Woah. I really don't get this.
I'm looking for the next regexp:
</span></p>\n\n <p><span>file:\/\/.*txt</span></p>\n\n <p><span>
and it gets found perfectly - no problem there.
But it won't replace! Not anything... I began with a space, and thought that was the problem, but a number of different strings yielded no results either. Other search/replace queries I've done work fine however (chapeau!).