mod_rewrite with subdomains

In my site, I need to have members.sgfcanada.com/XXXX/admin.php -and- URLXXXX/admin.php both point to URLXXXX. I thought the best method to do this would be using the REQUEST_FILENAME variable, since both of the above filename would point to /home/sgfcana/public_html/members/XXXX/admin.php.

Currently, this is my .htaccess file:

Now, I can go to URL and it works as it should (the correct page shows up). But, when I try to modify the code so it will work inside the members directory, like this:

then it doesn't work properly (returns a 404 error).

Also, I want it to instead of being lower-case letters and numbers, I want it instead to be and word character(A-Z, a-z, and _ [underscore]) in between 2 and 8 characters long. Any help would be greatly appreciated, for then maybe I'll understand mod_rewrite once and for all

It'd be much more efficient to not use REQUEST_FILENAME because it requires a stat() call. BTW, the default RewriteCond is REQUEST_URI, which can do what you needed.

That works thanks

But now I ran into another problem. Here is my .htaccess file now:

I want the first 4 redirects to go through, and if any of them are matched, stop rewriting (with the [L] tag). All files that do not match those 4 rules go onto the last rule. The problem is, if I go to admin.html, I see viewfile.php instead of admin.php (it's rewriting with the last rule). This shouldn't be the case in my understanding of mod_rewrite, for according to the Apache documentation:
Doesn't that mean if any of the rules are matched, none of the following rules will be carried out?

Thanks for your help,

>> if any of them are matched, stop rewriting (with the [L] tag)

Then you don't need to repeat the 2 parameter. Instead, you can specify it to - like so:

RewriteRule ^(.*)/admin.png - [L]

In addition, when you don't specify a RewriteCond, you'd want to specify a leading / like so:

RewriteRule ^/xxx/yyy.ext$ - [L]

>> RewriteRule ^(.*)/(.*) backend/viewfile.php?d=$1&f=$2

Not sure what you are you trying to do exactly. Though specifying ^(.*)/(.*) usually is not reliable and it doesn't always work. Also, you should set /full/system/path/to/backend/viewfile.php?d=$1&f=$2 [T=application/x-httpd-php,L]