get_domain_name.lua

local middle_words = {edu=1, gov=1, com=1, net=1, org=1}
function get_domain(url)
   return (
      url:gsub("(%.(%w+))(%.%w+)$",
         function(a,b,c)
            return a..(middle_words[b] and "\0" or "")..c
         end
      ):gsub("^(.-)(%w+%.%w+)%f[%z].?(.*)$", "%2%3")
   )
end

for _, url in ipairs{
   "safebrowsing.google.com",
   "i2.cdn.turner.com",
   "powerdns.13854.n7.nabble.com",
   "nabble.com",
   "nabble.com.br",
   "cdn.nabble.com.br"
} do
   print()
   print("Input:", url)
   print("Output:", get_domain(url))
end