Word characters (\w) might include local characters like German Ä ö ü ß on a German webserver. Although these Umlaut characters should be, in theory, valid in email adresses (in my interpretation of RFC 822), I know from experience that their occurence in email addresses usually causes problems sooner or later. At least one German provider (T-Online) used to allow for those chars, but I would rather disallow and have the user enter an email address which is safe for international use.