The pattern returned by "get_preg_expression" for the "url" mode does not allow for any non-ascii characters. An example URL is http://en.wikipedia.org/wiki/UK—US_extradition_treaty_of_2003.

This tag fails because of this:

[url=http://en.wikipedia.org/wiki/UK—US_extradition_treaty_of_2003]UK-US extradition treaty of 2003[/url]