Oracle 18c

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select * from V$VERSION;

BANNER
BANNER_FULL
BANNER_LEGACY
CON_ID

Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
Version 18.4.0.0.0
Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
0

create table t1 as 
    select trim (column_value) str 
    from xmlTable ('"123","abc","#abcdef/"')
3 rows affected

select 
    substr (str, 1, 1)||rpad ('*', length (str)-2, '*')||substr (str, -1, 1) res 
from t1

RES

1*3

a*c

#******/

select 
    regexp_replace (str, '.*(.$)', rpad ('*', length (str)-2, '*')||'\1', 2) res 
from t1

RES

1*3

a*c

#******/