Post relationship ettiquette

So today i went to the gym when i got off work and I saw my Best-Friends ex-boyfriend and wasn't the best break-up situations, but personally I have no beef with him, but for her I obviously don't communicate with him( I don't really have a desire to anyways). So I was like oh, man. I don't want him to recognize me so I avoided making eye contact. In a gym it's a little harder than I thought. I could see him in my line of sight at one point and I was like trying to look everywhere but there and then at another point I saw him at the water fountain and before he could look up I directed my attention else where. So this experience got me thinking about my ex's. Luckily I've only ran into two of them post break up. The 1st encounter was super awkward. I was walking into publix and this guy in a truck stopped and called out my name. I looked and I was like "HI" but he could clearly see all over my face that I didn't remember who he was. So he was like, It's"Bleep Bleep". Then I was like OMG, HEY! then i walked inside the store. It was really awkward and I did no justice in describing it to you, but whatever. Use your imagination.  At the time I felt embarrassed for not remembering who he was. I mean I'm pretty good about not forgetting people and I kissed this guy shouldn't I remember him?  It wasn't the best relationship. In fact it was very short lived. I don't think we got past two months. Our schedules conflicted. 

So the 2nd time I ran into an 'ex' I was outside the movie theater getting tickets early for a later showing of 'Twilight', I didn't want the movie to be sold out, and I was looking a hot mess because it was early after noon and I really didn't think I'd run into anyone I knew.I know I'm an idiot. So there I am waiting and I see him walk up with his new girlfriend. I tried everything to make my self not noticeable. Running into your Ex and their new person and you look like shit sucks. EMBARRASSING!! I couldn't wait to get my tickets and leave.

So When you run into your ex are you obligated to say hi? I'm really just not sure how it's supposed to go down. I really don't run into my ex's ever. I don't have many thank gosh(I think). Now running into 'Hook-ups' that's a different story. Again luckily I don't really run into them that much either, but sometimes it is a tiny bit strange. Do I hug you? Do we shake hands? Do I nod if you recognize me? Do I ignore you like a stranger I've never met? Can I say 'Hi' first? Uuumm Like I just have so many questions.