Postgres 14

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select version();

version

PostgreSQL 14.10 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-20), 64-bit

SELECT 1

CREATE TABLE test
(
  id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  t_field TEXT
);

CREATE TABLE

INSERT INTO test (t_field)
VALUES ('bar$foo$john$doe$xxx');  -- OP's own data

INSERT 0 1

INSERT INTO test (t_field)
SELECT 
  LEFT(MD5(RANDOM()::TEXT), FLOOR(RANDOM() * (5 - 3 + 1) + 3)::INT) || '$' ||
  LEFT(MD5(RANDOM()::TEXT), FLOOR(RANDOM() * (5 - 3 + 1) + 3)::INT) || '$' ||
  LEFT(MD5(RANDOM()::TEXT), FLOOR(RANDOM() * (5 - 3 + 1) + 3)::INT) || '$' ||
  LEFT(MD5(RANDOM()::TEXT), FLOOR(RANDOM() * (5 - 3 + 1) + 3)::INT) || '$' ||
  LEFT(MD5(RANDOM()::TEXT), FLOOR(RANDOM() * (5 - 3 + 1) + 3)::INT)
FROM 
  GENERATE_SERIES(1, 299999);  --<<== Vary here
   
  --
  -- Vary the GENERATE_SERIES(1, 299999) number if the fiddle run fails!
  --
  -- 299999 (+ 1 - OP) = 300,000 = large enough number to get reliable, consistent results
  -- and small enough so that the fiddle run doesn't fail too often.
  -- 

INSERT 0 299999

ANALYZE test;

ANALYZE

--
--
--
--
-- 3: Evan Carroll's solution.
--
-- REGEXP_REPLACE - only method that doesn't use REVERSE(). Elegant code-wise!
--

SELECT
  t_field,
  REGEXP_REPLACE(t_field, '(.*)\$', '\1-' )
FROM test
LIMIT 2;

t_field
regexp_replace

bar$foo$john$doe$xxx
bar$foo$john$doe-xxx

c92$b1eb$bd9e6$fa7$d83
c92$b1eb$bd9e6$fa7-d83

SELECT 2

EXPLAIN (ANALYZE, TIMING OFF)
SELECT
  REGEXP_REPLACE(t_field, '(.*)\$', '\1-' )
FROM test;

QUERY PLAN

Seq Scan on test  (cost=0.00..5968.00 rows=300000 width=32) (actual rows=300000 loops=1)

Planning Time: 0.070 ms

Execution Time: 534.461 ms

EXPLAIN

t_field	regexp_replace
bar$foo$john$doe$xxx	bar$foo$john$doe-xxx
c92$b1eb$bd9e6$fa7$d83	c92$b1eb$bd9e6$fa7-d83