RewriteEngine On
RewriteRule ^(a|gallery)/([a-zA-Z0-9]{5,})$ /?$1/$2
RewriteRule ^([a-zA-Z0-9]{5,32})$ /?$1
RewriteRule ^([a-zA-Z0-9]{5,})\.(png|jpe?g|gifv?|webm|mp4)$ /?$1.$2