rot13 = function(s)
    chars = s.values
    for i in chars.indexes
        c = chars[i]
        if c >= "a" and c <= "z" then chars[i] = char(97 + (code(c)-97+13)%26)
        if c >= "A" and c <= "Z" then chars[i] = char(65 + (code(c)-65+13)%26)
    end for
    return chars.join("")
end function

print rot13("Hello world!")
print rot13("Uryyb jbeyq!")