with qry as (select '[PSXXPPPPPPPPPSPSSSXPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPXXXXXXXXXXXXXXX]' str from dual)
select listagg(lvl, ',') within group(order by lvl) answ
from (select substr(str, level, 1) val,
             level lvl
      from qry
      connect by level <= length(str))
where val = 'X'

SQL> with
  2    data as (
  3      select '[PSXXPPPPPPPPPSPSSSXPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPXXXXXXXXXXXXXXX]' val
  4      from dual
  5    )
  6  select instr(val, 'X', 1, level) idx
  7  from data
  8  connect by level <= regexp_count(val, 'X')
  9  /
       IDX
----------
         4
         5
        20
        52
        53
        54
        55
        56
        57
        58
        59
        60
        61
        62
        63
        64
        65
        66

18 rows selected.

with qry as (select '[PSXXPPPPPPPPPSPSSSXPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPXXXXXXXXXXXXXXX]' str from dual)
select  xmlcast(
                xmlquery(
                         'fn:string-join(
                                         for $e at $i in string-to-codepoints(.)
                                           where codepoints-to-string($e) = "X"
                                           return xs:string($i),
                                         ","
                                        )'
                         passing str
                         returning content
                        )
                as varchar2(60)
               ) x_positions
  from  qry
/

X_POSITIONS
---------------------------------------------------
4,5,20,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66

SQL> 

with qry as (select '[PSXXPPPPPPPPPSPSSSXPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPXXXXXXXXXXXXXXX]' str from dual)
select  column_value x_positions
  from  qry,
        xmltable(
                 'fn:string-join(
                                 for $e at $i in string-to-codepoints(.)
                                   where codepoints-to-string($e) = "X"
                                   return xs:string($i),
                                 ","
                                )'
                         passing str
                )
/

X_POSITIONS
---------------------------------------------------
4,5,20,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66

SQL> 

with qry as (select '[PSXXPPPPPPPPPSPSSSXPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPXXXXXXXXXXXXXXX]' str from dual)
select  column_value x_positions
  from  qry,
        xmltable(
                 'for $e at $i in string-to-codepoints(.)
                    where codepoints-to-string($e) = "X"
                    return $i'
                 passing str
                )
/

X_POSITIONS
------------------
4
5
20
52
53
54
55
56
57
58
59

X_POSITIONS
------------------
60
61
62
63
64
65
66

18 rows selected.

SQL>