早知今日何必当初

zǎo zhī jīn rì hé bì dāng chūzaozhijinrihebidangchu

1. if I (you, she, he...) had known it would come to this, I (you, she, he...) would not have acted thus (idiom)
2. to regret vainly one's past behavior

Study it in Yanamino