If the number is in a valid format if returns 'True'. If the number is in an invalid format it returns 'False'. This algorithm should work with all credit cards. If a card validates with this code it doesn't mean that the card is actually good, it just means that the numbers are arranged in a valid format. If they don't validate then you've saved some time because you don't have to process the card to find out that it is definitely bad. I use this function in CGI forms that process credit card orders.

Code

Function CheckCard(CCNumber As String) As Boolean
    Dim Counter As Integer, TmpInt As Integer
    Dim Answer As Integer

    Counter = 1
    TmpInt = 0

    While Counter <= Len(CCNumber)
        If IsEven(Len(CCNumber)) Then
            TmpInt = Val(Mid$(CCNumber, Counter, 1))
            If Not IsEven(Counter) Then
                TmpInt = TmpInt * 2
                If TmpInt > 9 Then TmpInt = TmpInt - 9
            End If
            Answer = Answer + TmpInt
            'Debug.Print Counter, TmpInt, Answer
            Counter = Counter + 1
        Else
            TmpInt = Val(Mid$(CCNumber, Counter, 1))
            If IsEven(Counter) Then
                TmpInt = TmpInt * 2
                If TmpInt > 9 Then TmpInt = TmpInt - 9
            End If
            Answer = Answer + TmpInt
            'Debug.Print Counter, TmpInt, Answer
            Counter = Counter + 1
        End If
    Wend

    Answer = Answer Mod 10

    If Answer = 0 Then CheckCard = True
End Function