This selects the default regexp engine. |two-engines| The possible values are: 0 automatic selection 1 old engine 2 NFA engine 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.