Small fixes to regex rule

[oliparcol]

This pull request improves the way regex rules are handled by:

• making sure they start and end with parentheses (fixed some tests along the way)
• casting the keys to str before trying to match them against the regex. This unifies the way required parameter and key matching against the regex are handled.

I've also casted tmp_path fixture to str in 3b547fa to make the tests compatible with latest pytest version as it is now a pathlib (see https://docs.pytest.org/en/latest/tmpdir.html#the-tmp-path-fixture).

[Grokzen]

@oliparcol I manually resolved the conflicts and merged this PR manually and pushed it to master branch. Consider this PR merged/fixed even if it says closed :) Thank you for this contribution.