def replace_tags(cls, raw_filter):
        """
        Searches for known tags in the given string and replaces them with the
        corresponding regular expression.

        *raw_filter* is an (optionnaly tagged) regular expression.

        Returns the regular expression with known tags replaces by the
        corresponding regular expression.
        """
        for k, v in iter(cls.known_tags.items()):
            raw_filter = raw_filter.replace(k, v)

        return raw_filter