SQL> select  *
  2    from  tbl
  3  /

STR
------------------------------
15+50/(10+10*(5+5))+(20+30)

with r(
       rid,
       str,
       in_pos,
       out_pos
      ) as (
             select  rowid rid,
                     regexp_replace(
                                    str,
                                    '[^()]',
                                    ' '
                                   ) str,
                     0,
                     0
               from  tbl
            union all
             select  rid,
                     regexp_replace(str,'\(( +)\)',' \1 ',1,1),
                     regexp_instr(str,'\( +\)'),
                     regexp_instr(str,'\( +\)',1,1,1) - 1
               from  r
               where regexp_instr(str,'\( +\)') != 0
           )
select  row_number() over(partition by rid order by in_pos) row_num,
        in_pos,
        out_pos
  from  r
  where in_pos > 0
  order by rid,
           in_pos
/

   ROW_NUM     IN_POS    OUT_POS
---------- ---------- ----------
         1          7         19
         2         14         18
         3         21         27

SQL>