CREATE OR REPLACE FUNCTION stuff (
   char_expr1   IN   VARCHAR2
 , startpos     IN   NUMBER
 , strlen       IN   NUMBER
 , char_expr2   IN   VARCHAR2
)
   RETURN VARCHAR2 DETERMINISTIC
AS
   v_return   VARCHAR2 (4000);
BEGIN
   IF LEAST (NVL (startpos, 0), NVL (strlen, 0)) >= 0
   THEN
      v_return    :=
            SUBSTR (char_expr1, 1, startpos - 1)
         || char_expr2
         || SUBSTR (char_expr1, startpos + strlen, LENGTH (char_expr1));
   END IF;

   RETURN v_return;
END stuff;
/
sho err

col mystuff format A25

SELECT stuff('HelloWorld',6,0,' ') mystuff
     , 'Hello World'               expected
FROM   dual
/

SELECT stuff('HelloWorld',6,4,NULL) mystuff
     , 'Hellod'                     expected
FROM   dual
/

SELECT stuff('abcdef', 2, 3, 'ijklmn') mystuff
     , 'aijklmnef'                    expected
FROM dual
/

DROP FUNCTION stuff
/

SQL> @orafaq

Function created.

No errors.

MYSTUFF                   EXPECTED
------------------------- -----------
Hello World               Hello World


MYSTUFF                   EXPECT
------------------------- ------
Hellod                    Hellod


MYSTUFF                   EXPECTED
------------------------- ---------
aijklmnef                 aijklmnef


Function dropped.

SQL>