code: def _update_expression(self):
          '''Update internal expression.'''
          self._expression = re.compile(
              '^{0}(?P<index>(?P<padding>0*)\d+?){1}$'
              .format(re.escape(self.head), re.escape(self.tail))
          f _update_expression(self):
          '''Update internal expression.'''
          self._expression = re.compile(
              '^{0}(?P<index>(?P<padding>0*)\d+?){1}$'
              .format(re.escape(self.head), re.escape(self.tail))
          )
docs: Update internal expression.