code: def is_url():
      
      # Stolen from Django
      regex = re.compile(
          r'^(?:http|ftp)s?://' # http:// or https://
          r'(?:(?:[A-Z0-9](?:[A-Z0-9-]{0,61}[A-Z0-9])?\.)+(?:[A-Z]{2,6}\.?|[A-Z0-9-]{2,}\.?)|' #domain...
          r'localhost|' #localhost...
          r'\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})' # ...or ip
          r'(?::\d+)?' # optional port
          r'(?:/?|[/?]\S+)$', re.IGNORECASE)
  
      def validate(value):
          if not regex.match(value):
              return e("{} is not a valid URL", value)
      return validate
docs: Validates that a fields value is a valid URL.