Ignoring the flame bait, for me the only things I wish more regex engines supported (cough JavaScript) is the ability to ignore whitespace, and have named groups. Python has a flag to do this, and being able to have multi-line regexes with comments and named groups is phenomenal and greatly improves readability of more complex regexes.

In general I would say ~70% of regexes are highly readable. With tools like the above, you can probably go to like ~85%? There are some regexes that are super complicated and then likely should be refactored into a composition of simpler regexes. But that's just a guess. I wonder if there are any studies done about this...