code: function findInstanceOffset(anno, text) {
      var re = new RegExp('\\b'+anno.exact+'\\b', 'g'), cur = 1, match;
      while ((match = re.exec(text)) !== null) {
        if (cur === anno.instance) {
          return match.index;
        }
        cur++;
      }
    }
docs: find the starting position of an instance