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)))
            Total = Total + Result
        End If
        If j = 2 Then j = 1 Else j = 2
    If Total Mod 10 = 0 Then
        CCNumberValid = True
        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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