function _isIE() {
    var rv = -1 // Return value assumes failure.
      , ua = navigator.userAgent
      , re;
    if (navigator.appName == 'Microsoft Internet Explorer') {
      re = /MSIE ([0-9]{1,}[\.0-9]{0,})/;
      if (re.exec(ua) != null) {
        rv = parseFloat(RegExp.$1, 10);
      }
    }
    return rv;
  }
Will return the version number if the browser is IE. If not will return -1
TRY NEVER TO USE THIS AND USE FEATURE DETECTION IF POSSIBLE
@returns {Number} -1 if false or the version number if true