var test_value = document.getElementById('a').value;
    var reg = /(\<img)[^\<.]*(\/\>)/gi;
    var aaa = test_value.replace(reg,function($1,$2,$3){
        return $1.replace($2,'[img').replace($3,'\/]');
    });
    alert("aaa="+aaa);

------解决方案--------------------
var dom = document.body.innerHTML;
dom = dom.replace(/\<(img.*\/)\>/g, "[$1]");
document.body.innerHTML = dom;
可能不是太好，不过能够实现，望高人指点

------解决方案--------------------

JScript code
var s='<img src="xxxx" />';
alert(s.replace(/<img([^>]+)>/ig,'[img$1]'));