By default, a regex match in Varnish happens case sensitive. If you want to use a case insensitive check, you can use the (?i) flag.
This is a normal, case sensitive regex check:
if (req.http.host ~ "^domain.(be|nl|com)$") {
...
}
To make the same check case insensitive, add the (?i) modifier at the beginning.
if (req.http.host ~ "(?i)^domain.(be|nl|com)$") {
...
}
Odd as it may be, some people type in their domain names with caps, and the browser does not convert those to lowercase on submit. Though in practice, every Host-header is treated as a lowercase value in all webserver configs.