Javascript validation for special characters in ASP.NET/ C#

function validatespecailchars()
{

     var txtbox = document.getElementById('<%=txtsearch.ClientID %>').value;
   
      var str="<";
        var str1=">";
        var str2="--";
        var str3=";";
        var str4="'";

    if((txtbox.indexOf(str)>=0)||(txtbox.indexOf(str1)>=0)||(txtbox.indexOf(str2)>=0)||      (txtbox.indexOf(str3)>=0)||(txtbox.indexOf(str4)>=0))
    {
         alert('Special Characters are not Allowed.');
        // you can focus here that particular control
     }
}