sieve - content checks with special chars

[Hajo Locke]

Hello,

currently i try subject tests with special chars and german umlauts öäü etc.

if header :contains "subject" ["öööö äää"] {
   fileinto "umlauts";
}

iam surprised, this is working out of the box. great! in procmail i had 
to add a second step by manual coding the needle with perl.
is sieve reliable at this kind of tests or are there some special cases?
iam worried about this, because it was to easy.  confusing... ;)

Thanks,
Hajo