my pc battery is low help

-- time to remake the strange do all math algo.


str = "8*3 is my favorite number, but it's not as cool as 3*42.  827 Is a good one too."

function isnumber(x)
    local numbers = {"1","2","3","4","5","6","7","8","9","0","."}
    for i=1,#numbers do
        if tonumber(x) == tonumber(numbers[i]) then
            return true
        end
    end
    return false
end

function isoperator(x)
    local operators = {"*","-","+","/"}
    for i=1,#operators do
        if x == operator[i] then
            return {true,x}
        end
    end
    return {false,nil}
end

function calculate(x,sign,y)
    if sign == "+" then
        local sum = tonumber(x)+tonumber(y)
        return sum
    elseif sign == "-" then
        local sum = tonumber(x)-tonumber(y)
        return sum
    elseif sign == "/" then
        local sum = tonumber(x)/tonumber(y)
        return sum
    elseif sign == "*" then
        local sum = tonumber(x)*tonumber(y)
        return sum
    end
end

local sub1
local sub2
local sub3
local op

for i=1,#str do
    if isnumber(string.sub(str,i,i)) == true and sub1 == nil and op == nil then
        print('hi')
        sub1 = i
    elseif isnumber(string.sub(str,i,i)) == false and sub1 == true and isoperator(string.sub(str,i,i))[1] == true and op == nil then
        print('hi again')
        op = isoperator(string.sub(str,i,i))[2]
        sub3 = i
    elseif op ~= nil and isnumber(string.sub(str,i,i)) == false then
        sub2 = i
        local number1 = string.sub(sub1,sub3-1)
        local number2 = string.sub(sub3+1,sub2)
        local math = calculate(number1,op,number2)
        local idiff = #str
        str = string.sub(str,1,sub1-1)..math..(string.sub(str,sub3+1))
        idiff = idiff - #str
        i = i - idiff
    else
        --sub1 = nil
        --sub2 = nil
        --op = nil
    end
end

print(str)