SQL> with
  2    data as (
  3      select 'abc543jhk' val from dual
  4      union all
  5      select '123' val from dual
  6    )
  7  select val
  8  from data
  9  where translate(val, 'A0123456789', 'A') is null
 10  /
VAL
---------
123

WITH testascii AS
(
  SELECT 'aaaaaa'   AS colval FROM DUAL UNION
  SELECT 'aaa111'   AS colval FROM DUAL UNION
  SELECT '225'      AS colval FROM DUAL UNION
  SELECT 'B'        AS colval FROM DUAL UNION
  SELECT 'a1b2ffgs' AS colval FROM DUAL UNION
  SELECT '17'       AS colval FROM DUAL UNION
  SELECT '98710'    AS colval FROM DUAL
)
SELECT colval
FROM testascii t1
WHERE NOT EXISTS 
(
  SELECT t2.colval
  FROM testascii t2
  WHERE (t1.colval = t2.colval) AND
  EXISTS
    (
      SELECT ASCII(SUBSTR(t3.colval, LEVEL, 1))
      FROM testascii t3
      WHERE (t2.colval = t3.colval) AND
            (ASCII(SUBSTR(t3.colval, LEVEL, 1)) NOT BETWEEN 48 AND 57)
      CONNECT BY LEVEL <= LENGTH(t3.colval)
    )
);



COLVAL
--------
225
17
98710

SQL> 

SQL> WITH testascii AS
  2  (
  3    SELECT 'aaaaaa'   AS colval FROM DUAL UNION
  4    SELECT 'aaa111'   AS colval FROM DUAL UNION
  5    SELECT '225'      AS colval FROM DUAL UNION
  6    SELECT 'B'        AS colval FROM DUAL UNION
  7    SELECT 'a1b2ffgs' AS colval FROM DUAL UNION
  8    SELECT '17'       AS colval FROM DUAL UNION
  9    SELECT '98710'    AS colval FROM DUAL
 10  )
 11  SELECT colval
 12  FROM testascii 
 13  where translate(colval, 'A0123456789', 'A') is null
 14  /
COLVAL
--------
17
225
98710

SQL> WITH testascii AS
  2  (
  3    SELECT 'aaaaaa'   AS colval FROM DUAL UNION
  4    SELECT 'aaa111'   AS colval FROM DUAL UNION
  5    SELECT '225'      AS colval FROM DUAL UNION
  6    SELECT 'B'        AS colval FROM DUAL UNION
  7    SELECT 'a1b2ffgs' AS colval FROM DUAL UNION
  8    SELECT '17'       AS colval FROM DUAL UNION
  9    SELECT '98710'    AS colval FROM DUAL
 10  )
 11  select colval
 12  from testascii
 13  where regexp_like(colval, '^\d+$');

COLVAL
--------
17
225
98710

SQL>