do sth by the book郑人买履
A man of the state of Zheng wanted to buy a pair of shoes.
When he set out for the market,he forgot to bring it along.
It was after he had found the pair he wanted that this occurred to him.
"I forgot the measurement", said he.
He went home to get it. But when he returned, the market had closed and he did not get his shoes after all.
"Why didn't you try on the shoes with your feet?" He was asked.
"I'd rather trust the measurement than trust myself."
成语“郑人买履”形容了一个不折不扣“不知变通”的人，做人如果“一味只知道照本宣科”，那就会变得“墨守成规”，就像这个“买鞋的郑人”一样了。英文中“郑人买履”可以用“do sth by the book”来表达，意思就是“ 凡事都按照规章制度来办”，有时候引申开来意思就是“墨守陈规”了。
This is a private deal - we don't have to do everything by the book.