Using htaccess Redirect Rule

I will tell you right off the bat that I don’t “get” regular expressions, and I don’t get .htacess rewrite rules. I wish I understood them better, but there’s some part of my brain that just fights me every time I try to get a better all-around understanding. Still, I have to (and want to) do stuff with htaccess, so I end up digging for code online, and trying stuff until something works. I wish I knew more about WHY it worked, but I’m just happy that it works at all. Image may be NSFW.
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Now that I’m done with my disclaimer, on to the point of this post. I had to use a htaccess file to redirect a subdomain to a directory in the main domain. For example: http://blog.mysite.com needed to point to http://www.mysite.com/blog/

After much digging and trial and error, this seems to work:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^blog\.mysite\.com
RewriteRule ^(.*)$ /blog/$1 [L]