Substitute for SUBSTR

I am working on a system (SAP) which uses Oracle, but its language disallows the use of any Oracle functions, we can just use generic SQL. I need to do the following SUBSTR in SQL, without using SQL, comparing '010' against '010001'. Is there a way to do this in generic SQL?

create table t_1 (
  pos_nr varchar2(3)
);

create table t_2 (
  ref_posid varchar2(10)
);

insert into t_1 (pos_nr) values ('010'); insert into t_2 (ref_posid) values ('010001');

commit;

• this is what I need to do, but substr not allowed. select * from t_1 where exists (select 'match' from t_2 where t_1.pos_nr = substr(ref_posid,1,3));
• so, how can we use "LIKE" or "BETWEEN"? select * from t_1 where exists (select 'match' from t_2 where t_1.pos_nr like ref_posid);