getElementsByClassName兼容低版本浏览器

var getElementsByClassName = function (searchClass, node,tag) {
     if(document.getElementsByClassName){
         var nodes = (node || document).getElementsByClassName(searchClass),result = [];
         for(var i=0 ;node = nodes[i++];){
            if(tag !== "*" && node.tagName === tag.toUpperCase()){
                 result.push(node)
             }else{
                 result.push(node)
             }
         }
     return result
     }else{
         node = node || document;
         tag = tag || "*";
         var classes = searchClass.split(" "),
         elements = (tag === "*" && node.all)? node.all : node.getElementsByTagName(tag),
         patterns = [],
         current,
         match;
         var i = classes.length;
         while(--i >= 0){
             patterns.push(new RegExp("(^|s)" + classes[i] + "(s|$)"));
         }
         var j = elements.length;
         while(--j >= 0){
             current = elements[j];
            match = false;
            for(var k=0, kl=patterns.length; k<kl; k++){
                match = patterns[k].test(current.className);
                if (!match) break;
                }    
                if (match) result.push(current);
            }
         return result;
         }
     }