Regrettable; unfortunate — an adjectival noun expressing that something is unsatisfactory or lamentable, often used in formal statements.
主催者は開催中止を遺憾だと発表した。
The organizers announced that the cancellation was regrettable.
その結果を受けて遺憾の意を表した。
They expressed regret in response to that result.
遺憾ながら予定どおりには進められなかった。
Regrettably, it could not proceed as planned.