Regexes are silently anchored

[pete0emerson]

I'm seeing this in version 0.17.0.

Suppose __meta_consul_tags value is ,environment:stage,application:webapp,.

In relabel_configs:

    - source_labels: ['__meta_consul_tags']
      regex:         ',application:(.*),'
      target_label:  'application'
      replacement:   '$1'

I would expect this to succeed. However, it fails, and my suspicion is that the regular expressions are anchored silently, because this succeeds:

    - source_labels: ['__meta_consul_tags']
      regex:         '.*,application:(.*),'
      target_label:  'application'
      replacement:   '$1'

If my assumption is correct, my suggestion would be that anchoring be not silently added, so if I truly wanted to anchor it, I'd use the regex anchor ^ to anchor it to the beginning of the string.

[juliusv]

This behavior has been changed intentionally in 0.17.0 to the one you are seeing because wanting anchored regexes was seen as the more frequently desired use case.

See also: #1518 and the linked changelog entry.

[brian-brazil]

This was a change we made on purpose, as not everyone is fully aware of how regexes work and we'd seen a lot of patterns in use that could unintentionally overmatch (consul tags being a common one).

[pete0emerson]

That makes sense. I'd suggest that it's worth documenting well in the relabel_configs section to make that clear to the people who are more proficient at regular expressions. If you'd like me to submit a patch, I would be glad to create one.

[grobie]

It's already documented? https://prometheus.io/docs/operating/configuration/#relabel_config

[juliusv]

It'd make sense to briefly mention it in the code block comment above the regex field too.

[grobie]

prometheus/docs#370

[lock]

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