var html='ddfsdfsdfdsd dfsdfsdffds<img _src="http://localhost:8490/60E86EA7-FE7B-44BF-8270-4DF2036B2118.PNG" data-latex="x=\frac {-b\pm \sqrt {{b}^{2}-4ac}} {2a}" src="http://localhost:8490/60E86EA7-FE7B-44BF-8270-4DF2036B2118.PNG" class="kfformula">dfsdfsrewewrererew  ds<img _src="http://localhost:8490/60E86EA7-FE7B-44BF-8270-4DF2036B2118.PNG" data-latex="x=\frac {-b\pm \sqrt {{b}^{2}-4ac}} {2a}" src="http://localhost:8490/60E86EA7-FE7B-44BF-8270-4DF2036B2118.PNG" class="kfformula">dfsdfsdfsddfs<img _src="http://localhost:849060E86EA7-FE7B-44BF-8270-4DF2036B2118.PNG" data-latex="x=\frac {-b\pm \sqrt {{b}^{2}-4ac}} {2a}" src="http://localhost:8490/60E86EA7-FE7B-44BF-8270-4DF2036B2118.PNG" class="kfformula">ddfdsdfd<img _src="http://localhost:849060E86EA7-FE7B-44BF-8270-4DF2036B2118.PNG" data-latex="x=\frac {-b\pm \sqrt {{b}^{2}-4ac}} {2a}" src="http://localhost:8490/60E86EA7-FE7B-44BF-8270-4DF2036B2118.PNG" class="kfformula">dkjdfskjfdskl';

String.prototype.replaceAll = function(s1,s2){

　　return this.replace(new RegExp(s1,"gm"),s2);

}

var appendHtml="";

var keyword="s";

var partArray=html.split("<");

$.each(partArray,function(key,value){

   //console.log("value="+value+";key="+key);

   var secPartArray=value.split('>');

  $.each(secPartArray,function(key1,value1){

    if(value1.indexOf("img")==0)

    {

      appendHtml+="<"+value1+">";

    }

    else

    {

      appendHtml+=value1.replaceAll(keyword,"<strong>"+value1+"</strong>");

    }

   console.log(value1);

  });

});

console.log(appendHtml);