Subdomain based on CodeIgniter controller name

RewriteEngine on
RewriteCond %{HTTP_HOST} ^blog.example.com$
RewriteCond $1 !^(index.php|images|css|js|robots.txt)
RewriteRule ^(.*)$ /index.php/blog/$1 [L]

if(preg_match('/^blog/',$_SERVER['HTTP_HOST'])) {
    $route['(:any)/(:any)/(:any)/(:any)'] = 'blog/$1/$2/$3/$4';
    $route['(:any)/(:any)/(:any)'] = 'blog/$1/$2/$3';
    $route['(:any)/(:any)'] = 'blog/$1/$2';
    $route['(:any)'] = 'blog/$1';
    $route['blog'] = 'blog/index';
}