Regex parsing issues in matchers for expressions broken down into multiple lines.

[aned]

Alertmanager version 0.28.1

It appears that if the regex expression is broken into multiple lines, when the new line starts with .* it breaks things.
amtool config check doesn't result in any errors, no error logs when running the "broken regex" config file.

In the example bellow it doesn't match anything .*BGP.* and anything after it.

    - matchers:
        - team = "netops"
      routes:
      - matchers:
          - alertname =~ "ping_fail|PeerDownORFlappingJuniper|ServiceMonitorLowAvailability|ReadOnlyFileSystem|
                          PowerSupplyBad.*|*|TooMuchTrafficTowardsLBorFW|BGPTooManySessionsDown.*|BGPDrained.*|
                          .*BGP.*|FanStateBad.*|NodeLowFreeMem|InputDiscardsHighSonic|TempHigh.*|
                          TempHigh.*|.*Interface.*|.*ErrorsRate.*"
        receiver: abc
        continue: true

This is working as expected

    - matchers:
        - team = "netops"
      routes:
      - matchers:
          - alertname =~ "ping_fail|PeerDownORFlappingJuniper|ServiceMonitorLowAvailability|ReadOnlyFileSystem|
                          PowerSupplyBad.*|*|TooMuchTrafficTowardsLBorFW|BGPTooManySessionsDown.*|BGPDrained.*|
                          FanStateBad.*|NodeLowFreeMem|InputDiscardsHighSonic|TempHigh.*|.*BGP.*|
                          TempHigh.*|.*Interface.*|.*ErrorsRate.*"
        receiver: abc
        continue: true

To test, "bad" regex string:

./amtool config routes test --config.file=alertmanager-config.yml   alertname=BGPAdminDown   team=netops   production_state=production   severity=info
receiver_A,netops-info

"normal" regex string:

./amtool config routes test --config.file=alertmanager-config.yml   alertname=BGPAdminDown   team=netops   production_state=production   severity=info
receiver_A,abc,netops-info

[grobinson-grafana]

I think the regex is invalid for another reason as |*| is not valid regex:

msg="Loading configuration file failed" component=configuration file=config.yml err="error parsing regexp: missing argument to repetition operator: `*`"

R.E newlines, the following YAML:

matchers:
- alertname =~ "foo|
  .*BGP.*"

is equivalent to the following JSON:

{
  "matchers": [
    "alertname =~ \"foo| .*BGP.*\""
  ]
}

You've put an extra space in front of .*BGP.*, which means it will match alertname=" BPG" but not alertname="BGP".

[aned]

Indeed, the string that Alertmanager actually receives in its configuration struct is not:

ping_fail|...|BGPTooManySessionsDown.*|BGPDrained.*|.*BGP.*|FanStateBad.*|...

It is actually:

ping_fail|...|BGPTooManySessionsDown.*|BGPDrained.*| .*BGP.*|FanStateBad.*|...

In this case regex is parsed as: match a literal space , followed by any characters .*, followed by the literal string BGP, followed by any characters .*

In the case of

ping_fail|...|BGPTooManySessionsDown.*|BGPDrained.*| BGP.*|FanStateBad.*|...

the space between the | and the next part of the pattern is irrelevant for regex parsing.

@grobinson-grafana idk how you got it to error, but with my config above the amtool doesn't error.

msg="Loading configuration file failed" component=configuration file=config.yml err="error parsing regexp: missing argument to repetition operator: *"

From searching , the issue is the distinction between two types of multi-line strings in YAML in golang implementation:

1. Literal Block Scalar (using |): This preserves newlines exactly as they are.
2. Folded Block Scalar (using >): This is what I'm implicitly using. It folds newlines into spaces. The goal is to make long text more readable in the YAML file while treating it as a single continuous line of text in the application.

[grobinson-grafana]

Yes that's correct, the issue is in the YAML, but that is just how YAML works as per the spec, it's not specific to the Go YAML package.

[aned]

Thanks @grobinson-grafana for the input, closing it, hopefully it'll save some time for someone troubleshooting the same issue.