A feeling of regret or remorse about something done (or not done); repentance over something that cannot be undone.
もっと勉強しておけばよかったと深い悔いが残った。
I was left with deep regret that I should have studied more.
彼は自分の軽率な言動を悔い改めた。
He repented his rash words and actions.
人生に悔いのないように生きたい。
I want to live so that I have no regrets in life.