Use 'regrettable' when something could have been avoided and there's a sense of disappointment or missed opportunity. The project's failure is most regrettable; we could have succeeded with more time.
Pro Tip 2/3
Formal Tone
'Regrettable' is formal and often used in official statements to express sorrow or apology. We find the errors in our report most regrettable and assure you they will be corrected.
Pro Tip 3/3
Personal Feelings
It can describe personal emotions, expressing a polite way to say you wish something hadn't happened. It's regrettable that I missed your birthday party.