Binaryworld - IsValidCreditCardNumber - Check whether a credit card number is valid ... [ VB -> String ]

' Validate a credit card numbers
' Returns True if valid, False if invalid
'
' Example:
' If IsValidCreditCardNumber(Value:="1234-123456-12345", IsRequired:=True)

Function IsValidCreditCardNumber(Value As Variant, Optional ByVal IsRequired As _
  Boolean = True) As Boolean
  Dim strTemp As String
  Dim intCheckSum As Integer
  Dim blnDoubleFlag As Boolean
  Dim intDigit As Integer
  Dim i As Integer

  On Error GoTo ErrorHandler

  IsValidCreditCardNumber = True
  Value = Trim$(Value)

  If IsRequired And Len(Value) = 0 Then
    IsValidCreditCardNumber = False
  End If

  ' If after stripping out non-numerics, there is nothing left,
  ' they entered junk
  For i = 1 To Len(Value)
    If IsNumeric(Mid$(Value, i, 1)) Then strTemp = strTemp & Mid$(Value, i, _
      1)
  Next
  If IsRequired And Len(strTemp) = 0 Then
    IsValidCreditCardNumber = False
  End If

  'Handle different lengths for different credit card types
  Select Case Mid$(strTemp, 1, 1)
    Case "3"  'Amex
      If Len(strTemp) <> 15 Then
        IsValidCreditCardNumber = False
      Else
        Value = Mid$(strTemp, 1, 4) & "-" & Mid$(strTemp, 5, _
          6) & "-" & Mid$(strTemp, 11, 5)
      End If
    Case "4"  'Visa
      If Len(strTemp) <> 16 Then
        IsValidCreditCardNumber = False
      Else
        Value = Mid$(strTemp, 1, 4) & "-" & Mid$(strTemp, 5, _
          4) & "-" & Mid$(strTemp, 9, 4) & "-" & Mid$(strTemp, 13, 4)
      End If
    Case "5"  'Mastercard
      If Len(strTemp) <> 16 Then
        IsValidCreditCardNumber = False
      Else
        Value = Mid$(strTemp, 1, 4) & "-" & Mid$(strTemp, 5, _
          4) & "-" & Mid$(strTemp, 9, 4) & "-" & Mid$(strTemp, 13, 4)
      End If
    Case Else   'Discover - Dont know rules yet
      If Len(strTemp) > 20 Then
        IsValidCreditCardNumber = False
      End If
  End Select

  'Now check for Check Sum (Mod 10)
  intCheckSum = 0
      ' Start with 0 intCheckSum
  blnDoubleFlag = 0
      ' Start with a non-doubling
  For i = Len(strTemp) To 1 Step -1          ' Working backwards
    intDigit = Asc(Mid$(strTemp, i, 1))       ' Isolate character
    If intDigit > 47 Then              ' Skip if not a intDigit
      If intDigit < 58 Then
        intDigit = intDigit - 48        ' Remove ASCII bias
        If blnDoubleFlag Then
            ' If in the "double-add" phase
          intDigit = intDigit + intDigit   ' then double first
          If intDigit > 9 Then
            intDigit = intDigit - 9     ' Cast nines
          End If
        End If
        blnDoubleFlag = Not blnDoubleFlag    ' Flip doubling flag
        intCheckSum = intCheckSum + intDigit  ' Add to running sum
        If intCheckSum > 9 Then         ' Cast tens
          intCheckSum = intCheckSum - 10   ' (same as MOD 10 but
                            ' faster)
        End If
      End If
    End If
  Next

  If intCheckSum <> 0 Then              ' Must sum to zero
    IsValidCreditCardNumber = False
  End If

ExitMe:
  Exit Function
ErrorHandler:
  Err.Raise Err.Number, "IsValidCreditCardNumber", Err.Description

End Function