Phabricator linter can't find any lintable paths
So it turns out there's actually nothing wrong with the regular expression. I'm just not using arc lint correctly.
The arc lint command only tries to lint files that have been modified. I was just expecting it to behave like arc lint --everything.
$ arc lint --help lint [options] [paths] lint [options] --rev [rev] Supports: git, svn, hg Run static analysis on changes to check for mistakes. If no files are specified, lint will be run on all files which have been modified.