HTML Native Constraint Validation: required, pattern, and type

Imagine asking a friend for some information on a webpage. The field should explain exactly what you need.

What You Need to Know

Required, type=email, and pattern each check a different aspect of the entered data.

In this example: reject a malformed email in a native form.

Try the Example

Take a look at the example. We’ll explain the important lines right below it.

<form action="/join" method="post">
    <label for="club-code">Club code (3 capital letters)</label>
    <input id="club-code" name="club-code" pattern="[A-Z]{3}" required>
    <label for="email">Email</label>
    <input id="email" name="email" type="email" required>
    <button type="submit">Join</button>
</form>

Let’s Read the Code

type: Type changes what sort of control or resource the browser uses.
<form>: The form groups controls and identifies where submitted data goes.
<label>: The label tells people what the input is for. A matching for/id pair makes the label clickable.
for: For connects a label or output with a control by ID.

Read the named elements and attributes together. They explain how the browser connects this feature to the rest of the page.

What Happens in the Browser?

The HTML controls work in the browser, but saving an answer requires the example action path to be handled by a real server.

How can you test it? Submit once with an empty field, once with a clearly invalid value, and once with a valid value. Watch which rule blocks submission. These are helpful browser checks; your server must validate all received data again.

Your Turn

Try a matching value and a nonmatching one. Can you explain exactly which character fails the pattern?

Common Beginner Mistakes

Writing a pattern that rejects valid values because it was not tested with realistic examples.
Native validation helps people correct entries but cannot replace server validation.

Key Takeaways

Choose the HTML feature that explains this task clearly.
type: Type changes what sort of control or resource the browser uses.
Remember that HTML form rules help the visitor, but server-side code must decide whether received data is valid.