local letterdetect = setmetatable({ }, { __mode = "k" })

local Letters = { }

setmetatable(getfenv(1), { 
  __newindex = function (self, key, value)
    if letterdetect[value] then
      Letters[key] = value 
    else
      rawset(self, key, value)
    end
  end 
})

local function Letter(character, upper)
  local letter = {actualChar = character, uppercase = upper}
  letterdetect[letter] = true
  return letter
end

Asterisk = Letter("*", false)

print(Letters.Asterisk.actualChar)

*