Excel - Input mask issue

I would like to assign a custom input mask to a textbox like this:

Issue

"A.44.A.04.04"

Solution

Try this:

Dim lt As Long
lt = Len(TextBox1.Text)
If lt = 1 Or lt = 4 Or lt = 6 Or lt = 9 Then
  TextBox1.Text = TextBox1.Text & "."
End If

Thanks to ccm81 for this tip.