Attack them when they are unready, take them by surprise ： 这个成语有多少方法可以说？

Question

Could you please let me know which of these sound right and why the other ones sound wrong? I'm not sure about the difference between 不 and 无.

出其不意，攻其不备

攻其不备，出其不意

攻其无备，出其无意

攻其不备，出其无意

攻其无备，出其不意

都可以用吗

Answer 1

Usually you can use any of them, but "攻其无备，出其不意" is the best one.

The reason is this word is from 《孙子兵法》, an ancient military book by a famous military strategist Sun Wu(孙武), and it is "攻其无备，出其不意" originally in that book.

Also, in 《现代汉语词典》（第7版）, there is "攻其无备，出其不意" under "出其不意" as an example.