Oracle 23c

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create table test (variable varchar2(50));
insert into test values ('XBR*jkhehj8f22*3012367134*CVHI*N* X*G*P');
1 rows affected

-- original
SELECT
  SUBSTR(
    t.variable, 
    INSTR(t.variable, '*', 1, 2) + 1, 
    INSTR(t.variable, '*', 1, 3) - INSTR(t.variable, '*', 1, 2)
  ) 
FROM test t;


SUBSTR(T.VARIABLE,INSTR(T.VARIABLE,'*',1,2)+1,INSTR(T.VARIABLE,'*',1,3)-INSTR(T.VARIABLE,'*',1,2))


3012367134*


-- with -1
SELECT
  SUBSTR(
    t.variable, 
    INSTR(t.variable, '*', 1, 2) + 1, 
    INSTR(t.variable, '*', 1, 3) - INSTR(t.variable, '*', 1, 2) -1
  ) 
FROM test t;


SUBSTR(T.VARIABLE,INSTR(T.VARIABLE,'*',1,2)+1,INSTR(T.VARIABLE,'*',1,3)-INSTR(T.VARIABLE,'*',1,2)-1)


3012367134