Selfhosted

You might be able to setup a mod_rewrite rule to load a specific file path or other url based on the URL path, but a subdomain would probably be easier/cleaner.

From Apache mod_rewrite docs:

The mod_rewrite module uses a rule-based rewriting engine, based on a PCRE regular-expression parser, to rewrite requested URLs on the fly. By default, mod_rewrite maps a URL to a filesystem path. However, it can also be used to redirect one URL to another URL, or to invoke an internal proxy fetch.

Just remember the old adage about regular expressions: when you use a regular expression to try to solve one problem, you create two problems.

A subdomain would likely be cleaner and easier.

From a security point of view it is not a good idea to host multiple web applications in sub directories on the same hostname. With such a configuration, every application sees all cookies from all other applications. This also means that you can have collisions of cookie names between applications if the names are not unique.

So if one application would get compromised, it could easily steal all your sessions for all other applications.