Help configure a Reverse proxy with Substitute

Dear community,

I need an advice on how to configure a reverse proxy in which I need to replace the <img src=""> tag from the response html with an ENV variable defined previously. How do you recommend me to do this?

I tried with substitute, but unfortunately doesn't seem to support variables.

The variable I am setting with the command: SetEnvIf Request_URI "/lp\/([a-zA-Z0-9]*)" st=$

and now I would like to use the {st} variable in the substitute

Substitute "s|src=\"(.*?)\"|src=\"%{st}e/lp/$1\"|i"
I tried with %{st}e , ${st} in any combination it doesn't work.

Is there any way in which I can use e previous defined variable in rewriting the proxy response HTML?

Thank you,

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