Document regular expressions in sync_back.ts

[mbg]

Follow up to #3529 to address @esbena's review comment.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

Environments:

• Testing/None - This change does not impact any CodeQL workflows in production.

How did/will you validate this change?

• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).
• End-to-end tests - I am depending on PR checks (i.e. tests in pr-checks).

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Development/testing only - This change cannot cause any failures in production.

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• Other - Please provide details.

Are there any special considerations for merging or releasing this change?

• No special considerations - This change can be merged at any time.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull request overview

This PR improves the readability/maintainability of the pr-checks/sync_back.ts sync-back script by documenting and centralizing the regular expressions it uses to detect and update GitHub Action references.

Changes:

• Adds detailed documentation and names for the regexes used to extract and replace action references.
• Refactors regex construction/replacement into reusable SyncBackPattern builders plus a shared escaping helper.

Comments suppressed due to low confidence (2)

pr-checks/sync_back.ts:77

• The comment for YAML_PATTERN says it groups uses: actionName and rest_of_line, but the regex only captures uses: actionName (group 1) and does not capture the remainder of the line. Please correct the comment (or add a second capture group if that was the intent).

/**
 * Used to find lines containing action references in a PR check specification.
 *
 * Matches `uses: actionName@rest_of_line` in PR check specifications and groups `uses: actionName`
 * and `rest_of_line`, allowing `rest_of_line` to be replaced with a new version string.
 */
const YAML_PATTERN: SyncBackPattern = (actionName: string) =>
  new RegExp(`(uses:\\s+${actionName})@(?:[^@\n]+)`, "g");

pr-checks/sync_back.ts:52

• ESCAPE_PATTERN is used to escape regex metacharacters before interpolating actionName into a RegExp, not for escaping TypeScript/YAML string literals. Consider tweaking this comment to avoid implying it handles string escaping.

/**
 * Used to identify characters in `action_name` strings that need to
 * be escaped before inserting them into TypeScript or YAML strings.
 */
const ESCAPE_PATTERN = /[.*+?^${}()|[\]\\]/g;