var arr = {

    content: "你好#D12345#,hello#D321#world",

    lawitems: {

        "D12345": "aaaa",

        "D321": "bbbbb"

    }

}

arr.content = arr.content.replace(/#([^#]+)#/g, function (match, $1) {

    return arr.lawitems[$1]||match;//沒有在lawitems找到返回原字符串

})

var reg = /#(Dd+)#/g;//正則寫的不是很規(guī)范，如果你的編號是你給出的DEMO那樣，還是可以用的
answer.content = answer.content.replace(reg,function($1,$2){return answer.lawitems[$2];})