Thread: 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?

"Halley Pacheco de Oliveira" <halleypo@yahoo.com.br> writes:
> Maybe it would be easier to see the the problem I'm having with regular
> expressions this way:

> 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

SIMILAR TO patterns are required to match the whole data string; so
the above fails because it only matches 3 digit groups not 4.  The
others all fail because you put explicit ^ and $ into them.

The reason the first one works is that you put _ into the pattern, which
means "match anything" in SIMILAR-TO land; so it gets translated to "."
to be fed to the regular regexp engine.  (Arguably that should not
happen inside square brackets, but similar_escape() isn't smart enough
to distinguish.)  And that makes it possible for one of the
[]-expressions to match two digit groups plus the intervening dot.

            regards, tom lane