strflags = iflags
	}
	sub = out

	// Round 2: Factor out common simple prefixes,
	// just the first piece of each concatenation.
	// This will be good enough a lot of the time.
	//
	// Complex subexpressions (e.g. involving quantifiers)
	// are not safe to factor because that collapses their
	// distinct paths through the automaton, which affects
	// correctness in some cases.
	start = 0
	out = sub[:0]