I had an occasion where I needed to find all links of with a certain class which also did not have a blank title attribute. I thought to use the attributeNotEqual filter found here: http://docs.jquery.com/Selectors/attributeNotEqual

I ran something similar to this code:

$(".hello[title!='']").css("background-color","red");

And firebug returns:

uncaught exception: Syntax error, unrecognized expression: title!='']

Demo here: http://jsbin.com/ibupo

Editable demo: http://jsbin.com/ibupo/edit

Putting anything inside the quotes works as expected, but apparently you can't find elements without a blank attribute.