BUG #1686: Regular expression problem

The following bug has been logged online:

Bug reference: 1686
Logged by: Halley Pacheco de Oliveira
Email address: halleypo(at)yahoo(dot)com(dot)br
PostgreSQL version: 7.4 and 8.0
Operating system: Linux and Windows
Description: Regular expression problem
Details:

SELECT '200.222.197.219' SIMILAR TO
'([[:alnum:]_]+).([[:alnum:]_]+).([[:alnum:]_]+)';

?column?
----------
t
(1 row)

SELECT '200.222.197.219' ~
'([[:alnum:]_]+)\\.([[:alnum:]_]+)\\.([[:alnum:]_]+)';

?column?
----------
t
(1 row)

SELECT '200.222.197.219' SIMILAR TO '([\\w-]+).([\\w-]+).([\\w]+)';

?column?
----------
f
(1 row)

SELECT '200.222.197.219' ~ '([\\w-]+)\\.([\\w-]+)\\.([\\w]+)';

?column?
----------
t
(1 row)

Why does the third query gives a different output? It is not exactly the
same as the first query?