Change target host in proxy_pass

[Tomasz Kapek]

Hello,
I have NGINX acting as reverse proxy and I would like to achieve something
like this:

When I get a request like this GET http://app1.mydomain.aa.com/aaa/bbb it
should be converted to:
GET http://app1.mydomain.bb.com/aaa/bbb so such directive will do the job:

proxy_pass http://app1.mydomain.bb.com;
problem is that I want to convert host part automatically (regex) basing on
incoming requests to NGINX  - app1.mydomain are not fixed they are changing
very often.
Is it possible? Can anyone get a clue how proxy_pass statement should look
like?
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