Add an example .gitlab-ci.yml for the use case with Pipelines for merge requests and exclude a specific branch

Copied from gitlab-foss#15310 (comment 226748230).

Hi guys, I hope this is a good place for a small question. I need to run a pipeline for all merge requests except cherry picks and mass updates (which have a specific branch name prefix). I used the following code:

test:
  only: [ merge_requests ]
  except: [ /^(cherry-pick|mass-update)-.*/ ]

And it seems to not work. What an I doing wrong?

It seems we don't have an example in documentation for this common use case. We should clarify it with the solution.

MR refs are a bit special than ordinary branch refs e.g. refs/merge-requests/:iid/head not example as a feature branch. You'd need to use only:variables with CI_COMMIT_REF_NAME predefined variables in this case, e.g.

test:
  only: [ merge_requests ]
  except:
    variables:
      $CI_COMMIT_REF_NAME =~ /^(cherry-pick|mass-update)-.*/