tonumber只能用于实数字符串，不能用于算术表达式.

tonumber can be used only on a string that is a real number, not an arithmetic expression.

您可以加载字符串并运行它:

You can load the string and run it:

x = "5 + 5"
func = assert(load("return " .. x))
y = func()
print(y)

在Lua 5.1中，使用loadstring代替load.

In Lua 5.1, use loadstring instead of load.