Improved regex detection/delimiters

This regex detection detecs every valid regex. It's taken from the [symfony finder](https://github.com/symfony/Finder/blob/master/Expression/Regex.php#L64-71) component.

I also changed from using `/` to using `{}`, as that's the best delimiter when dealing with user input: As `{}` already are meta characters, people will escape it when they want to match `{` or `}` and all other characters are valid and will not break the regex (you don't have to escape the `/`, but it won't harm if you do)