Split DocumentRoot using Apache Alias per request path - apache

Question

I'm trying to slowly take over an existing website, route per route. I found Apache's Alias(Match) which seems to allow me to set a different (content) document root per request. But I'm failing with trailing slashes and more complex paths.
My goal now is to have the old website serve everything as it is used to. And to have a new website, serving a first specific request, say /foo and /foo/*.
I have my vhost setup like this:
<VirtualHost *:80>
ServerName example.com
UseCanonicalName on
AliasMatch ^/foo/(.*)$ /www/new/$1
AliasMatch ^/foo$ /www/new/$1
<Directory /www/new/>
Options +FollowSymLinks -MultiViews
AllowOverride All
Require all granted
</Directory>
DocumentRoot /www/old
<Directory /www/old/>
Options +FollowSymLinks -MultiViews
AllowOverride All
Require all granted
</Directory>
</VirtualHost>
In both directories I have an .htaccess with:
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-l
RewriteRule ^(.*)$ index.php/$1 [L]
The index.php's show 'old' or 'new' depending on their directory.
What happens to the different routes I test:
When requesting example.com/bar it shows 'old'.
When requesting example.com/foo it redirects too /foo/, and then shows 'new'.
When requesting example.com/foo/bar it shows 'old'.
I don't want the forced redirect of /foo to /foo/. And I want /foo/bar to show 'new'.
I've been following Apache's mod_alias to get the multiple AliasMatch directives to catch anything after /foo but apparently that's not working correctly. Also, I don't read anything there of the trailing slash being added magically.
Anyone knows the magic tricks?

Answer 1

I've been able to fix this by adjusting the Alias to point to the index.php directly:
AliasMatch ^/foo/(.*)$ /www/new/index.php
AliasMatch ^/foo$ /www/new/index.php
Then /bar and anything else goes to the old website, and /foo, /foo/ and /foo/bar goes to the new website.
There's two downsides to this method:
The .htaccess in the new website is skipped completely. But as I mainly use that for letting the index.php pick up every route, that's fine for now.
Frontend resources like css/js go to the old website. Fix that with another simple alias: example.com/frontend/app.css + Alias /frontend /www/new/public_html/frontend.