BUG #2391: "Similar to" pattern matching does not operate as documented

The following bug has been logged online:

Bug reference: 2391
Logged by: Eric Noriega
Email address: noriega(at)gwu(dot)edu
PostgreSQL version: 7.0.5
Operating system: Linux Fedora core 4
Description: "Similar to" pattern matching does not operate as
documented
Details:

As far as I can tell, this may be a bug in how the pattern matches.

db=# select 'tab' similar to '(a|b)';
?column?
----------
f

db=# select 'tab' similar to 'a|b';
?column?
----------
t

The doc says: Like LIKE, the SIMILAR TO operator succeeds only if its
pattern matches the entire string; this is unlike common regular expression
practice, wherein the pattern may match any part of the string.

If the second case is invalid as an expression (not clear in the
docs:Parentheses may be used to group items into a single logical item),
then the statement should fail, or return false, not return true.

"Eric Noriega" <noriega(at)gwu(dot)edu> writes:
> db=# select 'tab' similar to 'a|b';
> ?column?
> ----------
> t

Yeah, this is a bug ... the cause can be seen by looking at the
underlying similar_escape() function, which converts a SIMILAR TO
pattern into a POSIX regex pattern:

regression=# select similar_escape('(a|b)', null);
similar_escape
----------------
^(a|b)$
(1 row)

regression=# select similar_escape('a|b', null);
similar_escape
----------------
^a|b$
(1 row)

regression=#

I believe that in the second case, ^ and $ bind more tightly than |
per POSIX rules. So we need to put parens around the pattern to
prevent that.

Thanks for the report!

regards, tom lane