Do not allow mixture of forward and backslashes in install path match

If this idea is correct. Just an idea review.

Also, I was not sure about the `\\\\` pattern part, in which environments does it match, and if its ok also like `\\install\\index.php` . If not, a `\\{1,2}` pattern would be fine I think. Well, initially I was thinking about to use only a `\\`, but this syntax was throwing errors in my local tests (I don't understand why), while on online testers it was fine.