regrets

i had lunch with a very dear friend of mine recently and we discussed 'regrets' - we talked about the regret of things we had not done and then of course the things we had done.  we talked about what could have been and then decided regrets are useless.  frankly what can you do with regret? 

what?  change the past? no.  change the future? maybe (but doubtful). learn from the past? we hope.  put really what is the purpose of regret(s)?

according to wikipedia regret means:

Regret is often felt when someone feels sadness, shame, embarrassment, depression or guilt after committing an action or actions that the person later wishes that he or she had not done.

ah, so  many emotions in that sentence - so which is it?  shame? guilt? depression?  not sure i guess, depends on the exact thing you regret right?

again, a random thought for the universe - should we waste our emotion on regret?