Using CTE in Oracle

Question

How it is possible to make it in Oracle?
SQL Server:
WITH cte (col1, col2) AS
SELECT col1, col2
FROM dbo.tb1
WHERE col1 = 12
UNION ALL
SELECT c.col1, c.col2
FROM dbo.tb1 AS c INNER JOIN cte AS p ON c.col2 = p.col1
where c.col1 = c.col2
SELECT * FROM cte
It is probably necessary to use START WITH ... CONNECT BY PRIOR?
I tried to make so:
WITH cte AS(SELECT col1 col1, col2 col2
FROM tb12
WHERE col1 = 12
UNION ALL
SELECT c.col1 col1, c.col2 col2
FROM tb12 c INNER JOIN cte p ON c.col2 = p.col1
where c.col1 = c.col2)
SELECT * FROM cte
order by lvl NULLS FIRST;
Result:
ORA-32031: illegal reference of a query name in WITH clause

Answer 1

Boy, I'm in a strange mood today ;)
SQL> select * from t;
      COL1        COL2 VAL
         1             root
         2           1 user1
         3           1 user2
         4           1 user3
         5           2 user4
         6           2 user5
         7           3 user6
         8           3 user7
         9           4 user8
        10           5 user9
        11           5 user10
        12           6 user11
        13           6 user12
        14           7 user14
        15           7 user15
15 rows selected.
Elapsed: 00:00:00.15
SQL> with t2 as (
  2  select col1
  3  ,      to_number(substr(reverse(ltrim(sys_connect_by_path(col2, '/'), '/'))
  4                         , 1
  5                         , instr(reverse(ltrim(sys_connect_by_path(col2, '/'), '/')), '/')-1
  6                         )
  7                  ) col2
  8  from   t
  9  start with col2 is null
10  connect by col2 = prior col1
11  )
12  select *
13  from   t2
14  where  col1 = 12;
      COL1        COL2
        12           6
1 row selected.
Elapsed: 00:00:00.04
SQL> column col2 format a10
SQL> select col1
  2  ,      ltrim(sys_connect_by_path(col2, '/'), '/') col2
  3  from   t
  4  start with col2 is null
  5  connect by col2 = prior col1
  6  order by col1;
      COL1 COL2
         1
         2 1
         3 1
         4 1
         5 1/2
         6 1/2
         7 1/3
         8 1/3
         9 1/4
        10 1/2/5
        11 1/2/5
        12 1/2/6
        13 1/2/6
        14 1/3/7
        15 1/3/7
15 rows selected.