Minor changes to `phutil_var_export`

Summary: Consolidate some regular expressions in `phutil_var_export`.

Test Plan: `arc unit`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9934