rue - regret
To ‘rue’ something is to feel sorry about it and wish that it had not happened, usually because it has had negative consequences for you or others. The word strongly implies looking back at the past with remorse and a degree of bitterness, therefore it is used exclusively in a negative context.
I’m not going to let Adam get away with what he’s done. He will rue the day he crossed me!
When Emily realised the gravity of her mistake, she was immediately rueful and worked hard to put things right.
There was a touch of ruefulness in John’s voice as he explained how close he had been to securing that lucrative deal.