Great,
exactly what I need, combination of both does the trick !
First, input mask as I want, and then a custom Msgbox if user types anything that is not allowed in field 
Thanks to both of you !!!
EDIT:
I also added code for custom Msgbox on input mask violation :
Code:
Private Sub Form_Error (DataErr As Integer, Response As Integer)
Const INPUTMASK_VIOLATION = 2279
If DataErr = INPUTMASK_VIOLATION Then
MsgBox "There was an input mask violation!"
Response = acDataErrContinue
End If
End Sub
Maybe somebody will find It useful.