Convert numeric html entities to utf8

#!/usr/bin/env lua
-- convert numeric html entities to utf8
-- converts from stdin to stdout
-- example:  €    ->   €
 
local char = string.char
 
local function tail(n, k)
  local u, r=''
  for i=1,k do
    n,r = math.floor(n/0x40), n%0x40
    u = char(r+0x80) .. u
  end
  return u, n
end
 
local function to_utf8(a)
  local n, r, u = tonumber(a)
  if n<0x80 then                        -- 1 byte
    return char(n)
  elseif n<0x800 then                   -- 2 byte
    u, n = tail(n, 1)
    return char(n+0xc0) .. u
  elseif n<0x10000 then                 -- 3 byte
    u, n = tail(n, 2)
    return char(n+0xe0) .. u
  elseif n<0x200000 then                -- 4 byte
    u, n = tail(n, 3)
    return char(n+0xf0) .. u
  elseif n<0x4000000 then               -- 5 byte
    u, n = tail(n, 4)
    return char(n+0xf8) .. u
  else                                  -- 6 byte
    u, n = tail(n, 5)
    return char(n+0xfc) .. u
  end
end
 
for line in io.lines() do
  out =  string.gsub(line, '&#(%d+);', to_utf8)
  print(out)
end