Here is an idea. @URL as it stands already has regexp attribute which allows to optionally refine the URL. The default value is '.*'. I am thinking we could add a flag along the lines regexpOnly. If set to true we only use regexp validation. We then also offer a string constant RFC_3987_REGEXP which basically contains this monster regexp. This way to use pure regexp validation you need to configure a bit as a constraint user, but on the other hand we keep backwards compatibility and build on the existing regexp attribute. As another nice side effect one can always use a different regexp in case a user finds a problem with the one we offer. WDYT?