The Past is the textbook of tyrants; the Future the Bible of the Free. Those who are solely governed by the Past stand like Lot's wife, crystallized in the act of looking backward, and forever incapable of looking before. ~Herman Melville, White Jacket
There are plenty of things I could regret in my life though it gets me nowhere. The largest regret I have would be not telling my stepfather that he was a great person, one who did all the fatherly things that he never really had to, and that I loved him. I struggled with this after his death, despite those conversations with my mother reassuring me that he already knew whether I ever told him or not. The fact remains that I let our pissing war over who was the man of the house and my immature adolescent attitude get in the way of enjoying the all too brief time he was on this Earth.
I also regret the martyr-like ways I pursued relationships in the past. I seemed to find myself in relationship where I felt like if I cared about someone enough that I could help them through their internal issues and emotional hang-ups. In the end, I found that, more often than not, this is not the case. I did learn a great deal from these somewhat awful experiences and I have been able to apply what I've learned, so maybe it's not entirely bad.
There are other regrets. Some of which I am not entirely comfortable providing for all my devoted readers. So, we'll leave it at this: A person can regret any number of things, but they can also learn a great deal from these instances of regret. Regret should be for learning; not mourning.