Help with .htaccess file's rewrite rule

Question

I apologize in advance if I missed someone cover this already; I tried to see if someone had my own .htaccess dilemma:

I currently have the following website: www.example.com

I want to set it up like the following: somewebsite.example.com

And have the subdomain "somewebsite" point to a subfolder on my root. If that subdomain is not available, return them to an error.html in the root folder.

Example:

/
error.html
/img
/websites
    /somewebsite
        index.html

Here is my .htaccess for the root folder that I am working with:

RewriteEngine on
Options +FollowSymlinks
RewriteBase /

RewriteCond %{HTTP_HOST} !^$
RewriteCond %{HTTP_HOST} !^somewebsite\.example\.com$ [NC]
RewriteCond %{REQUEST_URI} !somewebsite/ !-d
RewriteRule ^/(.*)$ http://somewebsite.example.com/$1 [L,R=301]

I'm not amazing with RegExp or .htaccess file, but what am I missing to accomplish my goal?

Answer 1

Try this rule:

RewriteCond %{HTTP_HOST} ^([^./]+)\.example\.com$
RewriteCond %{DOCUMENT_ROOT}/%1 -d
RewriteRule ^ %{DOCUMENT_ROOT}/%1%{REQUEST_URI} [L]

This rule tests if foobar.example.com with an arbitrary foobar can be mapped to an existing directory located in the document root directory.