If you don't travel now, you might 'live to regret it'. [=you might feel sorry or disappointed about it in the future]
We 'regret' to inform you [=we are sorry to tell you] that we have offered the job to someone else.
It is to be 'regretted' [=it is regrettable/unfortunate] that the program has lost its funding.
(..) We must follow them back and repair the damage they've done. - Come on. - Good night, Eddie. Go home. You'll regret this tomorrow. You should have learned by now that I don't have regrets. - One more round, Lily. - Z, you've had enough. - I'm not flying with a drunken pilot. - I can't fly sober. (..)1996 Star Trek: First Contact