This returns what looks like plugins that are running, what should I use to get features in my product. I am specifically interested in those listed in "Installed Software" section of Installation Details dialog.
Also, when constructing an actual query, how can add regular expression to the query, something like:
Check out the UserVisibleRootQuery class in the org.eclipse.equinox.p2.engine plugin. This will return the list of things that have been installed (whether they are plugins or features).
You can also check out the QueryUtil class for a bunch of helper methods to create "typical" queries.