Small correction: the RE2 C++ library does have a backtracker, but it doesn't enable any extra expressive power in the regex syntax. It is purely an optimizatio

Small correction: the RE2 C++ library does have a backtracker, but it doesn't enable any extra expressive power in the regex syntax. It is purely an optimization. It uses memory to guarantee linear time search (assuming the size of the regex is held as a constant).