{{2}([a-zA-Z0-9]+)}{2}

let questionIndex = 0;
myString.replace(/{{2}([a-z0-9]+)}{2}/gmi,
  function (fullExpression, matchedGroup1) {
    // NOTE fullExpression is {{abc} for example
    //      matchedGroup1 is the 'abc' itself

    questionIndex++;

    return `{{Question ${questionIndex}}}`;
  }
);