programming-lua/exercises/ch04/ex4_9.lua

local function remove(str, start, _end)
    -- print(start, _end, str:sub(start, start), str:sub(_end, _end))
    local start_part = str:sub(1, start - 1)
    local end_part = str:sub(_end + 1, -1)
    return start_part .. end_part
end

local function remove_patterns(str, pattern)
    while true do
        local rem_start, rem_end, _ = str:find(pattern)
        if rem_start == nil then
            break
        end
        -- print(str, pattern, rem_start, rem_end)
        str = remove(str, rem_start, rem_end)
        -- print(str)
    end

    return str
end

local function ispali(str)
    str = str:lower()

    -- format
    str = remove_patterns(str, "%s")
    str = remove_patterns(str, "%p")

    for i = 1, utf8.len(str) do
        local idx = utf8.offset(str, i)
        local idx_rev = utf8.offset(str, -i)
        local ch_code = utf8.codepoint(str, idx)
        local ch_code_rev = utf8.codepoint(str, idx_rev)
        if ch_code ~= ch_code_rev then
            return false
        end
    end

    return true
end

local function test(str)
    print(("%s: %q"):format(str, ispali(str)))
end

test("step ön nö pets")
test("bänänä")

print()

test("step. ön nö   pets!")
test("bänänä??")