Modern regexes are fiendishly complex: beyond traditional features like alternations and loops, they include complex constructs such as backreferences, possessive groups, recursion, conditionals, or bounded (and sometimes even unbounded!) lookarounds.

Surprisingly, the complexity characteristics of these modern regex features are not fully known. Take lookarounds, for example: until recently, if you wanted a regex flavor that supported lookarounds, you had to use a backtracking-based algorithm.