code: def _split_scheme(expression):
          
          match = re.search(r'^([a-z]+):(.*)$', expression)
          if not match:
              scheme = 'plain'
              actual = expression
          else:
              scheme = match.group(1)
              actual = match.group(2)
  
          return scheme, actual
docs: Splits the scheme and actual expression
  
          :param str expression: The expression.
  
          :rtype: str