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.
(..) But I was wrong. Jon Snow avenged the Red Wedding. He is the White Wolf. The King in the North. I did not fight beside you on the field and I will regret that until my dying day. A man can only admit when he was wrong and ask forgiveness. There's nothing to forgive, my lord. (..)2016 TvShow: Game of Thrones Title: The Winds of Winter Season: 6 Episode 10