While it's true that Perl's patterns resemble the DFAs (deterministic
finite automata) of the egrep program, they are in fact implemented as NFAs
(non-deterministic finite automata) to allow backtracking and
backreferencing. And they aren't POSIX-style either, because those
guarantee worst-case behavior for all cases. (It seems that some people
prefer guarantees of consistency, even when what's guaranteed is slowness.)
See the book ``Mastering Regular Expressions'' (from O'Reilly) by Jeffrey
Friedl for all the details you could ever hope to know on these matters.