I'd like to add something to a certain namespace. Thus I entered "Namespacename:" for the page prefix option. The preview works fine however when trying to actually do the changes I get "Error: The MediaWiki API returned the error code invalidtitle: Bad title "File:"." I am not sure if this is a bug or feature request.
Topic on Extension talk:MassEditRegex
I am encountering this same issue. The Show Preview button shows exactly what I want, with all pages in the namespace getting altered, but attempting to Execute (with "Execute in Browser") results in the "invalidtitle" pop-up. However, running with "execute in browser" unchecked seems to work--it's just that I've got many tens of thousands of articles to affect, so it stops after the first 1000.