Outlook contextual addin can't find PLUS sign with regex

My rule looks like this

<ExtensionPoint xsi:type="DetectedEntity">
  <Label resid="contextLabel" />
  <SourceLocation resid="testResId" />
  <Rule xsi:type="RuleCollection" Mode="And">
    <Rule xsi:type="RuleCollection" Mode="Or">
      <Rule xsi:type="ItemIs" ItemType="Message" />
      <Rule xsi:type="ItemIs" ItemType="Appointment" />
    </Rule>
    <Rule xsi:type="ItemHasRegularExpressionMatch" RegExName="Test" RegExValue="\+1" PropertyName="BodyAsPlaintext" Highlight="All"/>
  </Rule>
</ExtensionPoint>

as you can see the regex is looking for a +1 to highlight (my real regex does more than that, but this example is enough to reproduce the issue)

It seems like the search works, since it returns the results, but i see errors in the dev console and highlight is not happening. My guess is Outlook is using the actual matches to highlight and put them into a new regex (which in fact cannot start with a plus sign). The exception is:

constants.ts:1 Uncaught (in promise) SyntaxError: Invalid regular expression: /+1/: Nothing to repeat
    at new RegExp (<anonymous>)
    at constants.ts:1
    at Array.forEach (<anonymous>)
    at markElements.ts:54
    at AAQkADAzYTZlYTYyLWQyMGMtNDFhOS05MTAzLTQyYjJhNjBhMTYwZgAQAINoPf%2BPCdhNj6AVUpiXz4w%3D:347
    at Object.next (AAQkADAzYTZlYTYyLWQyMGMtNDFhOS05MTAzLTQyYjJhNjBhMTYwZgAQAINoPf%2BPCdhNj6AVUpiXz4w%3D:347)
    at a (AAQkADAzYTZlYTYyLWQyMGMtNDFhOS05MTAzLTQyYjJhNjBhMTYwZgAQAINoPf%2BPCdhNj6AVUpiXz4w%3D:347)

I did not find any documentation on the office addin development sites if/how should i escape these. Any idea?