Describing something that turned out badly, that you wish had gone another way. A regrettable event is unfortunate, unpleasant, a shame — an accident, a remark, a delay, a decision that caused harm.

The useful thing to notice is what the word does socially. It is the word of official apology, and it apologises without confessing. A company says 'the incident was regrettable' — the sentence is sad about the event but does not say who caused it or that anyone is sorry. Compare 'we made a serious mistake and we apologise'. That is why you meet regrettable so often in company statements, government announcements, and news writing: it lets a speaker sound sorry while staying at a distance. Native listeners hear that distance. When a politician says something is 'deeply regrettable', many people hear it as a non-apology.

It is formal. In everyday talk people say 'it's a shame', 'it's too bad', 'what a pity'. Save regrettable for writing and for careful, serious speech.

Remember also that it attaches to the THING, never to the person feeling it. Events, decisions, remarks, delays, and consequences are regrettable. A person who feels sorry is regretful.