- PCRS documentation for the subtle differences to Perl behaviour. Most
- notably, the non-standard option letter <literal>U</literal> is supported,
- which turns the default to ungreedy matching.
+ PCRS documentation for the subtle differences to Perl behaviour.
+</para>
+
+<para>
+ Most notably, the non-standard option letter <literal>U</literal> is supported,
+ which turns the default to ungreedy matching (add <literal>?</literal> to
+ quantifiers to turn them greedy again).
+</para>
+
+<para>
+ The non-standard option letter <literal>D</literal> (dynamic) allows
+ to use the variables $host, $origin (the IP address the request came from),
+ $path and $url. They will be replaced with the value they refer to before
+ the filter is executed.
+</para>
+
+<para>
+ Note that '$' is a bad choice for a delimiter in a dynamic filter as you
+ might end up with unintended variables if you use a variable name
+ directly after the delimiter. Variables will be resolved without
+ escaping anything, therefore you also have to be careful not to chose
+ delimiters that appear in the replacement text. For example '<' should
+ be save, while '?' will sooner or later cause conflicts with $url.
+</para>
+
+<para>
+ The non-standard option letter <literal>T</literal> (trivial) prevents
+ parsing for backreferences in the substitute. Use it if you want to include
+ text like '$&' in your substitute without quoting.