updated for version 7.4.497

Problem:    With some regexp patterns the NFA engine uses many states and
            becomes very slow.  To the user it looks like Vim freezes.
Solution:   When the number of states reaches a limit fall back to the old
            engine. (Christian Brabandt)

runtime/doc/options.txt

@@ -5626,6 +5626,10 @@ A jump table for the options with a short description can be found at |Q_op|.
 	Note that when using the NFA engine and the pattern contains something
 	that is not supported the pattern will not match.  This is only useful
 	for debugging the regexp engine.
+	Using automatic selection enables Vim to switch the engine, if the
+	default engine becomes too costly.  E.g., when the NFA engine uses too
+	many states.  This should prevent Vim from hanging on a combination of
+	a complex pattern with long text.
 
 		*'relativenumber'* *'rnu'* *'norelativenumber'* *'nornu'*
 'relativenumber' 'rnu'	boolean	(default off)