Replace stream if condition is met

Hi - I have an iRule i need to draft, however I can only test it once it drafted, by sending it to my customer for testing... bummer.

So the problem is - I need to inject my .js in the body of the response, but only if the script is not already injected (there is a valid reason for this, I promise). 

My thinking process is based on this:
https://clouddocs.f5.com/api/irules/STREAM_MATCHED.html

The goal is:
1) Check if the body contains /js/app<10digits>.js
2) If it does - do nothing
3) If it does not inject /js/app.js just before closing </body> tag
or:
2) delete the /js/app<10 digits>.js
3) Insert /js/app.js just before closing </body>tag

I'm thinking option 2 might be simpler, please let me know if my thinking is good, and If possible suggest some more elegant solution

when HTTP_REQUEST {
    STREAM::disable
    HTTP::header remove "Accept-Encoding"
}
when HTTP_RESPONSE {
    if { [HTTP::header exists "Content-Type"] && [HTTP::header value "Content-Type"] contains "text/html" } {
        if { [HTTP::header exists "Content-Length"] } {
            HTTP::header remove "Content-Length"
        }
        STREAM::expression {
                =<script type="text/javascript" src="/secfense/js/eru-[0-9]{10}\.js"><\/script><\/body>== 
                @</body>@<script type="text/javascript" src="/secfense/js/eru-1744029915.js"></script></body>@
        } 
        STREAM::enable
 }

I'm not sure whether multi expression STREAM::expression is a thing... Will this work?