How to split string words with regexp_substr in Oracle SQL?

select  
regexp_substr('a-b--->d--->e f','[^--->]+',1,1) col1
,regexp_substr('a-b--->d--->e f','[^--->]+',1,2) col2
,regexp_substr('a-b--->d--->e f','[^--->]+',1,3) col3
,regexp_substr('a-b--->d--->e f','[^--->]+',1,4) col4
 from dual

output

Required output

You can use:

select regexp_substr(value,'(.*?)(-+>|$)',1,1, NULL, 1) AS col1
,      regexp_substr(value,'(.*?)(-+>|$)',1,2, NULL, 1) AS col2
,      regexp_substr(value,'(.*?)(-+>|$)',1,3, NULL, 1) AS col3
,      regexp_substr(value,'(.*?)(-+>|$)',1,4, NULL, 1) AS col4
 from  table_name

or, if you are looking for the fixed delimiter string ---> then you can use simple string functions (which are more to type but are likely going to be much faster):

SELECT CASE
       WHEN pos1 = 0 THEN value
                     ELSE SUBSTR(value, 1, pos1 - 1)
       END AS col1,
       CASE
       WHEN pos1 = 0 THEN NULL
       WHEN pos2 = 0 THEN SUBSTR(value, pos1 + 4)
                     ELSE SUBSTR(value, pos1 + 4, pos2 - pos1 - 4)
       END AS col2,
       CASE
       WHEN pos2 = 0 THEN NULL
       WHEN pos3 = 0 THEN SUBSTR(value, pos2 + 4)
                     ELSE SUBSTR(value, pos3 + 4, pos3 - pos2 - 4)
       END AS col3,
       CASE
       WHEN pos3 = 0 THEN NULL
                     ELSE SUBSTR(value, pos3 + 4)
       END AS col4
FROM   (
  SELECT value,
         INSTR(value, '--->', 1, 1) AS pos1,
         INSTR(value, '--->', 1, 2) AS pos2,
         INSTR(value, '--->', 1, 3) AS pos3
  FROM   table_name
)

Which, for the sample data:

CREATE TABLE table_name (value) AS
SELECT 'a-b--->d--->e f' FROM DUAL;

Both output:

fiddle