How to force www in htaccess with permalinks??

Hi. I have a wordpress blog installed at example.com/blog and also have permalinks setup. So the directory folder blog on my server has the htaccess entry below.
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<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
</IfModule>
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The problem is I cant get the force www entry to work with this. I have it working fine in the “public html” folder but not the “blog” folder. I have tried a ton of combinations with no success.

Anybody have this working? Thanks.