CREATE TABLE test_val
(  sname varchar2(1000)
)

INSERT INTO test_val values('alksjhdlfkjh12345 secure alkjshdf 12345 lakjhdf 1234567');


INSERT INTO test_val values('jkalhsdlkjhf12345lakjhsdlfjh89076');

INSERT INTO test_val values('12345jkashdlkjhf678909876');

commit;

should be converted into:

'alksjhdlfkjh12345 secure alkjshdf 12345 lakjhdf 1234567' -> 'alksjhdlfkjh***** secure alkjshdf ***** lakjhdf 1234567'

'jkalhsdlkjhf12345lakjhsdlfjh89076' -> 'jkalhsdlkjhf*****lakjhsdlfjh*****'

'12345jkashdlkjhf678909876' -> '*****jkashdlkjhf678909876'

select  sname,
        regexp_replace(sname,'([[:digit:]]{5})','#####')       
from    test_val

WITH test(val) AS
 (SELECT 'alksjhdlfkjh12345 secure alkjshdf 12345 lakjhdf 1234567' FROM dual UNION ALL
  SELECT 'jkalhsdlkjhf12345lakjhsdlfjh89076' FROM dual UNION ALL
  SELECT '12345jkashdlkjhf678909876' FROM dual)
SELECT regexp_replace(val,'(^|[^[:digit:]])([[:digit:]]{5})([^[:digit:]]|$)','\1*****\3') repval 
 FROM test;

repval
-------------------------------------------------------------
alksjhdlfkjh***** secure alkjshdf ***** lakjhdf 1234567
jkalhsdlkjhf*****lakjhsdlfjh*****
*****jkashdlkjhf678909876