There's no need to keep repeating information already given.
As it stands your validation won't work. What if the user enters a selection of ranges? Or an exclusion? Or just an "*" to get everything?
Use the correct function module to get the valid values for the domain LFSTK (see here: GET DOMAIN FIXED VALUE - Code Gallery - SCN Wiki) into an internal table. Then
DATA match TYPE abap_bool. match = abap_false. LOOP domain_values INTO domain_value. CHECK domain_value IN s_lfstk. match = abap_true. ENDLOOP.
IF match EQ abap_false.
..." Sorry, pal, not valid.
ENDIF.