It was regrettable that the research funding was cut before the project could be completed.

The regrettableness of his hasty decision became clear when the project failed.

The regretter watched the missed train disappear into the fog, wishing he had left earlier.

Regrettably, the conference was canceled at the last minute due to severe weather.

Despite their friends' reassurances, the regretters couldn't stop replaying their past decisions in their minds.