CREATE DOMAIN base.url
  AS character varying
   CONSTRAINT url_check CHECK (base."isUrl"((VALUE)::text));

CREATE OR REPLACE FUNCTION base."isUrl"(url text)
  RETURNS boolean AS
$BODY$
import re
return re.match('http[s]?://(?:[a-zA-Z]|[0-9]|[$-_@.&+]|[!*\(\),]|(?:%[0-9a-fA-F][0-9a-fA-F]))+', url)
$BODY$
  LANGUAGE plpython2u VOLATILE
  COST 100;