虽然用vhost可以支持子域名, 但不方便.
RewriteCond $1 !^bbs/
RewriteCond %{HTTP_HOST} bbs.example.com
RewriteRule (.*) /bbs/$1 [L]
RewriteCond $1 !^bbs/ 避免内部无限redirect.
或者支持无限子域名:
RewriteCond %{ENV:Rewrite-Done} !^Yes$
RewriteCond %{HTTP_HOST} !^www\\.example\\.com
RewriteCond %{HTTP_HOST} ^([^.]+)\\.example\\.com
RewriteRule (.*) /%1/$1 [E=Rewrite-Done:Yes,L]