글로만 설명하는 것은 의미가 모호합니다.
원하는 결과를 예를 들어 주세요.
위 예시 자료로 부터 원하는 결과가 어떤 값인지?
다른 예시도 다양하게 들어 주세요.

최종적으로 원하는 값은

SELECT '(12345(678))' AA FROM DUAL 의 결과값은

() 안에 ()가 있는 경우 ()안에 있는 ()를 없애고 싶습니다.

결과값 => (12345678) 입니다.

SELECT '(12345(678))' AA FROM DUAL  => (12345678)

SELECT 'abcde(12345)(678)' AA FROM DUAL  => abcde(12345)(678)  : ()안에 ()가 없으므로 그대로 표기

SELECT 'wrrt(12678(123)1234567)' AA FROM DUAL  => wrrt(126781231234567)

SELECT '(12345678012345)' AA FROM DUAL  => (12345678012345)

WITH t AS
(
SELECT '(12345(678))' TXT FROM DUAL UNION ALL
SELECT 'abcde(12345)(678)' FROM DUAl UNION ALL
SELECT 'wrrt(12678(123)1234567)' FROM DUAL UNION ALL
SELECT '(12345678012345)' FROM DUAL 
)
SELECT REGEXP_REPLACE(TXT,'(\([^()]*)(\()([^()]+)(\))([^()]*)(\))','\1\3\5\6') TXT
FROM T

잘 되네요..답변 너무나 감사합니다.

괄호 안의 괄호를 모두 없애야 하는 거네요.
괄호는 2개 뿐인가요? 괄호가 3개 이상 나오는 경우는 없는지?

WITH T AS
(
SELECT '(12345(678))' TXT FROM DUAL UNION ALL
SELECT 'abcde(12345)(6(7)8)' FROM DUAl UNION ALL
SELECT 'wrrt(12678(123)1234567)' FROM DUAL UNION ALL
SELECT 'wrrt(12678(123)123(4)567)' FROM DUAL UNION ALL
SELECT '((1234)5678012345)' FROM DUAL UNION
SELECT '(abc(abc(abc(abc)(a(b)c))abc)abc)' FROM DUAL UNION ALL
SELECT '(abc(abc)abc(abc)abc(abc)abc)' FROM DUAL UNION ALL
SELECT '(abc(abc(abc)abc)abc(abc)abc(abc)abc)' FROM DUAL UNION ALL
select '0(12(34))(56(7(8)01(23))45)' FROM DUAL
)
, T2(ORGTXT, BEFTXT, TXT) AS
(
SELECT TXT, '*', TXT
  FROM T
 UNION ALL
SELECT T2.ORGTXT, T2.TXT, REGEXP_REPLACE(TXT,'(\(|\))([^()]*)\(([^()]+)\)([^()]*)(\(|\))','\1\2\3\4\5') TXT
  FROM T2
 WHERE T2.BEFTXT != T2.TXT
)
SELECT ORGTXT, TXT
  FROM T2
 WHERE BEFTXT = T2.TXT 

WITH t AS
(
SELECT 1 pk, '0(12(34))(56(7(8)01(23))45)' v FROM dual --> 0(1234)(5678012345)
)
SELECT pk, v
     , LISTAGG(z) WITHIN GROUP(ORDER BY lv) x
  FROM (SELECT pk, v
             , lv
             , x, y
             , CASE WHEN x = '(' AND y != 1 THEN ''
                    WHEN x = ')' AND y != 0 THEN ''
                    ELSE x END z
          FROM (SELECT pk, v
                     , lv
                     , SUBSTR(v, lv, 1) x
                     , SUM(DECODE(SUBSTR(v, lv, 1), '(', 1, ')', -1, 0))
                       OVER(PARTITION BY pk ORDER BY lv) y
                  FROM t
                     , (SELECT LEVEL lv FROM dual CONNECT BY LEVEL <= 99)
                 WHERE lv <= LENGTH(v)
                )
        )
 GROUP BY pk, v
;