Cannot match regex capture group within same expression

Grafana Alloy
1

I’m using the discovery.relabel component on kubernetes service targets from the discovery.kubernetes component, in order to filter targets to only the services which have a particular annotation. These annotations contain the URL path for blackbox health checks. This part is working fine.

I then want to filter the service targets by a given port (given in another service annotation), so that I’m only left with the services whose ports match the port in the annotation.

I’m trying to create a discovery.relabel rule to do this. I created the regex shown below for this purpose. It’s designed to check the two port numbers are equal, and write ‘true’ to a new label.

image

I’ve attempted to put this regex into a relabel rule but can’t get it to work, see below (note that I have removed the ^/$ from the start/end for testing).

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image1104×137 14.2 KB

For the reference to the capture group in the regex arg, I’ve tried using \1, \\1, $1 and ${1}, but cannot get anything to work here. It may simply be that this regex engine doesn’t support using capture groups within the same expression.

Can anyone help with this? Have I just done something wrong? Or are there any alternatives to do what I want to do here?

Below are some of the targets where I would like to make sure the service port is equal to the blackbox port.

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image982×1089 223 KB

Thanks :slight_smile:

2

Ahhh yes I think I’ve just found out that Golang’s RE2 does not support backreferences. Does anyone know a different way to achieve this in Alloy?

3

In fact, I’ve figured it out now. I hadn’t realised that the keepequal action compares two label values. The below works how I want it :slight_smile:

rule {
  // Drop all services where the service port is not equal to the blackbox port
  source_labels = ["__meta_kubernetes_service_port_number"]
  action = "keepequal"
  target_label = "__meta_kubernetes_service_annotation_blackbox_match_port"
}
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