title: Using an expression in the mod_ext_filter module
body: <p>I need to configure an apache reverse proxy to return a js script on each page. For that, I use a perl script that injects the javascript to the pages.
  To do this, I used the following directive:
  <code>ExtFilterDefine fixtext mode=output intype=text/html cmd=&quot;/home/eloi/leanovia/observability/webserver-script/ttk-js-rum-injector.perl ${cookies_retrieved}&quot;</code>
  I succeeded until this step.</p>
  <p>However, I need to get the cookies from the request to use it in the javascript. I know I need to use <code>expr=%{req:Cookie}</code></p>
  <p>However, the expression does not evaluate when I use it like this:</p>
  <p><code>ExtFilterDefine fixtext mode=output intype=text/html cmd=&quot;/home/eloi/leanovia/observability/webserver-script/ttk-js-rum-injector.perl expr=%{req:Cookie}&quot;</code></p>
  <p>I know I'm really close to the solution, but I've been struggling for hours trying to get the expression evaluated, without success. I haven't found a solution to my problem. I know that it is possible to send a header in the response that evaluates this expression, since I could test it with this directive:</p>
  <p><code>Header set eloi &quot;${cookies_retrieved}&quot;</code></p>