programming-lua/exercises/ch04/ex4_4.lua

local function insert(str, i, o)
    i = utf8.offset(str, i)
    local first_half = str:sub(1, i - 1)
    local latter_half = str:sub(i, -1)
    return first_half .. o .. latter_half
end

local function tests()
    local res = insert("ação", 5, "!")
    print(('insert("ação", 5, "!") --> %s'):format(res))
end

tests()