We must support PCRE, but i don't think we do at the moment 👍

Something is definitely wrong 😃

According to https://docs.gitlab.com/ee/ci/jobs/job_control.html#compare-a-variable-to-a-regex-pattern the regex syntax is called RE2.

Great, we need to utilize this package if possible then.
https://www.npmjs.com/package/re2

@firecow I wish I would understood your first comment earlier. After digging into this subject for 3 hours I found out that the image you posted clearly shows that the specific regex mentioned in this issue should work using RegExp without the need for RE2 or PCRE . gitlab-ci-local handles the following case perfectly fine:

build-fake:
  stage: build
  script:
    - echo fake build
  rules:
    - if: $CI_COMMIT_TAG =~ /^(\d+(\.\d+)*|latest)$/

To be clear, yes we should support RE2 to allow things like '/^([[:digit:]]+(\.[[:digit:]]+)*|latest)$/' which are not supported by JS RegExp. But, there is another bug that causes the mentioned REGEX expansion to fail:

since REGEX is a variable it is replaced with JSON.stringify(envs[name]) which replaces every \ with \\. So, the evaluated result becomes: "1.11".match(/^(\\d+(\.\\d+)*|latest)$/) != null instead of "1.11".match(/^(\d+(\.\d+)*|latest)$/) != null.