Ensure the following before filing this issue

• I verified it reproduces with the latest version with - uses: ruby/setup-ruby@v1 (see Versioning policy)

• I tried to reproduce the issue locally by following the workflow steps (including all commands done by ruby/setup-ruby, except for Downloading Ruby & Extracting Ruby),
  and it did not reproduce locally (if it does reproduce locally, it's not a ruby/setup-ruby issue)

Are you running on a GitHub-hosted runner or a self-hosted runner?

GitHub-hosted runner

Link to the failed workflow job (must be a public workflow job, so the necessary information is available)

https://github.com/rileychh/repros/actions/runs/21517607329/job/61999846713

Any other notes?

setup-ruby crashes when parsing mise.toml if the ruby version line contains an end-of-line comment (which is valid TOML syntax).

Error

TypeError: Cannot read properties of undefined (reading 'match')
    at parseRubyEngineAndVersion

Steps to Reproduce

1. Create a mise.toml with an EOL comment:

   [tools]
   ruby = "3.4"  # For Fastlane and CocoaPods

2. Use setup-ruby without explicitly specifying ruby-version:

   - uses: ruby/setup-ruby@v1
     with:
       bundler-cache: true

3. Run the workflow

Expected Behavior

setup-ruby should parse ruby = "3.4" correctly and use version 3.4, ignoring the comment (as per TOML specification where # starts a comment).

Actual Behavior

Action crashes because the regex fails to match the line, resulting in undefined.match() call.

Root Cause

In index.js, the regex for parsing mise.toml anchors to end-of-line:

const regexp = /^ruby\s*=\s*['"](.+)['"]$/

This regex requires the line to end immediately after the closing quote:

• Matches: ruby = "3.4"
• No match: ruby = "3.4" # comment

When no lines match, filter()[0] returns undefined, causing the subsequent .match() call to crash.

Suggested Fix

Update the regex to allow optional trailing content:

const regexp = /^ruby\s*=\s*['"](.+?)['"]/

Or more explicitly handle comments:

const regexp = /^ruby\s*=\s*['"](.+)['"]\s*(?:#.*)?$/

Workarounds

1. Remove comments from the ruby line in mise.toml
2. Create a .ruby-version file (checked before mise.toml)
3. Explicitly specify ruby-version: 'X.Y' in workflow