Public Function Gruppentrenner(ByVal text As String, ByVal länge As _
      Integer, _
                                   ByVal trennzeichen As Char) As String
 
        If länge < 1 Then Return text
        If String.IsNullOrEmpty(text) OrElse text.Length < länge Then Return text
 
        Dim stb As New System.Text.StringBuilder(text)
        Dim z As Integer = -1
 
        For i As Integer = stb.Length - 1 To 0 Step -1
            If (i + 1) Mod länge = 0 Then stb.Insert(i + 1, trennzeichen)
        Next i
        Return stb.ToString.Trim(trennzeichen)
    End Function

    Private Sub TextBox1_TextChanged(sender As System.Object, _
        e As System.EventArgs) Handles TextBox1.TextChanged
 
        Dim t As String = TextBox1.Text.Replace("-"c, "")
        TextBox1.Text = Gruppentrenner(t, 5, "-"c)
        TextBox1.SelectionStart = TextBox1.Text.Length
    End Sub