multiple url/uri rewrite w/multiple default statements???

If you define a class with the appropriate app1, app2, app3, etc values, would this meet your needs:

when HTTP_REQUEST {
  set app_path [string tolower [getfield [HTTP::host] "." 0] ]
  if { [matchclass $::defined_apps equals $app_path] } {
    HTTP::redirect "https://internal_url.abc.com/$app_path[HTTP::uri]"
  }
}

I haven't tested this, but it should work like this:

http://app2.abc.com/my_path/internal?userid=citizen_elah

will be redirected to

https://internal_url.abc.com/app2/my_path/internal?userid=citizen_elah

Now if you also want to control pool selection, you could use findclass to extract the pool name associated to the app like so:

when HTTP_REQUEST {
  set app_path [string tolower [getfield [HTTP::host] "." 0] ]
  if { [matchclass $::defined_apps contains $app_path] } {
    HTTP::redirect "https://internal_url.abc.com/$app_path[HTTP::uri]"
  } elseif { [string tolower [HTTP::uri]] starts_with "/$app_path"} {
      pool [findclass $app_path $::defined_apps " "]
  }
}

This assumes a class such as:

"app1 app1-pool"

"app2 app2-pool"

"app3 app3-pool"

Not sure all my ducks are in a row, but perhaps this gives you a springboard to solve your problem.