`~' operator and indices

Hello,

I would like to ask you why do `~' gives the following results,
if there is an index on `string':

select string from indextbk_fti_fkey where string ~ '^IE';
string
--------
(0 rows)

select string from indextbk_fti_fkey where string ~ '^IECIA';
string
--------
IECIA
(1 row)

`E' here is a polish letter. I have set locale to `pl_PL' before
starting postgres.

Best regards

Artur Rataj