Question

Here is the table data with the column name as Ships.

+--------------+
Ships          |
+--------------+
Duke of north  |
---------------+
Prince of Wales|
---------------+
Baltic         |
---------------+

Replace all characters between the first and the last spaces (excluding these spaces) by symbols
of an asterisk (*). The number of asterisks must be equal to number of replaced characters.

Answer 1

Regular expressions are your friend 🙂

First match the space, followed by any other characters, ending in a space.
Then replace that with a string that consists of the starting and trailing space and, in between, a string of asterisks.

The string of asterisks is made by right padding a single asterisk with further asterisks to the appropriate length. That length is the length of the regular expression matched minus two characters for the leading/trailing space.

select regexp_replace(column_value,' .* ', 
          ' '||rpad('*',length(regexp_substr(column_value,' .* '))-2,'*')||' ')
from table(sys.dbms_debug_vc2coll(
       'Duke of north','Prince of Wales','Baltic','what if two spaces'));

Duke ** north
Prince ** Wales
Baltic
what ****** spaces