RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !^wiki
RewriteRule . /index.php [L]

# rewrite wiki URLs
RewriteRule ^wiki/?(.*)$ /w/index.php?title=$1 [L,QSA]