Rules for directing all subdomains to a folder (wildcard subdomains) - and exclude a specific subdomain, like dev.

.htaccess

<IfModule mod_rewrite.c>
Options +FollowSymLinks -MultiViews

RewriteEngine On
RewriteBase /

# Dev app sub domain masking
RewriteCond %{HTTP_HOST} ^dev\.yourdomain\.com$ [NC]
RewriteRule ^((?!dev/).*)$ /dev/$1 [L,NC]

# All sub domain masking
RewriteCond %{HTTP_HOST} ^([\w-]+)\.yourdomain\.com$ [NC]
RewriteRule ^(.*)$ /app/$1 [L,NC]
</IfModule>