JWT regex for a specific aud

The tooltip actually talks about validating the token, i.e., the header, before sending it to Cognito. I do not think it makes a lot of sense to use it for a Cognito authorizer. It makes more sense when using a Lambda authorizer to make sure that the header contains the value in the right format. This will save you cost in the cases the format is invalid and then API Gateway will not invoke the function.