Custom Alert When Checking Data Input, Javascript

I would like to displa开发者_C百科y a custom alert, rather than simply true or false. I have:

function isValid(test) {

     return /^[a-zA-Z]{2}(?:\d{6}|\d{8}|\d{10})$/.test(test);
}

function checkValid(){

     var userEntry = document.getElementById("entry1").value;

     alert(isValid(firstRef));

}

So depending on whether what the user is valid or not they get the message "true" or "false". I would like the user to get a customised message if their input returns false such as "Invalid format try again" and get no message displayed when they input the correct data. Could I somehow use an if statement along the lines of if true then.... else...?

function checkValid(){

     var userEntry = document.getElementById("entry1").value;

     if (!isValid(firstRef)) {
         alert("Invalid format try again.");
     }

}

Change your checkValid function to something like this one:

function checkValid(){

    var userEntry = document.getElementById("entry1").value;

    if (isValid(firstRef)) {
        alert("It is valid!");
    }
    else {
        alert("It is invalid");
    }
}