#!Lua-5.0.exe
-- ROT-13 text.
-- http://en.wikipedia.org/wiki/Rot13
-- by Philippe Lhoste <PhiLho(a)GMX.net> http://Phi.Lho.free.fr
-- v. 1.1 -- 2004/10/22 -- A more elegant algorithm, inspired by Roberto's one <http://www.miranda.org/~jkominek/rot13/lua/>
-- v. 1.0 -- 2004/10/21 -- Creation

local text = "This article have been written on April first!"

-- Old code, all mine!
function Rotate13(t)
	return (string.gsub(t, "[%u%l]",
			function (char)
				local bUpper = (char < 'a')
				local b = string.byte(string.upper(char)) - 65 -- 0 to 25
				b = math.mod(b  + 13, 26)
				if bUpper then
					return string.char(b + 65)
				else
					return string.char(b + 97)
				end
			end
		))
end

-- New code, more concise and more elegant (not relying on numerical codes).
function Rotate13(t)
	local byte_a, byte_A = string.byte('a'), string.byte('A')
	return (string.gsub(t, "[%a]",
			function (char)
				local offset = (char < 'a') and byte_A or byte_a
				local b = string.byte(char) - offset -- 0 to 25
				b = math.mod(b  + 13, 26) + offset -- Rotate
				return string.char(b)
			end
		))
end

print(Rotate13(text))