// HTTP 1.1 is essentially 1.0 so any future version of HTTP will work with our code.
var weSupportThis = new Regex("^HTTP/\d+\.\d+").IsMatch(header);
There are many coders out there and many broken ways of detecting protocols. Only changing the version number might run into trouble if one side of the conversation assumes that everything starting with "HTTP" is going to be pretty much equivalent to HTTP/1.0. So at least the "PRI" part makes sense.