Postgres 10

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CREATE OR REPLACE FUNCTION some_custom_function(prefix TEXT, number TEXT)
RETURNS TEXT AS $$
  SELECT concat((upper(prefix) || '='), CASE number
    WHEN '123' THEN '"one-two-three"'
    WHEN '234' THEN '"two-three-four"'
    ELSE CASE
      WHEN number IS NOT NULL THEN '#foo'
    END
  END)
$$ LANGUAGE SQL IMMUTABLE PARALLEL SAFE;
CREATE OR REPLACE FUNCTION my_replace(description TEXT)
RETURNS TEXT AS $$
  SELECT string_agg(concat(l, some_custom_function(u[1], u[2])), '')
  FROM ROWS FROM (
    regexp_split_to_table(description, '\[(?:([is]):)?(\d+)\]', 'i'),
    regexp_matches(description, '\[(?:([is]):)?(\d+)\]', 'gi')
  ) AS m(l, u)
$$ LANGUAGE SQL IMMUTABLE PARALLEL SAFE;
SELECT unnest, my_replace(unnest)
FROM unnest(array[$$Lorem ipsum dolor sit amet [123], consectetur adipiscing elit [42],
sed do eiusmod [i:123] tempor incididunt ut labore et [S:234] magna aliqua.$$])


unnest
my_replace


Lorem ipsum dolor sit amet [123], consectetur adipiscing elit [42],
sed do eiusmod [i:123] tempor incididunt ut labore et [S:234] magna aliqua.
Lorem ipsum dolor sit amet "one-two-three", consectetur adipiscing elit #foo,
sed do eiusmod I="one-two-three" tempor incididunt ut labore et S="two-three-four" magna aliqua.