sample #1 @ https://groups.google.com/forum/#!topic/lua-br/-10MQVxPyy8

sample1.lua

---
--- license: public domain
--- author: denis dos santos silva
--- date  : 20/05/17
--- lua 5.1+ 
---

function processa(data)
	local _c3 = {};
	local _c;

	for i=1, #data do
	    _c = data[i];
	    if         (_c == 'A') then _c3[#_c3+1] = 'T';   -- a->t
		elseif (_c == 'T') then _c3[#_c3+1] = 'A';   -- t->a
		elseif (_c == 'C') then _c3[#_c3+1] = 'G';   -- c->g
		elseif (_c == 'G') then _c3[#_c3+1] = 'C';   -- g->c
	    else
		_c3[#_c3+1] = '?';
    	    end
	end

	return _c3;
end

function implode(str)
	local r = {};
	str:upper():gsub('.' , function (c) table.insert(r, c); return end);
	return r;
end

--
-- main
--
local c1, c2, c3;

c1 = io.read();
c2 = implode(c1);

print('\n\n');
print('input', table.concat(c2, ","));
c3 = processa(c2);
print('output', table.concat(c3, ","));