Is there any way to edit all pages in the Main namespace, even though those pages don't have a prefix? If not, I think that would be a good addition to the extension.
Topic on Extension talk:MassEditRegex
I have the same question—tried to play around with a regex that would capture all titles but that didn't work.
I found a good solution. Go to Special:AllPages, and just select then copy the list of pages. And paste all that into the list of pages box on MassEditRegex.