create table t1 (
r1 int4,
r2 character varying( 255 )
);
insert into t1 values ( 2 ,'');
insert into t1 values ( 2 ,'a');
insert into t1 values ( 5 ,'');
insert into t1 values ( 20 ,'');
insert into t1 values ( 26 ,'');
insert into t1 values ( 49 ,'');
select r1||r2 from t1 order by r1,r2;

create table t2 (
r2 character varying( 255 )
);
insert into t2 values ('2');
insert into t2 values ('2a');
insert into t2 values ('5');
insert into t2 values ('20');
insert into t2 values ('26');
insert into t2 values ('49');
insert into t2 values ('2/8');
select * from t2 order by substring(r2 from '^\\d*')::int4

select * from t2 order by case when substring(r2 from '^\\d*')::int4 = null then  0  end 

select * from t2 order by case when substring(r2 from '^\\d*')='' then  1  else substring(r2 from '^\\d*')::int4 end

CREATE INDEX t2_substr_index
  ON t2
  USING btree
  ("substring"(r2::text, '^\\d*'::text));