Sub check_format()
Dim ok As Boolean
Dim str As String
 
'select the cell to check and run the macro - "ok" is the boolean that will...
'...determine whether the pattern is matching. It is set to true if the cell matches...
'...one of the allowed patterns.
 
str = Selection.Value
ok = False
 
Select Case True
    Case str Like "##[.]": ok = True
    Case str Like "##[.]##[.]": ok = True
    Case str Like "##[.]##[.]##[.]": ok = True
    Case str Like "##[.]##[.]##[.]##[.]": ok = True
End Select
 
If ok = False Then
    MsgBox ("Wrong pattern!")
Else
    MsgBox ("Pattern is ok!")
End If
 
End Sub