Perl 6 "regular expressions" are so far beyond the formal definition of regular expressions that we decided it was time for a more meaningful name . [1] We now call them "rules." Perl 6 rules bring the full power of recursive descent parsing to the core of Perl, but are comfortably useful even if you don't know anything about recursive descent parsing. In the usual case, all you'll ever need to know is that rules are patterns for matching text.
[1] Regular expressions describe regular languages, and consist of three primitives and a limited set of operations (three or so, depending on the formulation). So even Perl 5 "regular expressions" weren't formal regular expressions.