FTS don't work

Hi, all!

There is one full text search solution contributed with postgres
source (I use 7.1.3). And there is sed that

select id from fti where string ~ '^substring'

will use indexes.
But it don't.

As I find out index works only if I search like
SELECT id from fti where string = 'string';

How can I search for strings like 'strin%' and use index?


Mark