一个简单的dom查询函数

    var regid = /^#([w-]*)$/,
      regClass = /^.([w-]*)$/,
      regName = /^(div|a|p|ul|li|input|select|document|body|iframe)$/,
      regWhite = /s*/g;
//主要方法
 function _B(name, doc) {
        var selector, match, result = [];
        var doctemp;
        if (doc) {
            doctemp = (typeof doc == "object" && doc.length > 0 && doc[0] && doc[0].nodeType || doc && doc.nodeType) ? doc : _B(doc);
        } else {
            doctemp = document;
        }
        selector = name && name.replace(regWhite, "");
        if (selector) {
            match = regid.exec(selector);
            if (match && match[1]) {
                result = document.getElementById(match[1]);
            }
            match = regClass.exec(selector);
            if (match && match[1]) {
                var elems = doctemp.getElementsByTagName("*");
                var len = elems && elems.length || 0;
                for (var i = 0; i < len; i++) {
                    var currentelem = elems[i];
                    var classlist = currentelem.className.split(/s+/);
                    var classlen = classlist.length;
                    for (var j = 0; j < classlen; j++) {
                        if (classlist[j] == match[1]) {
                            result.push(currentelem);
                            break;
                        }
                    }
                }
            }
            match = regName.exec(selector);
            if (match && match[1]) {
                result = doctemp.getElementsByTagName(match[1]);
            }
        }
        return result ? result : null;
    }

支持_B("#id")  _B(".class")  _B("div")    _B("#id",dom)   _B(".class",dom)   _B("div" ,dom) dom默认是document