The easy approach to split up an URL into its constitutent parts is to use URI. The module returns you an object that has convenient accessors for the protocol, host, port, path and query.

If GREP filtering in Privoxy uses (Perl) regular expressions, but does not allow you to load Perl modules, maybe you can use the regular expression from jsUri.js and do further matches appropriately.

If GREP filtering in Privoxy is not based on Perl and does not allow you to load modules and/or uses a regular expression syntax different from Perls, I'm not sure how it really relates to Perl.