Postgres 13

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drop table if exists some_table;
create table some_table(id int primary key, description text);
insert into some_table values
(123, 'Lorem ipsum dolor sit amet [i:123], consectetur adipiscing elit, 
sed do eiusmod tempor incididunt ut labore et [234] magna aliqua.');

DROP TABLE

CREATE TABLE

INSERT 0 1

create or replace function the_function(text, text)
returns text language sql as $$
	select format('=replacement of %s and %s=', $1, $2)
$$;

CREATE FUNCTION

select regexp_replace(
    description, 
    '\[([is]?:?)(\d+)\]', 
    the_function('\1', '\2'), 
    'gi' ) 
from some_table
where id = 123;

regexp_replace

Lorem ipsum dolor sit amet =replacement of i: and 123=, consectetur adipiscing elit, 
sed do eiusmod tempor incididunt ut labore et =replacement of  and 234= magna aliqua.

SELECT 1