[Users] Using subst() of Textops module

[Leonid Fainshtein]

Hi,
In the textops module doc
(http://openser.org/docs/modules/1.0.x/textops.html#AEN124) there is an
example that demonstrates how to replace URI in the header "To" with the
message URI:


# replace the uri in to: with the message uri (just an example) if (
subst('/^To:(.*)sip:[^@]*@[a-zA-Z0-9.]+(.*)$/t:\1\u\2/ig') ) {};


After I've fixed a small mistake (instead of /t: I put /To:) it worked.
But I don't understand how does it work. It looks like "u" is
substituted by URI from R-URI but  I didn't find such core variable.
Could somebody explain me?

Thank you in advance,
Leonid Fainshtein