[UK-AL]

A regular expression implemented as a DFA would literally be looping over the string, and a state transition table. I don't see how performance could be bad.

It is highly dependent on the regular expression engine you use, most don't use automata because of extra features.

[raiph]

Perl 6 unifies "regexes" and recursive descent grammars at the syntax level and then compiles them to a hybrid of DFAs, NFAs, and regular code as necessary. The idea is to maintain the simplicity of simple regexes and of parser combinators for the user but retain as much of the performance benefits of true DFA-able regular expressions as is possible.

At least, that's the theory. In practice, while the benefit of syntactic usability is available today, the Perl 6 rules engine is still very slow and it'll likely take years to optimize the heck out of this approach and really harvest the performance benefit.