RewriteEngine on
RewriteCond     %{HTTP_HOST}                    !^www.*
RewriteCond     %{HTTP_HOST}                    ^[^.]+\.myhost\.com$
RewriteRule     ^(.+)                           %{HTTP_HOST}$1                  [C]
RewriteRule     ^([^.]+)\.myhost\.com(.*)        /home/$1/public_html$2