Filter the current expression.

def Filter(self, match=None, **_):
    """Filter the current expression."""
    arg = self.stack.pop(-1)

    # Filters can be specified as a comma separated list.
    for filter_name in match.group(1).split(","):
      filter_object = ConfigFilter.classes_by_name.get(filter_name)
      if filter_object is None:
        raise FilterError("Unknown filter function %r" % filter_name)

      if not filter_object.sensitive_arg:
        logging.debug("Applying filter %s for %s.", filter_name, arg)
      arg = filter_object().Filter(arg)
      precondition.AssertType(arg, Text)

    self.stack[-1] += arg