vmagent relabeling error re-writing regex

[cskinfill]

Describe the bug
Regression when vmagent loads a relabeling config it is failing to parse regex because it is incorrectly rewriting it.

To Reproduce
It appears to be any relabeling regex with an escaped $ (so \$) at the end of the regex string.

For example

- source_labels: [__name__]
  action: replace
  regex: '(.*)\$$'
  target_label: "__name__"
  replacement: "$1"

Expected behavior
The relabeling config will continue to work.

Logs

2022-09-19T17:37:16.055Z	fatal	VictoriaMetrics/app/vmagent/remotewrite/remotewrite.go:139	cannot load relabel configs: cannot load -remoteWrite.relabelConfig="/etc/vm/relabeling/global_relabeling.yaml": cannot unmarshal `relabel_configs` from "/etc/vm/relabeling/global_relabeling.yaml": error when parsing `relabel_config` #5: cannot parse `regex` "^(?:(.*)\\)$": error parsing regexp: missing closing ): `^(?:(.*)\)$`

Version
The line returned when passing --version command line flag to the binary. For example:

vmagent-20220908-110742-tags-v1.81.2-0-gdd1126dcc

Used command-line flags

-graphiteListenAddr=":2003"
-httpListenAddr=":8429"
-influx.maxLineSize="8MB"
-promscrape.cluster.memberNum="1"
-promscrape.cluster.membersCount="5"
-promscrape.config="/etc/vmagent/config_out/vmagent.env.yaml"
-promscrape.maxScrapeSize="1GiB"
-promscrape.streamParse="true"
-promscrape.suppressDuplicateScrapeTargetErrors="true"
-promscrape.suppressScrapeErrors="true"
-remoteWrite.maxDiskUsagePerURL="1073741824"
-remoteWrite.relabelConfig="/etc/vm/relabeling/global_relabeling.yaml"
-remoteWrite.tmpDataPath="/tmp/vmagent-remotewrite-data"
-remoteWrite.url="secret"

It appears to be this function

VictoriaMetrics/lib/regexutil/regexutil.go

Line 11 in e9a0b80

func RemoveStartEndAnchors(expr string) string {

which will loop and remove any $ in the regex string or substrings.

[valyala]

The issue should be fixed in the commit fda60b3 . This commit will be included in the next release. In the mean time it is possible to build vmagent from this commit according to these instructions and verify whether it correctly handles regex ending with escaped $ in relabeling rules.

[valyala]

vmagent should properly handle escaped $ char at the end of regex in relabeling configs starting from v1.82.0. Closing the issue as fixed.