htaccess question: http://username.site.tld to show http://www.site.tld/user/username

Hi,

How can i get the following functionality:

The subdomain "http://USERNAME.site.tld" should show users the same info as the now existing path "http://www.site.tld/user/USERNAME".
This should work dynamically, so i don't have to make a subdomain each time a new user registers.
Can i enforce this through htaccess?
And is this possible without using redirect, so "http://USERNAME.site.tld" shows in the browsers address bar?

Kind regards

Sven