Case-insensitivity in regex breaks CI only statement

Summary

Using a /i flag in an only: regex statement breaks regex matching.

Steps to reproduce

1. Create a new project with a gitlab-ci.yml containing the following statements

build:
  only:
    - /^R\d+S\d+$/
    - /BRANCH/
  script:
    - echo "Hello World"

2. Create a new branch called BRANCH
3. Verify that the CI job is executing.
4. Delete BRANCH branch
5. Amend gitlab-ci.yml to the following

  only:
    - /^R\d+S\d+$/i
    - /BRANCH/i
  script:
    - echo "Hello World"

6. Create BRANCH branch.
7. Verify that the CI pipeline was not registered.

Example Project

https://gitlab.com/DylanBartlett/regex-test/

What is the current bug behavior?

Regex statements with the /i flag set do not work at all

What is the expected correct behavior?

Regex statement should match and become case insensitive.

Output of checks

This bug happens on GitLab.com