An alternate regex implementation. It differs from "re" in that

* Zero-width matches are handled like in Perl and PCRE:
* ``.split`` will split a string at a zero-width match.
* ``.sub`` will handle zero-width matches correctly.
* Inline flags apply to the end of the group or pattern, and they can
be turned off.
* Nested sets and set operations are supported.
* Case-insensitive matches in Unicode use full case-folding by
default.