failed thing

-- algorithm to do all math in a string

str = "I'm not really sure why I made this but 128*2 should be 256. Yeet."

-- look for mAth

local function is_number(x)
    x = tostring(x)
    if x == "0" or x == "1" or x == "2" or x == "3" or x == "4" or x == "5" or x == "6" or x == "7" or x == "8" or x == "9" then
        return true
    else
        return false
    end
end

local function is_operator(x)
    x = tostring(x)
    if x == "+" or x == "*" or x == "-" or x == "/" then
        return {true,x}
    else
        return {false,x}
    end
 end

for i=1,#str do
    local find_number1
    local find_number2
    local sub1
    local sub2
    local sign
    if is_number(string.sub(str,i,i)) == true then
        find_number1 = i
        sub1 = i
        for k=i,#str do
            if is_number(string.sub(str,k,k)) == false then
                find_number1 = string.sub(str,i,k-1)
                print(find_number1)
                local op = is_operator(string.sub(str,k,k))
                if op[1] == true then
                    sign = op[2]
                    if is_number(string.sub(str,k+1,k+1)) then
                        for l=k+1,#str do
                            find_number2 = l
                            if is_number(string.sub(str,l,l)) == false then
                                find_number2  = string.sub(str,find_number2,l-1)
                                sub2 = l-1
                            end
                        break
                        end
                    end
                end
            break
            end
        end
    end
    local sum
    if find_number1 ~= nil and find_number2 ~= nil and sub1 ~= nil and sub2 ~= nil then
        if sign == "+" then
            sum = tonumber(find_number1) + tonumber(find_number2)
        elseif sign == "-" then
            sum = tonumber(find_number1) - tonumber(find_number2)
        elseif sign == "*" then
            sum = tonumber(find_number1) * tonumber(find_number2)
        elseif sign == "/" then
            sum = tonumber(find_number1) / tonumber(find_number2)
        end
        str = string.sub(str,1,sub1)..tostring(sum)..string.sub(str,sub2)
        i = sub2
    end
end