RewriteCond %{HTTP_HOST} ^(bbs|blog|download).domain.com$
RewriteRule ^(.*)$ /%1/$1 [L, NC]

这样，会自动把bbs对应到bbs目录，blog对应到blog目录，download对应到download，要加新的域名时，只需要在上面那行按格式添加即可

-------------------------------------------

RewriteEngine on

RewriteCond %{HTTP_HOST} ^bbs.domain.com$
RewriteCond %{REQUEST_URI} !^/bbs/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /bbs/$1
RewriteCond %{HTTP_HOST} ^bbs.domain.com$
RewriteRule ^(/)?$ bbs/index.php [L]