To clarify my comment in bug 1497725, there was no intention to file a follow-up bug to implement regular expression filtering, given the extremely low usage: 33 times in absolute terms, 0.005% in relative terms. That was over a time span of a month or so and for Nightly users, if I remember correctly. The only other use case was to preselect a set of multiple preferences, which was something the "favorites" feature in bug 1497728 could cover in a better, more visible way.
That said, I know various tasks have been added to the scope of the project since then, including bug 1544195 that was filed to still allow some sort of wildcard. While my opinion on the usefulness of these has not changed, it may be the case that as part of the new scope of work the final version might include some form of regular expression filtering. At the moment, the project is on hold, so it's difficult to say what will be in scope for the first release.