Mod_rewrite: How do I omit subdomains from rules?

Complete Apache newbie here. I'm trying to get my main URL to redirect to the www. Here's the code I'm using:

Rewri开发者_开发问答teCond %{HTTP_HOST} !^www\.domain\.com 
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=permanent,L] 

The problem is, this has wrecked my subdomains. sub.domain.com goes to www.sub.domain.com, which doesn't work. So what do I write to fix that?

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This might be enough:

RewriteCond %{HTTP_HOST} ^domain\.com 
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=permanent,L]

If all you're trying to do is make domain.com go to www.domain.com, then just use a RewriteCond that only matches domain.com at the start.