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