Fixed phrase used to express regret or disappointment about a situation. Often placed at the beginning of a sentence.
See also: 残念
Unfortunately, he couldn't come.
Regrettably, the shop was closed.
Standard kanji spelling.
Variant with 事 instead of こと; less common but still seen.
From 残念 (regrettable) + な (attributive) + こと (thing) + に (adverbial particle), literally 'as a regrettable thing'.