var strUserAgent = navigator.userAgent.toLowerCase(); 
var isIE = strUserAgent.indexOf("msie") > -1; 
var isNS6 = strUserAgent.indexOf("netscape6") > -1; 
var isNS4 = !isIE && !isNS6  && parseFloat(navigator.appVersion) < 5; 
var pattern = /\d/;
var reBkSpce = /[\x00\x08]/;
var iKeyCode, strKey
function validateKeyStroke(objEvent) {
  if (isIE) {
    iKeyCode = objEvent.keyCode;
  } else {
    iKeyCode = objEvent.which; 
  }
  strKey = String.fromCharCode(iKeyCode);
  if (pattern.test(strKey) || reBkSpce.test(strKey)) { return true; } else { return false; }
}

// usage  onkeypress="return validateKeyStroke(event)"