Oh, wow, I had no idea there was a GitHub action. That's great to hear! Happy to do this as a follow-up, as it's a larger change.

Regarding the whitespace issue, I think we can change branchLabelMapping from the default ^backport-to-(.+)$ to ^backport (.+)$.

@pquentin Sure. I think the action automatically reads the config that you added in this PR.

In the .NET generator repository I don't have a config yet, but I used the provided option on the action:
https://github.com/sorenlouv/backport-github-action/blob/929f69d04adbc196d982e60f02837b6cc00b3129/action.yml#L7-L9

The default value does not seem to contain a regex:

auto_backport_label_prefix: backport-to-

I tried with:

auto_backport_label_prefix: 'backport '

but this was not correctly recognized when I did an initial test:

"results": [
    {
      "targetBranch": " 9.0",
      "status": "handled-error",
      "error": {
        "name": "BackportError",
        "errorContext": {
          "code": "invalid-branch-exception",
          "branchName": " 9.0"
        }
      }
    }
  ]

It parsed the prefix incorrectly and assumed the whitespace to be part of the branch name. That's probably something that we could fix upstream or ask to get fixed upstream, but I didn't had time to check that any further so I just used backport- which works perfectly fine.

If we are lucky, we don't have to use this option in the GHA at all and the tool will read it from the config file.