[5.6] Fix validation of wildcard rules with slashes

[staudenmeir]

#16309 fixed the validation of "normal" rules with slashes.

This is still an issue with wildcard rules:

$validator = Validator::make(['foo/' => ['bar']], ['foo/.*' => 'required']);
$validator->passes();

This throws an exception:

preg_match(): Unknown modifier '\'

Fixes #25146.

[taylorotwell]

Don't use inputs with slashes.