How to configure apache as forward proxy server to regex-replace domain names

Probably the ProxyPass / directive you have in your config routes all requests to the Tomcat backend, bypassing all your mod_rewrite directives. You can try to remove the ProxyPass directive and use RewriteRule with the [P] flag after your other rewrites:

• mod_rewrite: P|proxy

For the expire headers the situation is worse — in another similar question a solution was not found.

Option +FollowSymLinks would be needed for using mod_rewrite in .htaccess files; in your case it is not required, because you can put everything in the Apache config. Moving rules from config to .htaccess is pointless and can only make things slower. However, there is an important difference between .htaccess and the Apache config — in .htaccess patterns in RewriteRule are matched against relative filesystem paths (which never start with /), while in the VirtualHost context these patterns are matched against the URL path after the hostname, which always starts with /. Therefore at least one of your rules is incorrect:

RewriteRule ^content/(js|css)/([a-z]+)-([0-9]+)\.(js|css)$ /content/$1/$2.$4

should be

RewriteRule ^/content/(js|css)/([a-z]+)-([0-9]+)\.(js|css)$ /content/$1/$2.$4

(note the additional slash in the beginning).

You'll have to create two virtual hosts for apache one for each subdomain, acting as a reverse proxy to the specified ip.

something along the lines of

NameVirtualHost *:80
<VirtualHost *:80>
    ServerName sub1.example.com

    ErrorLog "var/log/sub1_error_log"
    CustomLog "var/log/sub1_access_log" common

    ProxyRequests Off
    <Proxy *>
        Order deny,allow
        Allow from all
    </Proxy>
    ProxyPass / http://10.10.10.10/
    ProxyPassReverse / http://10.10.10.10/
</VirtualHost>

<VirtualHost *:80>
    ServerName sub2.example.com

    ErrorLog "var/log/sub2_error_log"
    CustomLog "var/log/sub2_access_log" common

    ProxyRequests Off
    <Proxy *>
        Order deny,allow
        Allow from all
    </Proxy>
    ProxyPass / http://10.20.20.20/
    ProxyPassReverse / http://10.20.20.20/
</VirtualHost>