A somewhat formal or literary noun for deep regret or remorse over one's own actions. Often used in set phrases like 悔いが残る or 悔いのない人生.
I still regret not having spoken honestly back then.
I will give it my all so that I have no regrets.
He was left with regret, feeling he could have tried harder.
I want to live a life without regrets.
She harbors regret over the choice she made when she was young.
I still feel regret about that decision.