What does regrettable mean?

: causing sorrow or disappointment a regrettable mistake. Other Words from regrettable. regrettably \ -​blē \ adverb.

Is it regrettable or regretful?

Regrettable is an adjective that means “able to be regretted.” It describes the nature of an event or idea. ... Regretful is an adjective that means “full of regret.” It describes a specific person who feels sorry for the fact that an event or idea occurred.