Originally Posted by Fris

Options +FollowSymLinks

RewriteEngine On

# mod_dir fix
RewriteCond %{HTTP_HOST} !^(www\.)?domain\.com$ [NC]
RewriteCond %{HTTP_HOST} ^(.*\.)?([^\.]+)\.domain\.com$ [NC]
RewriteCond %{DOCUMENT_ROOT}/subs/%2%{REQUEST_URI}/ -d
RewriteRule [^/]$ http://%2.domain.com%{REQUEST_URI}/ [R=301,L]

# strip sub subdomains
RewriteCond %{HTTP_HOST} ^.*\.([^\.]+\.domain\.com)$ [NC]
RewriteRule ^(.*)$ http://%1%{REQUEST_URI} [R=301,L]

# abc.domain.com/ --> /subs/abc/
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{HTTP_HOST} !^(www\.)?domain\.com$ [NC]
RewriteCond %{HTTP_HOST} ^([^\.]+)\.domain\.com$ [NC]
RewriteCond %{DOCUMENT_ROOT}/subs/%1/ -d
RewriteRule ^(.*)$ /subs/%1%{REQUEST_URI} [QSA,L]