TBpnav

thank you! I found what I need:

[ Moderator Note: Please post code or markup snippets between backticks or use the code button. ]

"alt-string-pre": function ( a ) {
        return a.match(/alt="(.*?)"/)[1].toLowerCase();
    },

    "alt-string-asc": function( a, b ) {
        return ((a < b) ? -1 : ((a > b) ? 1 : 0));
    },

    "alt-string-desc": function(a,b) {
        return ((a < b) ? 1 : ((a > b) ? -1 : 0));
    }

Works perfekt!

Best R.
Tobias