Private Sub TextBox1_TextChanged(sender As System.Object, _
        e As System.EventArgs) Handles TextBox1.TextChanged
 
        With TextBox1
            Dim cursorposition As Integer = .SelectionStart
            Dim textlength As Integer = .TextLength
            Dim t As String = TextBox1.Text.Replace("-"c, "")
            .Text = Gruppentrenner(t, 5, "-"c)
            If cursorposition > 0 AndAlso cursorposition < .TextLength _
                AndAlso .Text(cursorposition - 1) = "-" Then cursorposition += 1
            .SelectionStart = cursorposition
        End With
    End Sub