1.3.5. subst_uri('/re/repl/flags')

   Runs the re substitution on the message uri (like subst but
   works only on the uri)

   Meaning of the parameters is as follows:

     * '/re/repl/flags' - sed like regular expression. flags can
       be a combination of i (case insensitive), g (global) or s
       (match newline don't treat it as end of line).

   Example 1-5. subst usage
...
# adds 3463 prefix to numeric uris, and save the original uri (\0 match)
# as a parameter: orig_uri (just an example)
if (subst_uri('/^sip:([0-9]+)@(.*)$/sip:3463\1@\2;orig_uri=\0/i')){$

...