regret to do　对要做的事遗憾。　(未做)
regret doing　对做过的事遗憾、后悔。　(已做)

I regret to have to do this, but I have no choice.
我很遗憾必须这样去做，我实在没有办法。

I don't regret telling her what I thought.　
我不为告诉她我的想法而后悔。

典型例题

---You were brave enough to raise objections at the meeting.
---Well, now I regret ___ that.
A. to do　B. to be doing　C.　to have done　D.　having done

答案：D。regret having done sth. 对已发生的事感到遗憾。regret to do sth. 对将要做的事感到遗憾。本题为对已说的话感到后悔，因此选D。