Xpath for Drupal form_build_id in jmeter

I often load test Drupal webapps with jmeter. One thing that I need to do very often is grab the form_build_id (unique for every form invocation) and use it for the form submit.

For a very simple form (only one form_build_id on the page), this works:

//input[@name="form_build_id"]/@id

in the Xpath extractor, set the reference name to whatever you want. I often set a default value of, e.g., form_build_id_no so I can easily tell (in a Debug Sampler) that no value was found and therefore the xpath isn't quite right.

The @id extracts just the id attribute from the input element.