WITH test (expr) AS (
  /*incorrect*/
  SELECT '((((a)g)q)' FROM dual UNION ALL
  SELECT 'z)(s)(' FROM dual UNION ALL
  SELECT '(((f)e)w))(h(g(w))' FROM dual UNION ALL
  SELECT ')(()' FROM dual UNION ALL
  SELECT '()))' FROM dual UNION ALL
  SELECT '(()()(()' FROM dual UNION ALL
  /*correct*/
  SELECT '(()((())()))' FROM dual UNION ALL
  SELECT '((t)!!er(q(qer()g)ffd())///)' FROM dual UNION ALL
  SELECT '(5 + x) * (y + 1) <= 10^(z + 2)' FROM dual UNION ALL
  SELECT '(((())))' FROM dual UNION ALL
  SELECT 'ффффвфывьав' FROM dual UNION ALL
  SELECT '(()()()())' FROM dual
)
SELECT expr, (SELECT MIN(INSTR(expr, ')', 1, level) - INSTR(expr, '(', 1, level))
     FROM dual
     CONNECT BY INSTR(expr, ')', 1, level) != 0
             OR INSTR(expr, '(', 1, level) != 0)
FROM test
WHERE
    /* Количество открывающих равно количеству закрывающих */
    (SELECT COUNT(SUBSTR(expr, level, 1))
     FROM dual
     WHERE SUBSTR(expr, level, 1) = '('
     CONNECT BY level <= LENGTH(expr))
    =
    (SELECT COUNT(SUBSTR(expr, level, 1))
     FROM dual
     WHERE SUBSTR(expr, level, 1) = ')'
     CONNECT BY  level <= LENGTH(expr))
  /* Level-тая закрывающая скобка всегда правее level-той открывающей */
  AND
    (SELECT MIN(INSTR(expr, ')', 1, level) - INSTR(expr, '(', 1, level))
     FROM dual
     CONNECT BY INSTR(expr, ')', 1, level) != 0
             OR INSTR(expr, '(', 1, level) != 0)
    >= 0;