RewriteEngine on
RewriteCond %{HTTP_HOST} !^www.example.org$ [NC]
RewriteCond %{HTTP_HOST} ^([a-z0-9]+).example.org$ [NC]
RewriteCond /home/example/public_html/%1 !-d
RewriteCond /home/example/public_html/%1 !-l
RewriteRule ^(.*)$ http://www.example.org/%1 [R,L]