javascript to avoid special characters ?

Question

i want to write a javascript code to avoid special characters in the textbox but,

it should allow  comma , round brackets and forward slash 

 

below is the javascript code which i have used it avoid all the special characters

<script type="text/javascript">  // added by subin to block special characters
   function blockSpecialChar(e){
       var k;
       document.all ? k = e.keyCode : k = e.which;
       return ((k > 64 && k < 91) || (k > 96 && k < 123) || k == 8 || k == 32 || (k >= 48 && k <= 57));
       }
</script>