Display validation message if TextBox is Empty (Null) using jQuery in ASP.Net

Can anyone tell me how to display "Please enter meter reading" if textbox is empty in below code

            if($('#DynamicTable .clsRegisterReading').length > 0)
            {
                var rv = true;
                $('#DynamicTable .clsRegisterReading').each(function (key, val) {
                    debugger;
                    var metR = $(this).val();
                    var regex = /((\d+)(\.\d{1}))$/;
                    if (regex.test(metR)) {              
                        var itemRow = "<ul id='listError'><li>" + "The Meter Reading is Valid'" + " " + "</li></ul>";
                        FUNMessageBox(itemRow, "E", 700,200, "", "");
                        //$('#txt10IV_11').css('border-color', 'red');
                       
                    } else {               
                        var itemRow = "<ul id='listError'><li>" + "The Meter Reading is Invalid" + " " + "</li></ul>";
                        FUNMessageBox(itemRow, "E", 700,200, "", "");
                        // $('#txt10IV_11').css('border-color', 'red');  
                        rv = false;
                        return false;
                    }
                });
                if(rv == false)
                    return rv;
            }