.htaccess force "www." on everything but subdomains and remove trailing slashes - .htaccess

Question

Here's what I have so far:
#Force www.
RewriteCond %{HTTPS} !=on
RewriteCond %{HTTP_HOST} !^www\..+$ [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
#Remove trailing slash
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} (.*)$
RewriteRule ^(.+)/$ http://www.domain.com/$1 [R=301,L]
However, this messes up all subdomains doing the following redirect:
sub.domain.com -> www.sub.domain.com
And also, its dependant on the domain written on the remove trailing slash bit.
So... two questions.
How do I rewrite the rule on the "remove trailing slash" bit to exclude writing the domain on it?
How do I make a rewritecond to exclude subdomains, without explicitly writing them down, on the "force www." bit?
Examples of desired results -
sub.domain.com/something/ -> sub.domain.com/something
domain.com/something/ -> www.domain.com/something
www.domain.com/ -> www.domain.com
sub.domain.com -> sub.domain.com
Thanks!

Answer 1

Change the www to check for the actual domain:
#Force www.
RewriteCond %{HTTPS} !=on
RewriteCond %{HTTP_HOST} ^domain\.com$ [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
Or, if you're hosting a bunch of domains, you can check for a name before the TLD:
RewriteCond %{HTTPS} !=on
RewriteCond %{HTTP_HOST} ^([^.]+)\.([a-z]{2,4})$ [NC]
RewriteRule ^ http://www.%1.%2%{REQUEST_URI} [L,R=301]
As for the trailing slash, you have to be careful that the request isn't made for a directory. Because if it is, and you have DirectorySlash turned on (by default it is on), then you'll cause a redirect loop.
To exclude subdomains, we assume that the first rule redirected the browser to ensure that it started with "www", and since subdomains aren't being redirected to start with "www", we can just check for that:
#Remove trailing slash
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{HTTP_HOST} ^www\. [NC]
RewriteRule ^(.+)/$ /$1 [R=301,L]