If the optional ``asfilter`` parameter to ``re.regex()`` is true, the
vmod registers itself as a Varnish Fetch Processor (VFP) for use in
`beresp.filters` and as a Varnish Delivery Processor (VDP) for use in
`resp.filters`. In this setup, the `xregex.substitute_match()` and
`xregex.substitute_all()` methods can be used to define replacements
for matches on the body.

Example:

	    sub vcl_init {
		new reiher = re.regex("r(ei)h(er)", asfilter = true);
	    }
	    sub vcl_deliver {
		unset req.http.Accept-Encoding;
		set resp.filters += " reiher";
		reiher.substitute_match(1, "czapla");
		reiher.substitute_match(2, "\1\2");
		reiher.substitute_match(0, "heron");
	    }