You were afraid to do it. The sale of the domain later for a large amount of money resolved that fear inside you, only leaving the desire to have bought it. This is regret defined. While it serves a purpose in our lives in teaching a lesson, it makes very little logical sense to wish for a future scenario where you are less afraid, or more intent without considering the reasons why you were to begin with.
To avoid regret, realize and accept you are going to make mistakes. To avoid regret, face your fears when they are presented to you. The choice to do what scares you the most is the single most important ability you have in controlling your fate in this universe.
> Couldn't they claim it was copyright infringement?
No, though they could claim trademark infringement (which is an important difference).
Trademarks can (but do not need to be) registered in order to be protected, and an unregistered trademark can trump a registered trademark if it precedes the registered one, if it is in a sufficiently different "category", if it is geographically distinct, or for a number of other reasons. One interesting example is the Burger King in Mattoon, Illinois[0].
The USPTO is very clear to note that registering a trademark is not a guarantee of validity of the trademark, and they do not issue refunds on the application if the trademark is successfully challenged and invalidated later.
In this particular case he'd be less likely to win against McDonald's, especially if all he's doing is domain squatting.... but you never know - take a look at http://www.chaseonline.com .
You can be too late, with respect to certain windows.
For a while, people made significant money selling highly desired domain names, including to big companies.
Now, those companies just "take them away" via trademark infringement claims. The legal structure for doing so has since been constructed and tested by them.
One of my concerns is that more and more such windows are being closed. It's not the isolated case, but the overall trend towards a corporate and corporate state restricted "push" implementation.
I don't really see how disallowing people from registering a name already in use by a person or company goes against "openness". If you're using someone else's name, I think they're justified in trying to forcibly retake control of it.
To avoid regret, realize and accept you are going to make mistakes. To avoid regret, face your fears when they are presented to you. The choice to do what scares you the most is the single most important ability you have in controlling your fate in this universe.