非常遗憾，我公司不得不通知，在此同一时期内，需要取消数种订单。
I regret that I have to notify you of so many order being cancelled at the same time.

遗憾的是 ...

（他）竟然提出此事，令人遗憾。
I regret (his) ever having raised the matter.

很遗憾，（关於那事）你听到的情况不确。
I regret to say you have been misinformed (about that).

很遗憾，我不能帮助您。
I regret (that) I can't help you.

我对自己对这个课题的无知感到遗憾。
I regret my ignorance on the subject.

我们遗憾地把我们的决定通知她。
We informed her with regret of our decision.

我们很遗憾无法再考虑你的情况。
We regret that we are unable to reconsider your case.