Optimizing query?

Hi,

I am trying to match items from 2 tables based on a common string.
One is a big table which has one column with entries like XY123, ABC44, etc
The table has an index on that column.
The second table is, typically, much smaller

select .... from tab1, tab2 where tab1.code = tab2.code;

This works fine and fast.
Now, as a variant, I have some entries like XY423A, XY423B, GF55A, GF55D in the
big table and want them to match XY423, GF55 in the second table

Variants I have tried

select .... from tab1, tab2 where tab1.code ~ (tab2.code||'($|[A-Z])');
select .... from tab1, tab2 where tab1.code ~ ('^'||tab2.code||'($|[A-Z])');

both take an enormous time. In the better case that I can subset (e.g. all candidates in table 2
share initial "AX") I get back to manageable times by adding
and tab1.code ~ '^AX'
into the recipe. Actual runtime with about a million entries in tab1 and 800 entries in tab2
is about 40 seconds.

Regards
Wolfgang Hamann