var html = "<div>div and other content</div>";
       var word = "div";
       var replaceContent = "111";
       var reg1 = new RegExp("([\\<][^\\<\\>]*)(" + word + ")([^\\>\\<]*[\\>])", "ig");
       var str1 = html.replace(reg1, "$1{" + "$2" + "}$3");
       alert(str1);
       var reg2 = new RegExp("(^|[^\\{]+)" + word + "([^\\{]+|$)", "ig");
       var str2 = str1.replace(reg2, "$1" + replaceContent + "$2");
       alert(str2);
       var reg3 = new RegExp("\\{" + word + "\\}", "ig");
       var str3 = str2.replace(reg3, word);
       alert(str3);

------解决方案--------------------
替换了之后你要把值赋回去啊 要不怎么替换呢 
  
 [code=JScript] 
 //关键字高亮 
   function highLight(keyWords)  
 {   
   if(keyWords !=null) 
   { 
   var array = keyWords.substring(1,keyWords.length-1); 
   array = array.split( ", "); 
  var html = document.getElementById( "tds ").innerHTML; 
   for(var i=0;i <array.length;i++) 
   {   
   var oneKey = array[i]; 
   var replaceContent =  " <a style= 'color:red '>  "+oneKey+ " </a>  "; 
   var reg1 = new RegExp( "([\\ <][^\\ <\\> ]*)( " + oneKey +  ")([^\\> \\ <]*[\\> ]) ",  "ig "); 
   var str1 = html.replace(reg1,  "$1{ " +  "$2 " +  "}$3 "); 
   var reg2 = new RegExp( "(^|[^\\{]+) " + oneKey +  "([^\\{]+|$) ",  "ig "); 
   var str2 = str1.replace(reg2,  "$1 " + replaceContent +  "$2 "); 
   var reg3 = new RegExp( "\\{ " + oneKey +  "\\} ",  "ig "); 
   html = str2.replace(reg3, oneKey); 
  
   } 
   document.getElementById( "tds ").innerHTML = html; 
   } 
     
   } 
  
 [/code]