A different understanding leads me to a better solution (my apologies for jumping to a  conclusion):

**[Demo](http://jsfiddle.net/pVLFN/)**

    (function($){
        $.expr[':'].classes = function(o,i,m,s){
            var classes = parseInt(m[3] || 1, 10);
            return o.className.split(/\s+/).length >= classes;
        };
    })(jQuery);
    
    // Elements with 2+ classes
    $('div:classes(2)').css('color','red');
    
    // Elements with 3+ classes
    $('div:classes(3)').css('font-weight', 'bold');
    
    // Elements with 4+ classes
    $('div:classes(4)').css('background-color','blue');