Use connect by by in REGEXP_SUBSTR without breaking result to multiple rows

SELECT CHR(91)||'a-zA-Z0-9._%-'||CHR(93)||'+'|| listagg(REGEXP_SUBSTR('aaa@yahoo.com, bbb@hotmail.com', '@'||CHR(91)||'^,'||CHR(93)||'+', 1, LEVEL), ', ') within group (order by level) as domain
FROM DUAL
CONNECT BY REGEXP_SUBSTR('aaa@yahoo.com, bbb@hotmail.com','@'||CHR(91)||'^,'||CHR(93)||'+', 1, LEVEL) IS NOT NULL
order by 1;

Above script only has the regular expression in front of @yahoo.com

 [a-zA-Z0-9._%-]+@yahoo.com, @hotmail.com

Expected result:

[a-zA-Z0-9._%-]+@yahoo.com, [a-zA-Z0-9._%-]+@hotmail.com

Sure; aggregate them back.

SQL> SELECT listagg(REGEXP_SUBSTR('aaa@yahoo.com, bbb@hotmail.com', '@'||CHR(91)||'^,'||CHR(93)||'+', 1, LEVEL), ', ') within group (order by level) as domain
  2  FROM DUAL
  3  CONNECT BY REGEXP_SUBSTR('aaa@yahoo.com, bbb@hotmail.com','@'||CHR(91)||'^,'||CHR(93)||'+', 1, LEVEL) IS NOT NULL
  4  order by 1;

DOMAIN
----------------------------------------------------------------------------------------------------
@yahoo.com, @hotmail.com

SQL>

If you want to put the regexp prefix to all domains, then

SQL>     SELECT LISTAGG (   '[a-zA-Z0-9._%-]+'
  2                      || REGEXP_SUBSTR ('aaa@yahoo.com, bbb@hotmail.com',
  3                                        '@' || CHR (91) || '^,' || CHR (93) || '+',
  4                                        1,
  5                                        LEVEL),
  6                      ', ')
  7             WITHIN GROUP (ORDER BY LEVEL) AS domain
  8        FROM DUAL
  9  CONNECT BY REGEXP_SUBSTR ('aaa@yahoo.com, bbb@hotmail.com',
 10                            '@' || CHR (91) || '^,' || CHR (93) || '+',
 11                            1,
 12                            LEVEL)
 13                IS NOT NULL;

DOMAIN
-----------------------------------------------------------------------------------------------
[a-zA-Z0-9._%-]+@yahoo.com, [a-zA-Z0-9._%-]+@hotmail.com

SQL>