regexp_replace puzzle - Mailing list pgsql-general

I've got a problem with regexp_replace which I could reduce to the following:

  CREATE FUNCTION digest(text, text) RETURNS bytea
      LANGUAGE c IMMUTABLE STRICT
      AS '$libdir/pgcrypto', 'pg_digest';

  CREATE FUNCTION sha224enc(text) RETURNS text AS $$
  BEGIN
    RAISE WARNING 'arg=»%«', $1;
    RETURN encode(digest($1, 'sha224'), 'hex');
  END;
  $$ LANGUAGE plpgsql IMMUTABLE;

  CREATE TABLE t1 (
    id serial NOT NULL,
    val text NOT NULL,
    PRIMARY KEY (id)
  );

  INSERT INTO t1 (val) VALUES ('d111111');
  INSERT INTO t1 (val) VALUES ('xd222222');
  INSERT INTO t1 (val) VALUES ('x d333333');

  SELECT val,
         regexp_replace(val,
          '^(.*\W)?(C\d{7}|[DI]\d{6}|S\d{10})(\W.*)?$',
          '\1' || '»\2«='|| sha224enc('\2') || '\3', 'i')
  FROM t1
  WHERE val ~*
          '^(.*\W)?(C\d{7}|[DI]\d{6}|S\d{10})(\W.*)?$';

(I want to replace patterns within a string by their SHA-224 hash.)
However, when I run this example I get:

  WARNING:  arg=»\2«
      val    |                            regexp_replace
  -----------+----------------------------------------------------------------------
   d111111   | »d111111«=da2c99be8170ce02b04ee7d90877ae9d86fea467abb973c74c708b27
   x d333333 | x »d333333«=da2c99be8170ce02b04ee7d90877ae9d86fea467abb973c74c708b27
  (2 rows)

i.e. the first '\2' gets properly expanded by the second paren match,
but the second '\2' doesn't get expanded.

What am I overlooking?