WP_Rewrite (Again)

I am trying to revive an old plugin which was last updated for version 2.1. The general idea is to make pages into subdomains using Apache’s wildcard subdomains.

The page rule I currently have is (.+?)(/[0-9]+)?/?$==>index.php?pagename=$matches[1]&page=$matches[2]

However, this seems to only capture what’s after HTTP_HOST. Is it possible to parse https://page1.mydomain.com into https://www.mydomain.com/?pagename=page1?

Thanks a million!