How to check the screen input on selection screen smartly?

Hi all,

I've a selection options plant:

select-option s_werks for marc-werks.

i want to display the error message if user inputs plant = 30xx or 30xx is within the range.

Currently, I use the codes below but i don't think it's smart enough, Is there a smarter way to do so?

AT SELECTION-SCREEN.

loop at s_werks.

if s_werks-sign = 'I'.

case s_werks-option = 'bt'.

...

endcase.

elseif s_werks-sign = 'E'.

case s_werks-option = 'bt'.

...

endcase.

endif.

endloop.