To regret or not to regret?

Molly Ringwald said: "I don't really believe in regret. I think you can always learn from the past, but I wouldn't want a different life."

I've been having a little debate with my BF. and I've also discussed this with my best friend.

How does regret work?

Do we look back at past bad decisions and regret them? Do we wish we had done differently? Do we regret sin or poor decision making skills?

Or do we not regret? whatever hard time, bad decision or misstep actually led us all to the places we are today. 

I tend to think there's a middle ground.

Had I not moved to MS instead DC, I wouldn't be dating Dave or friends with the girls I am close to.  Had I gone to Ole Miss straight out of high school, my life would be different. Sometimes I may have wished I made a diferent choice...what would my life be like if X? but how can I regret the steps that led me here, even if they were difficult.  Even broken relationships, I regret that I spent so much time into making them work, but can I regret that they happened? Didn't I learn and grow?

But, I disagreed some with my friends during this debate.... I must regret sometimes. I must regret sin.  When it wasn't just a decision, but a sinful choice, I must regret that past decision, even if God redeemed it and used it to bring me to Him. I must regret sin so I can "go and sin no more".  I regret the feelings I've hurt, the gossip I've spread, the angry thoughts I've had.

Regret is one thing, dwelling is an entirely different thing. Where's the line? Where does regret end and dwelling begin? And what should we regret and what should we take as a decision that made us who we are today? 

Any thoughts?