get only the last match? .match(word)

My suggestion is that you show only the last entry of your results.

You can do that by changing the line: var name = content.match(word);

to

var names = content.match(word);
var name = names[names.length - 1];

On more detail, what this does is it gets all the results from your regex, then it attributes the last item of the array to the name variable.

Hope this was helpful.

Besides getting all matches and obtain the last one, you can use capture groups to get the last match:

var word=/.*(@\w+)/i;
var name = content.match(word)[1];  

Or using exec, the whole would look like:

var word=/.*(@\w+)/i;

$("#comment").on("input",function() { //changed keyup to input

var content=$(this).val();
var match = word.exec(content); 

if(match){
 $("#result").text(match[1]);
}

});

Fiddle

PS, if your goal is a more generic approach and you need to switch between getting all words and a single one, I'd recommend keeping the global match and getting the last as in Jonas' answer.

You can simply select or pop the last match in the array of match returned by .match()

var word=/@(\w+)/ig;

$("#comment").on("keyup",function() {

    var content=$(this).val();
    var matches = content.match(word);
    var lastmatch = matches.pop();

    //IF YOU NEED TO KEEP INTACT THE VAR MATCHES
    //var lastmatch = matches[matches.length - 1];


    if(name.length>0){
        $("#result").text(lastmatch);
    }
    return false();
});

JSFiddle