•http://example.com – it returns example.com
•IANA — Example domains/ – it returns example.com
•http://www.example.com – it returns IANA — Example domains
•IANA — Example domains/ – it returns IANA — Example domains
•http://example.com/vb – it returns example.com
•http://www.example.com/vb/index.php – it returns example.com
•http://example.com/vb/index.php – it returns example.com

RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTP_HOST} ^([^.]+\.[a-z]{2,6})$ [NC]



RewriteRule ^(.*)$ http://www.%1/$1 [R=301,L]