const string input = "Lorem ipsum dolor sit %download%#456 amet, consectetur adipiscing %download%#3434 elit. Duis non nunc nec mauris feugiat porttitor. Sed tincidunt blandit dui a viverra%download%#298. Aenean dapibus nisl %download%#893434 id nibh auctor vel tempor velit blandit.";

static void Main(string[] args)
{
    Regex expression = new Regex(@"%download%#(?<Identifier>[0-9]*)");
    var results = expression.Matches(input);
    foreach (Match match in results)
    {
        Console.WriteLine(match.Groups["Identifier"].Value);
    }
}

Regex filter = new Regex(@"(\d+)");

foreach (var item in checkedListBox1.CheckedItems)
{
    var match = filter.Match(item.ToString());
    if (match.Success)
    {
        MessageBox.Show(match.Value);
    }    
}

var regex = new Regex(@"(?<=%download%#)\d+");
return regex.Matches(strInput);