I think this is a bug with the internal

isSimple

RegExp that jQuery uses to determine whether a selector is "simple" or not. I think the original intent was for selectors with a comma in them to be not simple.

Try running these:

var isSimple = /^.[^:#\\[\\.]*$/;
isSimple.test('code, #mark'); // false
isSimple.test('#mark, code'); // true, when it should be false

This doesn't cause any problems in jQuery 1.2.6, but it does break in 1.3b1 with Sizzle integrated.

This can be fixed by adding a comma into the RegExp:

isSimple = /^.[^:#\\[\\.,]*$/

This doesn't affect any tests with jQuery 1.2.6 and fixes a bug in 1.3b1 where this works:

jQuery('#ap *').not('code, #mark')

but this breaks with a "match[3] is undefined" error:

jQuery('#ap *').not('#mark, code')

I don't have a patch but I do have my patch and tests on my Github fork of Sizzle at http://github.com/chuyeow/sizzle/commit/0eb905887b7f20533d1d6945d9dc8a58ba0bc7f7 and http://github.com/chuyeow/sizzle/commit/243327d1bcf3f2009e837ec8a8004010887192d2.