Thread: regexp strangeness

I'm using PostgreSQL 6.4.2 regexps, and they don't seem to work the way
I'm used to (Perl).

Here's the records in a simple table:

select * from t5 where name ~ 'bar';
name
--------
bar
abar
xbar
not abar
(4 rows)

I'd like to match the ones that begin with 'bar' or 'abar'.

select * from t5 where name ~ '^a?bar';
name
----
abar
(1 row)

select * from t5 where name ~ '^bar|^abar';
name
----
(0 rows)

Neither of these works.  I looked at
pgsql/src/backend/regex/re_format.7, and it claims that ^, ?, and | all
work as expected.  The type of name is varchar(30), in case that is
relevant.

Am I misunderstanding PostgreSQL's use of regexps, or is this a bug
others have seen?

Thanks for your help.

Andrew Merrill

This is a question I've seen a few times, and
had to research, so I figured I'd share the
answer.


-------------------------------------------------


drop table test;
--
create table test ( a text, b text );
--  unique values
insert into test values ( 'x', 'y');
insert into test values ( 'x', 'x');
insert into test values ( 'y', 'y' );
insert into test values ( 'y', 'x' );
-- duplicate values
insert into test values ( 'x', 'y');
insert into test values ( 'x', 'x');
insert into test values ( 'y', 'y' );
insert into test values ( 'y', 'x' );
-- one more double duplicate
insert into test values ( 'x', 'y');
--
select oid, a, b from test;
--
-- select duplicate rows
--
select o.oid, o.a, o.b from test owhere exists ( select 'x'                  from test i                where i.a = o.a
                and i.b = o.b                  and i.oid < o.oid            );
 
--
-- delete duplicate rows
--
-- Note: PostgreSQL dosn't support aliases on
--       the table mentioned in the from clause
--       of a delete.
--
delete from test where exists ( select 'x'                  from test i                where i.a = test.a
  and i.b = test.b                  and i.oid < test.oid            );
 
--
-- Let's see if it worked.
--

select oid, a, b from test;

--
-- Delete duplicates with respect to a only, ignoring
-- the value in b.  Note, the first deletion leaves the 
-- first oid with the unique values and removes subsequent
-- ones, in this delete we reverse the direction of the <
-- to save the last oid, and remove the previous ones.
--

delete from test where exists ( select 'x'                  from test i                where i.a = test.a
  and i.oid > test.oid            );
 

--
-- Let's see if it worked.
--

select oid, a, b from test;