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.
(..) MIA: Could you, um... roll me one of those, cowboy? You can have this one, cowgirl. Thanks. RICKY NELSON SINGING: ... with regret Think nothin' of it. RICKY NELSON SINGING: ... lonesome town I can learn to forget MIA: So... Marsellus said you (..)1994 Pulp Fiction