Yes.
In 5.6 the engine keeps track of how long "complex RE's"
are taking to match. If it is too long, then it redoes it
while keeping a history of visited positions. This will
reduce the worst case scenario from exponential to
polynomial (unless, of course, you use backreferences).
There is, however, an optimization that I know of which
would turn this into straight linear behaviour.
I gave a basic sketch of the idea at Re (tilly) 1: Research ideas, but to
the best of my knowledge nobody has ever implemented it
in any real RE engine...
