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)