CLN: Regular expression clean
- [x] ref https://github.com/pandas-dev/pandas/pull/31091#discussion_r367732954
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry


Thanks @MomIsBestFriend 