Oracle SUBSTRING USING SPECIFIC CHARACTER

Just combine two function as follows.

Example:

SELECT SUBSTR('ABCD_EFG', 0, INSTR(TABLE_NAME, '_')-1) AS output
FROM dual

Result:
ABCD

Oracle - Date Conversion Sample

For those who always needs :

 SELECT TO_CHAR(TO_DATE('19981215','YYYYMMDD'),'YYYY' )  FROM DUAL  --Result  1998
   
 SELECT TO_CHAR(TO_DATE('19981215','YYYYMMDD'),'MM' )  FROM DUAL    --Result  12
   
 SELECT TO_CHAR(TO_DATE('19981215','YYYYMMDD'),'DD' )  FROM DUAL     --Result  15

Oracle - Trim Leading Zeros

*Assuming the data you are working on is CHARACTER :

TO_CHAR( TO_NUMBER( COLUMN_A ) )