RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]
Redirecting www to non-www
If you want to do the opposite, the code is very similar:
RewriteEngine On
RewriteCond %{HTTP_HOST} !^domain\.com.do$ [NC]
RewriteRule ^(.*)$ http://domain.com.do/$1 [R=301,L]