How to Redirect non-www to www for Codeigniter

RewriteEngine On                           

#This bit rewrites your host name to include www
RewriteCond %{HTTP_HOST} !^www\.doopromotion\.com [NC]
RewriteRule ^(.*)$ http://www.doopromotion.com/$1 [R=301,NC,L]

#This bit does the codeigniter magic
RewriteCond %{REQUEST_FILENAME} !-f  
RewriteCond %{REQUEST_FILENAME} !-d  
RewriteRule ^(.*)$ /index.php/$1 [L]

สำหรับคนที่ develop บน codeigniter จะได้ไม่มี content duplicate ลองดูนะครับ