Learning tools like this don't really help you to learn much as you end up having to learn twice as much in the end. It is better to teach by presenting people with /x regexes, and examples of what they match and why.

That said, a regex language would allow for creation of regular expressions in multiple syntaxes. There already is a 'standard' for these though, so I don't know if it would be worth the extra effort to implement this.