UPDATE using subquery with joined tables

Hi,

I need to update field1 in table1, gathering data from field1 in table2.
The following SELECT shows the data as it needs to be updated:

SELECT a.field1 || regexp_replace(b.field1, '.*(mypattern)', e'. \\1')
FROM table1 a JOIN table2 b USING (id)
WHERE a.field1 NOT LIKE '%mypattern%' AND b.field1 LIKE '%mypattern%';

I am not sure how to translate this into an UPDATE statement for fiel1
in table1 efficiently.  Any tips welcome.

Cheers,

--
Seb