gistfile1.txt

function Calculator:expression(expr)
    local res
    local function match(pattern)
        local pos
        _, pos, res = expr:find(pattern)
        if pos then
            expr = expr:sub(pos+1)
        end
        return pos
    end
    while true do
        print("Expr: <" .. expr .. ">")
        print(table.unpack(self.stack))
        if match("^%s*(%d+%.?%d*[eE]?%d*)") then
            table.insert(self.stack, tonumber(res))
        elseif match("^%s*([%+-%*/^%w])") then
            if res == "l" then
                local x = table.remove(self.stack)
                table.insert(self.stack, math.log(x))
            elseif res == "e" then
                local x = table.remove(self.stack)
                table.insert(self.stack, math.exp(x))
            else
                local b = table.remove(self.stack)
                local a = table.remove(self.stack)
                if a == nil or b == nil then
                    return "Not enough arguments"
                end
                if res == "+" then
                    table.insert(self.stack, a + b)
                elseif res == "-" then
                    table.insert(self.stack, a - b)
                elseif res == "*" then
                    table.insert(self.stack, a * b)
                elseif res == "/" then
                    table.insert(self.stack, a / b)
                elseif res == "^" then
                    table.insert(self.stack, math.pow(a, b))
                elseif res == 's' then
                    table.insert(self.stack, b)
                    table.insert(self.stack, a)
                elseif res == 'p' then
                    table.insert(self.stack, string.format('%s %s', a, b))
                else
                    return "Invalid operator: " .. res
                end
            end
        else
            break
        end
    end
    if not expr:match("^%s*$") then
        return "Unparsed: " .. expr
    elseif #self.stack then
        return "Result: " .. self.stack[#self.stack]
    else
        return "No result"
    end
end