local cats = "TANK SUB ZEPPELIN HORSE AIR"
local noTargetCategory = "ZEPPELIN AIR"
local Onlytargetcategory = ""

for c in cats :gmatch("%w+") do
 if string.find(noTargetCategory, c) == nil then
  Onlytargetcategory = Onlytargetcategory .. c .. " "
 end
end

io.write ("\nnoTargetCategory was:"..noTargetCategory .. "\n")
io.write ("and now Onlytargetcategory is:"..Onlytargetcategory )
---
noTargetCategory was:ZEPPELIN AIR
and now Onlytargetcategory is:TANK SUB HORSE