Changed format for named groupings
In [Python named back references](https://docs.python.org/3/library/re.html) must take the form of `(?P<name>...)` but in [Perl](http://perldoc.perl.org/perlretut.html#Named-backreferences) and Julia named back references can look like `(?<name>...)`, `(?'name'...)`, `(?P<name>...)`, etc.
I think we should stick to recommending the `(?<name>...)` syntax as it is clear and concise.
