Function pruef(strAusdruck As String)
    Dim i As Long
 
    For i = 1 To length(strAusdruck)
        Select Case Mid(strAusduck, i, 1)
            Case Is = "0"
                pruef = True
            Case Is = "1"
                pruef = True
            Case Is = "2"
                pruef = True
            Case Is = "3"
                pruef = True
            Case Is = "4"
                pruef = True
            Case Is = "5"
                pruef = True
            Case Is = "6"
                pruef = True
            Case Is = "7"
                pruef = True
            Case Is = "8"
                pruef = True
            Case Is = "9"
                pruef = True
            Case Is = "+"
                pruef = True
            Case Is = "-"
                pruef = True
            Case Is = "*"
                pruef = True
            Case Is = "/"
                pruef = True
            Case Else
                pruef = False
                Exit For
        End Select
    Next i
End Function

Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. (Brian W. Kernighan)