Mine

RewriteCond %{REQUEST_URI} !.*asm.*$      [C]
RewriteCond %{REQUEST_URI} !.*wm.*        [C]
RewriteCond %{REQUEST_URI} !.*error.html$ [C]

RewriteCond %{REQUEST_URI} portal_search
RewriteRule ^/portal_search/([^/]*) - [E=what:$1] 

RewriteCond %{REQUEST_URI} !portal_search
RewriteRule ^/([^/]*) - [E=what:$1]

RewriteRule ^/portal_search(.*)$ http://localhost:3500$1 [P]
RewriteRule ^(.*)$ http://localhost:3500$1 [P]

ProxyPassReverse / http://localhost:3500/
ProxyErrorOverride On