R1:cf092781ae50

Fix regexp issue with Diffusion remarkup rule

Summary: My `\w` shenanigans don't work with Diffusion, which has no explicit prefix.

Test Plan: Typed `rXnnn` and got the whole thing properly marked up.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5984