.htaccess - 如果URL中不存在字符串，如何将字符串插入地址

I am looking to redirect url's in a multi-lingual site if it doesn't contain a language string. For example, if the url comes in as:

http://www.example.com/overview/features

I would like to redirect it to

http://www.example.com/en/overview/features

... i.e. insert the 'en' language string into the url. If it does contain the string then it is left alone.

I hazard a guess this is using negative look backs/forwards, but my .htaccess skills don't seem to be up to the above scenario.

Any ideas?

Try:

RewriteEngine On
RewriteCond %{REQUEST_URI} !^/[a-z]{2}/
RewriteRule ^(.*)$ /en/$1 [L,R=301]