.htaccess - rewrite url to subdomain and include full url

i'm trying to prepend all of my urls from mysite.com/sub to sub.mysite.com and retain the full url using mod_rewrite ad it doesn't seem to be working.

example

if someone goes to - mysite.com/sub/1/2/3, the url in the address bar should be sub.mysite.com/1/2/3

i moved a wordpress install to my root that was previously installed to a subdirectory with a subdomain pointing to it and want to retain my url structure for posts.

thanks!