Codeigniter htaccess重定向到非www

I am using codeigniter.I want to redirect all users from www to non-www. I mean if user does a request to http://www.example.com it should be redirected to http://example.com or http://www.example.com/url/1/2 to http://example.com/url/1/2

How can I achive this ?

My htacess code:

RewriteEngine On
RewriteCond %{REQUEST_URI} ^/system.*
RewriteRule ^(.*)$ index.php/?$1 [QSA,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.+)$ index.php/?$1 [QSA,L] 

Try this way,

RewriteEngine On
RewriteCond %{REQUEST_URI} ^/system.*
RewriteRule ^(.*)$ index.php/?$1 [QSA,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteBase /
RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]

    RewriteEngine on
    RewriteBase /

    #for redirect non www to www

    RewriteCond %{HTTP_HOST} !^www\. [NC]
    RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]

    #for redirect www to non www

    RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
    RewriteRule ^(.*)$ http://%1/$1 [R=301,L]

    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule ^(.*)$ index.php?/$1 [L]

    #Also for http to https redirect

    RewriteEngine On
    RewriteCond %{HTTPS} !=on
    RewriteRule ^.*$ https://%{SERVER_NAME}%{REQUEST_URI} [R,L]