It comes in the morning after the night you've decided you won't go to training just because you let a petty stuff get the better of you.
Yes, regret. Especially if that night is one of those few precious nights you have that you really are available to train - few nights that you have the time free.
Note to self: When you're having a bad day, don't run away from jiujitsu because JIUJITSU is the way!