The original code in the .attr method:

return elem.filter ?

(parseFloat( elem.filter.match(/opacity=([^)]*)/)[1] ) / 100).toString():"";

If we defined a filter to the animated element, but don't modifie the opacity under jQuery amination, then the match will return null. This problem like http://dev.jquery.com/ticket/1101#comment:5, but we can't use this expression.

(parseFloat( (elem.filter.match(/opacity=([^)]*)/) || [])[1] )/100).toString();

This returns a "NaN" string. We should use this:

return elem.filter && /opacity=/.test(elem.filter)?
(elem.filter.match(/opacity=(\\d*)/)[1] / 100).toString() : "";

Now the division always returns a number, then we don't call the parseFloat function. Here you are the corrected code:

if ( jQuery.browser.msie && name == "opacity" ) {
  if ( value != undefined ) {
    // IE has trouble with opacity if it does not have layout
    // Force it by setting the zoom level
    elem.zoom = 1;

    // Set the alpha filter to set the opacity
    elem.filter = (elem.filter || "").replace(/alpha\\([^)]*\\)/, "") +
      (isNaN(parseFloat(value)) ? "" : "alpha(opacity=" + value * 100 + ")");
  }

  return elem.filter && /opacity=/.test(elem.filter)?
    (elem.filter.match(/opacity=(\\d*)/)[1] / 100).toString() : "";
}

And there are two .match callings in the curCSS method, but the .test is faster like .match to checking.

if (prop.match(/float/i))
if (/float/i.test(prop))