Index use for case insensitive query

I've got a table where one of the main lookup access is by email address, which is generally a case insensitive lookup.

With the constraints that: some of the entries aren't reall email addresses and I can't change the case of all the data because of that:

Is there a way to do a case insensitive index and appropriate query so that when searching for an email address, I get the benefit of an index?

In the following queries, there's an index on dl_profile(_email).

test=# explain analyze select _donorNum from dl_profile where _email~'^foo(at)bar(dot)org$' ;
NOTICE: QUERY PLAN:

Index Scan using dl_profile_email on dl_profile (cost=0.00..467.75 rows=1 width=4) (actual time=14.59..14.63 rows=1 loops=1)
Total runtime: 14.97 msec

EXPLAIN
test=# explain analyze select _donorNum from dl_profile where _email~*'^foo(at)bar(dot)org$' ;
NOTICE: QUERY PLAN:

Seq Scan on dl_profile (cost=0.00..10607.28 rows=1 width=4) (actual time=4196.43..5078.86 rows=1 loops=1)
Total runtime: 5079.42 msec

thanks

eric