how To Use Filesmatch In .htaccess with Regular Expressions?

Hello,

I have been trying to protect a virtual file/directory using .htaccess for a while now, but I'm getting nowhere.

What I am trying to do is protect the ADMIN area of a site that is dynamically created using rewrites.

So, there is no physical admin directory. One can access admin using:

index.php/admin
or just plain
admin/

Is there a way to password-protect anything with "admin" in it using FilesMatch within a .htaccess file, and how would one do that?

Many thanks!
Lisa