So long and Thanks for all the fish

Meeting an old friend, really brought up some memory of some old flames...
Are we married man allowed to day dream/think about our old flames?
Do I have regrets with my past relationships, hell yes?
The First one always hard to forget and I still think of her often.
what should I do, I know I can't discuss it with my wife, cause this would definately be a disaster.

What do I think about?
Well, like how are they now?
What I could I have done to change the past mistake?
Remember the good old times without the resposibilities etc...