早知今日何必當初 [－－－－－－当－]

Jyutping zou2 zi1 gam1 jat6 ho4 bit1 dong1 co1
Pinyin zǎo zhī jīn rì hé bì dāng chū

Definitions (CC-CEDICT)
  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