1. Two-character string substitutions can begin with ^ as well as $. If ^ is used, and the substitution is a URL component, the component will be the one in the pristine, rather than the remapped, URL. So, for example, $h is the host in the remapped URL, and ^h is the host in the pristine URL. (^ seems intuitive as "before URL" with $ as "after URL", because ^ means start of line and $ means end of line in regular expressions.)
2. Add the string substitutions $$ (translating to $) and ^^ (translating to ^).