Check for valid domain

Question

I want to add to this function Domain checking:

GetType = function(ip)
 local Split = function(s,sep)
  local t={};i=1
  for str in string.gmatch(s,"([^"..sep.."]+)") do
   t[i] = str
   i = i + 1
  end
  return t
 end
 local R = {ERROR=0,IPV4=1,IPV6=2,DOMAIN=3,STRING=4}
 if type(ip) ~= "string" then return R.ERROR end
 local chunks = {ip:match("(%d+)%.(%d+)%.(%d+)%.(%d+)")}
 if #chunks == 4 then
  for _,v in pairs(chunks) do
   if tonumber(v) > 255 then return R.STRING end
  end
  return R.IPV4
 end
 local chunks = {ip:match(("([a-fA-F0-9]*):"):rep(8):gsub(":$","$"))}
 if #chunks == 8 then
  for _,v in pairs(chunks) do
   if #v > 0 and tonumber(v, 16) > 65535 then return R.STRING end
  end
  return R.IPV6
 end
 -- Domain check
 -- Ensuring only alphanumeric characters and . and -
 -- Ensuring no --
 -- Ensuring - is not at the start or end of part
 -- Ensuring there is more than one part
 if string.gsub(ip,"[%w%-%.]","") ~= "" then return R.STRING end
 if string.find(ip,'--',1,true) ~= nil then return R.STRING end
 local t = Split(ip,"%.")
 if #t <= 1 then return R.STRING end
 for i,v in pairs(t) do
  if string.find(t[i],'-',1,true) == 1 or string.find(t[i],'-',#t[i],true) ~= nil then return R.STRING end
 end
 return R.DOMAIN
end

It's doing IPV4/6 well but I'm a bit lost on how to do the domain checking so the questions are:

What is the best method to do the checking?

Have I got all the things to check for correct?

Answer 1

I'm not sure the requirement you described are correct, but this logic follows your requirements:

-- Only characters A-Z and numbers 0-9 and the - so long as it's not at the start or end of string.
-- Be at least 3 characters excluding the extension.
-- No spaces allowed (wont be any in the string anyway).
-- Have an extension with at least 2 characters.
local domain, ext = ip:match("([A-Z0-9][A-Z0-9%-]+[A-Z0-9])%.([A-Z0-9][A-Z0-9]+)")
if not domain or not ext then return R.STRING end
-- Hyphen can't be used in consecutive positions.
if domain:match '--' then return R.STRING end
-- Be 63 characters max excluding the extension.
if #domain > 63 then return R.STRING end
-- return R.DOMAIN if domain
return R.DOMAIN

Obviously, you can combine all three conditions together:

return domain and ext and not domain:match('--') and #domain <= 63
and R.DOMAIN or R.STRING