Sort PostgreSQL in alphabetical and numeric ascending order

create table mytable(
  mycolumn char(4)
);

insert into mytable (mycolumn) values 
('0001'), ('0003'), ('0010'), ('a001'), ('d001'), ('k001'), ('i001'), ('z001');

 select * from mytable
order by case when mycolumn~'^\d'then2 else1end, mycolumn;

mycolumn 
----------
 a001
 d001
 i001
 k001
 z001
 0001
 0003
 0010
(8 rows)