Public Function StrFilter(ByVal StringToFilter As String, _
                          ByVal AllowedCharacters As String, _
            Optional ByVal PlaceHolder As String = "") As String
 
    Const Standardfilter As String = _
            "ABCDEFGHIJKLMNOPQRSTUVWQYZ" & _
            "abcdefghijklmnopqrstuvwxyz" & "0123456789 " & _
            "ÄÖÜäöüß"
 
 
    If StringToFilter Is Nothing Or _
       AllowedCharacters Is Nothing Then Return Nothing
    If PlaceHolder Is Nothing Then PlaceHolder = String.Empty
    If AllowedCharacters = String.Empty Then _
       AllowedCharacters = Standardfilter
 
    Dim stb As New System.Text.StringBuilder(StringToFilter.Length)
 
    For i As Integer = 0 To StringToFilter.Length - 1
       If AllowedCharacters.Contains(StringToFilter(i)) Then
          stb.Append(StringToFilter(i))
       Else
          stb.Append(PlaceHolder)
       End If
    Next i
 
    Return stb.ToString
End Function