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(at)oit(dot)gatech(dot)edu