How to count how many checkboxes has been checked

I have an HTML table the first column consists of checkboxes etc when the user clicks a button I want to check if any of the checkboxes have been checked before going to the code behind etc.

This is what I have but it keeps throwing an error

Microsoft JScript runtime error: Syntax error, unrecognized expression: input.checkbox:checked"]

This is my code I just want to return the count of how many checkboxes are actually checked.

       $('#BtnExportToExcel').click(function () {
              var check = ('#gvPerformanceResult').find('input.checkbox:checked"]').length;
              alert(check);
        return false;
        }):

Thanks for your help.