After enabling the "Accessibility enabled" option for your personal user settings you observe that the Regex Markup button is now missing from the Event Rule editor.

This is expected behavior.

1. Log in to a London instance
2. Navigate to Event Rules
3. Select the "AWS LB binding" rule
4. Go to "Transform and Compose Alert Output"
5. Select a Regex expression from the "Expressions" section
6. Note that Regex Markup button is present: 
7. Open the User Settings (top-right cogwheel), enable "Accessibility enabled"
8. Reload the page
9. Repeat steps 2 to 6, note that the markup button is missing

Disable the "Accessibility enabled" option in the user settings