using GNAT.Regexp with non-ASCII characters

Hi,

I am using the GNAT.Regexp package to filter a list of strings matching
them against a regular expression containing non-ASCII characters such as
á (a acute), ñ (n tilde), ü (u diaeresis) etc. I get it OK when the
input string doesn't contain any of these characters, but the Match
function returns False otherwise. Any help?

Example: the regexp "[u|ú]ltimo" matches succesfully against "ultimo" but
it doesn't against "último".

Thanks in advance,

Xabier Artola.
Univ. of The Basque Country.