In Oracle, how to replace surplus part of string by a dot?

My requirement is following simple handling of strings using Oracle 23c:

• If the string is NULL or has a length <= 5, it should be returned as it is.
• Otherwise, the first 4 characters followed by a dot should be returned, the rest should be removed.

So for following sample input data...

CREATE TABLE test (col VARCHAR2(100));
INSERT INTO test VALUES 
  ('hello!'), ('world'), ('worl.'), ('x'), ('long text'), ('alsotoolong'), (NULL);

...following result must be returned:

I was able to get the expected outcome with following query:

SELECT
  CASE WHEN LENGTH(col) > 5 
    THEN SUBSTR(col,1,4) || '.' 
    ELSE col END AS col
FROM test;

What I just wonder is if we really need a CASE expression here to check the length. I guess there should be a more trivial way which is shorter and better to read. A solution that just replaces the rest after the fourth character by a dot in one step.

Maybe you have an idea?

I searched here on Stackoverflow and also generally in the net, but couldn't find something related to this use case, so sorry in case there is a duplicate question here which I haven't spotted or in case there is no simpler option.

Anyway, you can use this sample fiddle to try out possible solutions.

This can be done with a regex replace function call, by matching strings of length 6 or more and replacing it with the first 4 characters plus a dot.

SELECT REGEXP_REPLACE(col, '(.{4}).{2,}', '\1.') AS col
FROM test;

Fiddle

This will probably be slower than a case + substring statement though.