mod_rewrite - add www - apache

Question

I'm trying to force www for my site address with .htaccess:
RewriteCond %{HTTP_HOST} ^mysite\.com$ [NC]
RewriteRule ^(.*)$ http://www.mysite.coml/$1 [R=301,L]
If I open mysite.com this working ok - it adds www. It becomes http://www.mysite.com/index.php.
But if I open mysite.com/subpage I redirected to http://www.mysite.com/index.php again, instead of http://www.mysite.com/subpage.
But if I open http://www.mysite.com/subpage I don't get redirect (which is the expected behaviour).
How can I fix this? I would like the redirect all requests to my site to the www subdomain.

Answer 1

From the superb HTML5 Boilerplate .htaccess:
<IfModule mod_rewrite.c>
RewriteCond %{HTTPS} !=on
RewriteCond %{HTTP_HOST} !^www\..+$ [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
</IfModule>

Answer 2

The problem is right here:
RewriteCond %{HTTP_HOST} ^mysite\.com$ [NC]
^ specifies the beginning of the string while $ specifies the end.
In order to make it work you need to have a wildcard select after the .com or .+.
to reiterate what Alex said:
<IfModule mod_rewrite.c>
RewriteCond %{HTTPS} !=on
RewriteCond %{HTTP_HOST} !^www\..+$ [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
</IfModule>

Answer 3

Actually, I have to disagree with Mr. Szanto.
It would appear the problem was not with the:
RewriteCond %{HTTP_HOST} ^mysite\.com$ [NC]
Through some additional research (via Google) I found that this worked great for me:
<IfModule mod_rewrite.c>
RewriteCond %{HTTPS} !=on
RewriteCond %{HTTP_HOST} ^mysite\.com$ [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
</IfModule>
Mr. Szanto's answer, while well formatted, seems to redirect all requests missing the 'www' to be redirected with the 'www' attached. This was not a desirable outcome with most subdomains. The above code only redirects if there is no 'www' AND no subdomain before the TL domain name.
Hope this helps others.