Is there a way to configure the Squid proxy server in order to make it select a different parent proxy (specified by the cache_peer directive) based on the hostname in the requested URL, say in the outgoing HTTP(S) request?
For example, if the requested URL contains *.example1.com then select proxy1.host. If it contains *.example2.com and *.example3.com then select proxy2.host.
yes
the directives you'll need are
acl example1 dstdomain example1
acl example2 dstdomain example2
cache_peer proxy1.host parent 3128 0 no-query default
cache_peer proxy2.host parent 3128 0 no-query default
cache_peer_access proxy1.host allow example1
cache_peer_access proxy2.host allow example2
You might also want to know how these directives work
never_direct
always_direct