code: def is_email():
      
  
      email = (
          ur'(?!^\.)'     # No dot at start
          ur'(?!.*\.@)'   # No dot before at sign
          ur'(?!.*@\.)'   # No dot after at sign
          ur'(?!.*\.$)'   # No dot at the end
          ur'(?!.*\.\.)'  # No double dots anywhere
          ur'^\S+'        # Starts with one or more non-whitespace characters
          ur'@'           # Contains an at sign
          ur'\S+$'        # Ends with one or more non-whitespace characters
      )
  
      regex = re.compile(email, re.IGNORECASE | re.UNICODE)
  
      def validate(value):
          if not regex.match(value):
              return e("{} is not a valid email address", value)
      return validate
docs: Validates that a fields value is a valid email address.