source: trunk/www.guidonia.net/wp/wp-content/plugins/simple-tags/2.3/inc/functions.js@ 44

function addTag(tag) {
        var tag_entry = document.getElementById("tags-input");
        if ( tag_entry.value.length > 0 && !tag_entry.value.match(/,\s*$/) ) {
                tag_entry.value += ", ";
        }
        var re = new RegExp(tag + ",");
        if ( !tag_entry.value.match(re) ) {
                tag_entry.value += tag + ", ";
        }
}

Event.observe(window, 'load', function() {
        Event.observe('post', 'submit', trimTagsBeforeSend);
});
function trimTagsBeforeSend() {
        var tag_entry = document.getElementById("tags-input");
        var taille = tag_entry.value.length;

        if ( tag_entry.value.substr(taille - 2 , 2) == ', ' ) {
                tag_entry.value = tag_entry.value.substr( 0, taille - 2);
        }

        if ( tag_entry.value.substr(taille - 1, 1) == ',' ) {
                tag_entry.value = tag_entry.value.substr( 0, taille - 1);
        }
}