-- If string is empty, returns zero.
local function to_double_or_zero(s)
    local n = tonumber(s)
    return n ? n : 0
end

local function multiply(s)
    local b = s:split("*"):map(|c| -> to_double_or_zero(c))
    return tostring(b[1] * b[2])
end

local function divide(s)
    local b = s:split("/"):map(|c| -> to_double_or_zero(c))
    return tostring(b[1] / b[2])
end

local function add(s)
    local t = s:gsub("^%+", ""):gsub("%++", "+")
    local b = t:split("+"):map(|c| -> to_double_or_zero(c))
    return tostring(b[1] + b[2])
end

local function subtract(s)
    local t = s:gsub("(%+%-|%-%+)", "-")
    if "--" in t then  return add(t:replace("--", "+")) end
    local b = t:split("-"):map(|c| -> to_double_or_zero(c))
    return tostring(#b == 3 ? -b[2] - b[3] : b[1] - b[2])
end

local function eval_exp(s)
    local t = s:gsub("[()]", "")
    local pmd = "%d+%.?%d*%s*[*/]%s*[+-]?%d+%.?%d*"
    local pm  = "%*"
    local pas = "%-?%d+%.?%d*%s*[+-]%s*[+-]?%d+%.?%d*"
    local pa  = "%d%+"

    while true do
        local exp = t:match(pmd)
        if !exp then break end
        local exp2 = exp:match(pm)
        if exp2 then
            t = t:replace(exp, multiply(exp))
        else
            t = t:replace(exp, divide(exp))
        end
    end

    while true do
        local exp = t:match(pas)
        if !exp then break end
        local exp2 = exp:match(pa)
        if exp2 then
             t = t:replace(exp, add(exp))
        else
             t = t:replace(exp, subtract(exp))
        end
    end

    return t
end

local function eval_arithmetic_exp(s)
    local t = s:gsub("%s", ""):gsub("^%+", "")
    local ppara = "%([^()]*%)"
    while true do
        local exp = t:match(ppara)
        if !exp then break end
        t = t:replace(exp, eval_exp(exp))
    end
    return to_double_or_zero(eval_exp(t))
end
({
    "2+3",
    "2+3/4",
    "2*3-4",
    "2*(3+4)+5/6",
    "2 * (3 + (4 * 5 + (6 * 7) * 8) - 9) * 10",
    "2*-3--4+-0.25",
    "-4 - 3",
    "((((2))))+ 3 * 5",
    "1 + 2 * (3 + (4 * 5 + 6 * 7 * 8) - 9) / 10",
    "1 + 2*(3 - 2*(3 - 2)*((2 - 4)*5 - 22/(7 + 2*(3 - 1)) - 1)) + 1"
}):foreach(|s| -> do
    local r = eval_arithmetic_exp(s)
    if r % 1.0 == 0 then r = math.round(r) end
    print($"{s} = {r}")
end)