Thread: BUG #17407: trim trims more than expected

The following bug has been logged on the website:

Bug reference:      17407
Logged by:          trim combined with substring from pattern for gives inconsistent output
Email address:      unmesh.joshi126@gmail.com
PostgreSQL version: 12.4
Operating system:   Linux 5.15.4-051504-generic
Description:

Expected output

demo=# select trim ('nextval(''' from
substring('nextval(''database_table_id_seq''::regclass)' from
'%#"%_id_seq#"%' for '#' ));
         btrim         
-----------------------
 database_table_id_seq
(1 row)


Unexpected output

demo=# select trim ('nextval(''' from
substring('nextval(''auth_table_id_seq''::regclass)' from '%#"%_id_seq#"%'
for '#' ));
      btrim       
------------------
 uth_table_id_seq
(1 row)

On 2022-Feb-16, PG Bug reporting form wrote:

> Unexpected output
> 
> demo=# select trim ('nextval(''' from
> substring('nextval(''auth_table_id_seq''::regclass)' from '%#"%_id_seq#"%'
> for '#' ));
>       btrim       
> ------------------
>  uth_table_id_seq

Not a bug.  trim() receives a *set* of characters to remove, not a
*substring* to remove, so the 'a' in 'auth' is matched to the 'a' in
'nextval(' and gets removed.

You can probably achieve what you want by combining substring() with
position() and char_length(), or by using a regular expression in
regexp_replace().

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"La conclusión que podemos sacar de esos estudios es que
no podemos sacar ninguna conclusión de ellos" (Tanenbaum)