initially this worked well: (\x{386}-\x{3ff} for Greek chars)
[code]
location ~ "^(/optionalwebappname)?/ProcessImageServlet.*$" {
root /opt/myfilerepository/;
rewrite ^(.+)$ http://static-dev.myhost.eu/$arg_hotel_id/$th$fn break;

set $hid '';
set $filename '';
set $th '';

if ($request_uri ~ "^(.*)[\\?|&]hotel_id=([0-9]+)(&.*)?$") {
set $hid $2;
}
if ($request_uri ~ "(*UTF8)^(.*)[\\?|&]filename=([% ,a-zA-Z0-9\x{386}-\x{3ff}_\-\.]+)(&.*)?$") {
set $fn $2;
}
if ($request_uri ~ "^(.*)[\\?|&]type=th(&.*)?$") {
set $th 'th_';
}
rewrite ^(.+)$ http://static-dev.myhost.eu/$hid/$th$fn break;
access_log logs/site-pis.log main;
expires 1h;
}

[/code]


however, later I found this to work better, including of course utf8 arguments - you would better check this out first... much more elegant

[code]
location ~ "^(/optionalwebappname)?/ProcessImageServlet.*$" {
set $th '';

if ($request_uri ~ "^(.*)[\\?|&]type=th(&.*)?$") {
set $th 'th_';
}
rewrite ^(.+)$ http://static-dev.myhost.eu/$arg_hotel_id/$th$arg_filename break;
expires 1d;
}
[/code]