huǐ bù dāng chū
to regret one's past deeds (idiom)
zǎo zhī jīn rì hè bì dāng chū
if I (you, she, he...) had known it would come to this, I (you, she, he...) would not have acted thus (idiom)
to regret vainly one's past behaviour
jì yǒu jīn rì hé bì dāng chū
see 早知今日何必當初|早知今日何必当初[zao3 zhi1 jin1 ri4 he2 bi4 dang1 chu1]
zǎo zhī jīn rì hé bì dāng chū
if I (you, she, he...) had known it would come to this, I (you, she, he...) would not have acted thus (idiom)
to regret vainly one's past behavior