Tutorial Add :nth-of-type to jQuery

$.expr[':']['nth-of-type'] = function(elem, i, match) {
    var parts = match[3].split("+");
    return (i + 1 - (parts[1] || 0)) % parseInt(parts[0], 10) === 0;
};

See the original article for more info on adding the other "of type" selectors.