<script>
var keywords=[];//要替换的部分
var replaces=[];//替换成什么
var signs=[];//特定标记，不用管它

keywords[0]=/结婚用品/g;
keywords[1]=/结婚/g;

replaces[0]="<a href='http://www.sina.com.cn'>结婚用品</a>";
replaces[1]="<a href='http://www.163.com'>结婚</a>";

function convert(){
    var contentValue=content.innerHTML;
    
    //设置signs，并替换为signs
    for(var i=0;(e=keywords[i]);i++){
        signs[i]="thePartWhichWillBeReplace"+i;
        contentValue=contentValue.replace(keywords[i],signs[i]);
    }

    for(var i=0;(e=signs[i]);i++){
        while(contentValue.indexOf(signs[i])>=0){
            contentValue=contentValue.replace(signs[i],replaces[i]);
        }
    }

    content.innerHTML=contentValue;
}
</script>

------解决方案--------------------

JScript code

<script type="text/javascript">
var a='aaabaa<img src="xxx.gif" alt="aaabaa">aaabaa<img src="xxx.gif" alt="aaabaa">aaabaa<Span>ssss<Span>ssss</Span>aaabaa</Span>'
alert(a.replace(/(^|>)([^>]*?)aaab([^>]*?)(^|<)/gi,"$1$2mm$3$4"))
</script>