[Finder] () is also a valid delimiter

The `Regex` class should also accept `()` as delimiters, as they are valid PREG delimiters: http://3v4l.org/D8v54

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -