Nginx rewrite non-www-prefixed domain to www-prefixed domain

As noted in the Nginx documentation, you should avoid using the if directive in Nginx where possible, because as soon as you have an if in your configuration your server needs to evaluate every single request to decide whether to match that if or not.

A better solution would be multiple server directives.

server {
        listen 80;
        server_name website.com;
        return 301 $scheme://www.website.com$request_uri;
}

server {
        listen 80;
        server_name www.website.com;
        ... 
}

If you're trying to serve an SSL (HTTPS) enabled site, you got more or less three different options.

1. Set up multiple IP addresses having each server directive listening on their own IP (or different ports if that's an option for you). This options needs SSL certificates for both website.com and www.website.com, so either you have a wild card certificate, a UNI certificate (multiple domains) or just plainly two different certificates.
2. Do the rewrite in the application.
3. Use the dreaded if directive.

There is also an option to use SNI, but I'm not sure this is fully supported as of now.

if ($host ~* ^[^.]+\.[^.]+$) {
    rewrite ^(.*)$ https://www.$host$1 permanent;
}

It's only possible to get valid hostnames because the request will never make it to your server otherwise, so there's no need to build your own validation logic.

Well I guess I don't really need the outer "if" statement since I'm only checking for domains of the form xxx.xxx anyways. The following works for me, though it's not robust. Let me know if there is a better solution.

    if ($host ~* ^([a-z0-9\-]+\.(com|net|org))$) {
        set $host_with_www www.$1;
        rewrite ^(.*)$ http://$host_with_www$1 permanent;
    }

Edit: Added hyphen to the regular expression since it is a valid character in a hostname.

if ($host !~* ^www\.) {
    rewrite ^(.*)$ http://www.$host$1 permanent;
}