Regex issue in Urlizer class

The problem is that the caret `^` is use several times in the same regex so the caret caracter isn't catch.

`preg_replace('/[^A-Z^a-z^0-9^\/]+/', '_', 'abc^def')` return `abc^def` instead of `abc_def`.
The good regex is `/[^A-Za-z0-9\/]+/`