http://jstv.codefairy.org/qunit?base=https%3A%2F%2Fgist.github.com%2Fraw%2F751419%2F;script=jquery.replaceClass.js;script=qunit.replaceClass.js

jquery.replaceClass.js

(function($) {
	$.fn.replaceClass = function(proc) {
		return this.each(function() {
			var classes = this.className.split(/\s+/), new_classes = [];
			for (var i = 0, klass, ret; klass = classes[i]; ++i) {
				ret = proc.call(this, i, klass);
				if (ret) {
					new_classes.push(typeof ret == 'string' ? ret : classes[i]);
				}
			}
			this.className = new_classes.join(' ');
		});
	};	
})(jQuery);

qunit.replaceClass.js

module('$.replaceClass');

test('replaceClass', 3, function() {
	var div;
	div = $('<div class="foo bar baz"/>');
	div.replaceClass(function(i, klass) {
		return klass == 'foo';
	});
	equal(div.attr('class'), 'foo');

	div = $('<div class="foo bar baz"/>');
	div.replaceClass(function(i, klass) {
		return /a/.test(klass);
	});
	equal(div.attr('class'), 'bar baz');

	div = $('<div class="foo bar baz"/>');
	div.replaceClass(function(i, klass) {
		return klass+='2';
	});
	equal(div.attr('class'), 'foo2 bar2 baz2');
});