A fixed adverbial phrase used to express regret or disappointment about a situation. Often appears at the beginning of a sentence or clause.
Unfortunately, tomorrow's event has been canceled.
Regrettably, he was unable to pass the exam.
残念ながら is more common in everyday speech and expresses personal disappointment, while 不幸にも sounds more formal and focuses on the unfortunate nature of the event itself.
残念ながら introduces a regrettable fact, contrasting with 幸いなことに, which introduces a fortunate one.
From 残念 (regrettable) + ながら (while, though), literally 'while it is regrettable'. The exact historical derivation is straightforward.