function removeNonHTML(str) {
  // 匹配HTML注释的结束标记
  var regex = /<!--[\s\S]*?-->/g;
  
  // 提取HTML注释的结束标记
  var matches = str.match(regex);
  
  // 将提取到的HTML注释的结束标记拼接成字符串
  var result = matches ? matches.join("") : "";
  
  return result;
}

// 示例用法
var html = "<div>This is a <b>sample</b> HTML <!-- comment --> string.</div>";
var cleaned = removeNonHTML(html);
console.log(cleaned);