mid$(CCNumber, i, 1)) * j)
       
        If Result >= 10 Then
            Total = Total + (CInt(Mid$(CStr(Result), 1, 1)) _
                + CInt(Mid$(CStr(Result), 2, 1)))
        Else
            Total = Total + Result
        End If
       
        If j = 2 Then j = 1 Else j = 2
    Next
       
    If Total Mod 10 = 0 Then
        CCNumberValid = True
    Else
        CCNumberValid = False
    End If
End Function

The fact that a credit card number is valid does not mean that it is actually assigned to an account or that the account is in good standing. But checking the validity of a number is the best and fastest way to weed out errors in number entry.

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