AREs in substring(from)

As the documentation leads me to expect, my Postgresql 7.4
installation produces:

   select substring('Bar, Foo' FROM '.')  ->  'B'

but even though my regex_flavor = advanced,

   select substring('Bar, Foo' FROM '\\w')  ->  NULL
   select substring('Bar, Foo' FROM '***:\\w')  ->  NULL

whereas I expect these to produce 'B' as well.  What am I missing?

--
Brandon Craig Rhodes                         http://www.rhodesmill.org/brandon
Georgia Tech                                            brandon@oit.gatech.edu