BUG #1687: Regular expression problem (II)

The following bug has been logged online:

Bug reference:      1687
Logged by:          Halley Pacheco de Oliveira
Email address:      halleypo@yahoo.com.br
PostgreSQL version: 7.4 and 8.0
Operating system:   Linux and Windows
Description:        Regular expression problem (II)
Details: 

Maybe it would be easier to see the the problem I'm having with regular
expressions this way:

SELECT '192.168.0.15' SIMILAR TO
'([[:alnum:]_-]+).([[:alnum:]_-]+).([[:alnum:]_]+)';
 ?column?
----------
 t

SELECT '192.168.0.15' SIMILAR TO '([\\w-]+).([\\w-]+).([\\w]+)';
 ?column?
----------
 f

SELECT '192.168.0.15' ~
'^([[:alnum:]_-]+)\\.([[:alnum:]_-]+)\\.([[:alnum:]_]+)$';
 ?column?
----------
 f

SELECT '192.168.0.15' ~ '^(([[:alnum:]_-]+)\\.){2}([[:alnum:]_]+)$';
 ?column?
----------
 f

SELECT '192.168.0.15' ~ '^([\\w-]+)\\.([\\w-]+)\\.([\\w]+)$';
 ?column?
----------
 f

SELECT '192.168.0.15' ~ '^(([\\w-]+)\\.){2}([\\w]+)$';
 ?column?
----------
 f

Why does the first query gives a different output? It is not exactly the
same as the second query and similar to the others?