I'm at two of these and while I can't say they were much fun, I don't regret them either.  They were both good for a while, and good things happened in them, and then bad things happened and they were over. 

I learned a long time ago that spending your life going "if only..." is a zero sum game.

First marriage failed but led (not directly) to a new long lasting and very beneficial relationship.

Nope, still married and no regrets.

It all depends on the situation. If you have broken up because your partner was abusive or because they were unfaithful, then this doesn't feel like such a big regret. But if someone divorced you for no reason or just because they didn't think you were giving them enough or they did the dirty on you, or whilst you were in marriage, you were horrible to the partner but now you realized what you nad, but it's not too late. Then this is a big regret and a lot of love songs have been built around this regret.

I have been married to the same lovely woman for almost 28 years. Marriage requires a lot of give and take, but mostly give before you take. Be willing to do the little things that mean so much to your mate. Remember those 3 not so little words; communicate, communicate and communicate. In order for a marriage to make it you must learn to forgive, but you must also learn how to accept forgivness. Harboring guilt prevents you from experiencing joy and making the most of your relationship. Everyday is a new day, enjoy it and long live marriage.