PL/SQL - stored substring procedure - stored-procedures

Question

I am doing a job interview PL/SQL question, and I am really stuck. Could someone help?
Create stored procedure to return substrings from input string value. Substring delimiter shall be input parameter.
Task description
Stored procedure shall have input parameters:
• STRING
• DELIMITER //any symbol
• STRING_NUMBER //number of substring to be returned
Input example: STRING => ‘one,two,three’,
DELIMITER => ‘e’,
STRING_NUMBER => null
Output shall be: ‘on’
‘,two,thr’
‘’
‘’
If STRING_NUMBER => 2, output shall be: ‘,two,thr’
EDIT:
First, I am trying to trim a string using TRIM function, but that doesn't work. Why?
CREATE OR REPLACE PROCEDURE substring
(STRNG IN VARCHAR2,DELIMITER IN VARCHAR2)
IS
instring VARCHAR2(100);
BEGIN
instring:= TRIM(DELIMITER FROM STRNG);
DBMS_OUTPUT.PUT_LINE(instring);
END;
set serveroutput on
BEGIN
substring('marc','a');
END;
EDIT 2:
This does the part on the job:
CREATE OR REPLACE PROCEDURE substring
(STRNG IN VARCHAR2,DELIMITER IN VARCHAR2)
IS
instring VARCHAR2(100);
BEGIN
instring:= REPLACE(STRNG,DELIMITER);
DBMS_OUTPUT.PUT_LINE(instring);
END;
set serveroutput on
BEGIN
substring('marc','a');
END;

Answer 1

Here is my own solution:
CREATE OR REPLACE PROCEDURE substring
(STRNG IN VARCHAR2,DELIMITER IN VARCHAR2, STRING_NUMBER IN NUMBER)
IS
replace_string VARCHAR2(100);
instring NUMBER;
comma_counter NUMBER;
substring VARCHAR2(100);
BEGIN
replace_string:= REPLACE(STRNG,DELIMITER);
comma_counter:=REGEXP_COUNT(STRNG,',');
instring:=INSTR(STRNG,',',1,comma_counter-STRING_NUMBER+1);
substring:=SUBSTR(replace_string,instring);
DBMS_OUTPUT.PUT_LINE(replace_string);
DBMS_OUTPUT.PUT_LINE(','||substring);
END;
set serveroutput on
BEGIN
substring('one,two,three,four','e',2);
END;