Edit in JSFiddle

$('input').on('change', function(){

    var index = parseInt( $('input:first').val(), 10 ),
    search1 = $('input:eq(1)').val(),
    search2 = $('input:last').val(),
    
    // basic replacement
    result1 = search1.replace(/\{index\}/, index),
    
    // regex math
    result2 = search2
        .replace(/\{index([-+]\d+)?\}/g, function(fullstring, match){
            return index + (match ? parseInt(match, 10) : 0);
        });

    $('span:first').html(result1);
    $('span:last').html(result2);

}).change();