Understanding Javascript Regex: Fixing Common Display Name Validation Issues

If you’ve recently ventured into web development, you might find yourself grappling with regex (regular expressions) in Javascript. One common use is validating user inputs, such as a display name during registration. In this blog post, we tackle a particular problem: how to correctly validate a display name to allow only letters, numbers, and underscores.

The Problem at Hand

When creating a registration form, it’s crucial to ensure that the display name adheres to specific criteria. Here’s a snippet of code that was meant to achieve this goal:

<form method="post" action="/" onsubmit="return check_form()">
    <input type="text" id="display-name" name="display-name" maxlength="255" />
    <input type="submit" />
</form>
<script type="text/javascript">
    var name_regex = /^([a-zA-Z0-9_])+/;

    function check_form() {
        if (!name_regex.test(document.forms[0].elements[0].value)) {
            document.forms[0].elements[0].focus();
            alert("Your display name may only contain letters, numbers and underscores");
            return false;
        }
    }
</script>

Despite the best intentions, this code failed to restrict the display name correctly. Let’s explore why this happened and how to fix it.

Breaking Down the Regex Issue

Current Regex Explanation

The current regex used is: /^([a-zA-Z0-9_])+/

Here’s what it does:

1. Starts with (^): The pattern begins with the start of the string.
2. Matches (([a-zA-Z0-9_])+): It checks for one or more letters (both cases), numbers, or underscores.

While this logic seems sound, the problem lies in that the regex does not verify what comes after the initial match. It will allow any characters after a valid starting sequence, leading to unexpected results.

The Solution: Modifying the Regex

The goal is to ensure the entire string consists of valid characters. To achieve this, we need to adjust the regex to enforce the following criteria:

• It must start with a valid character.
• It must end right after a valid character, disallowing anything else.

The modified regex should look like this:

/^([a-zA-Z0-9_]+)$/

• Added ($): The dollar sign at the end matches the end of the string. This inclusion confirms that only letters, numbers, or underscores are allowed throughout the entire string.

Improving the Code: A More Robust Approach

Accessing Form Elements

Moreover, while the current code uses document.forms, a more standard and reliable way would be to directly access the input by its ID. This can make your code cleaner and less prone to issues if you decide to change the structure of your HTML.

Here’s how you could refactor the check_form function:

function check_form() {
    var displayName = document.getElementById('display-name').value;
    if (!name_regex.test(displayName)) {
        document.getElementById('display-name').focus();
        alert("Your display name may only contain letters, numbers, and underscores");
        return false;
    }
}

Conclusion

Ensuring that user inputs are valid is paramount in web development. With just a few adjustments to your regex and how you access form elements, you can enhance your registration forms significantly. By adopting the revised regex pattern /^([a-zA-Z0-9_]+)$/ and using document.getElementById, your application will be more reliable and user-friendly.

Armed with this knowledge, you’re one step closer to mastering Javascript and providing users with a smoother registration experience. Happy coding!