.htaccess - 如何添加http：// www。 在多种条件下

I need to know how do I write the following htaccess so that it does the following:

• If the website link has no http://www. it'll add http://www. to the link.
• If the website link has just www. it'll change it to http://www.
• If the website link has just http:// it'll change it to http://www.

I've tried doing the .htaccess code as I've enabled it to load as I'm using Apache 2.4.7 on Ubuntu.

I've resolved this now by installing a2enmod with my htaccess.

Thanks