Public Sub CleanUpZahl(ByRef zahl As String)
        zahl = Me.formelschreibentxt.Text
        Dim chars() As String = {"+", Chr(34)}
        For Each c As String In chars
            If zahl.IndexOf(c) > -1 Then
                While zahl.IndexOf(c) > -1
                    zahl = zahl.Remove(zahl.IndexOf(c), 1)
                End While
            End If
 
        Next
 
    End Sub

Dim chars() As String = {"+-<>*()/", Chr(34)}