selector.js

var rx = {
    id: /^#([\w\-]+)$/,
    className: /^\.([\w\-]+)$/,
    tagName: /^[\w\-]+$/,
    name: /^\[name=["']?([\w\-]+)["']?\]$/
};

var ConciseSelector = function(selector, context) {
    var result, m;
    if(!context) {
        context = document;
    }
    if(m = rx.id.exec(selector)) {
        result = [document.getElementById(m[1])];
    } else if(m = rx.className.exec(selector)) {
        result = context.getElementsByClassName(m[1]);
    } else if(m = rx.tagName.exec(selector)) {
        result = context.getElementsByTagName(m[1]);
    } else if(m = rx.name.exec(selector)) {
        result = context.getElementsByName(m[1]);
    } else {
        result = context.querySelectorAll(selector);
    }
};
var $ = function(selector, context) {
    return new ConciseSelector(selector, context);
};