required pattern min max และ maxlength

<label for="email">Email (required)</label>
<input type="email" id="email" name="email" required>
<!-- Empty on submit → browser shows "Please fill in this field" -->
<!-- Also: field is :invalid until filled -->

<label for="username">Username (letters and numbers only)</label>
<input
  type="text"
  id="username"
  name="username"
  pattern="[a-zA-Z0-9]{3,20}"
  title="3-20 letters and numbers"
>
<!-- pattern anchored at both ends automatically -->
<!-- title provides the error message hint -->

<!-- Phone: digits, spaces, hyphens -->
<input type="tel" pattern="[0-9 ()+-]{7,20}">

<!-- Postal code: US zip code -->
<input type="text" pattern="[0-9]{5}(-[0-9]{4})?">

<!-- Password: min 8 chars, one uppercase, one digit -->
<input type="password" pattern="(?=.*[A-Z])(?=.*[0-9]).{8,}">

<!-- Twitter handle: -->
<input type="text" pattern="@?[A-Za-z0-9_]{1,15}">

<!-- Number range -->
<input type="number" name="qty" min="1" max="100">

<!-- Date range -->
<input type="date" name="checkin" min="2025-01-01" max="2025-12-31">

<!-- Range slider -->
<input type="range" name="volume" min="0" max="100" step="5">

<!-- Time -->
<input type="time" name="meeting" min="09:00" max="17:00">

<input type="text" name="username" minlength="3" maxlength="20">
<!-- minlength: minimum number of characters required -->
<!-- maxlength: maximum characters allowed (browser enforces hard stop) -->

<textarea name="bio" minlength="50" maxlength="500"></textarea>
<!-- Same attributes work on textarea -->

<input type="number" name="price" min="0" step="0.01">
<!-- Accepts decimal prices: 0.00, 0.01, 4.99, etc. -->

<input type="number" name="qty" min="0" step="5">
<!-- Only multiples of 5: 0, 5, 10, 15... -->

<input type="range" min="0" max="100" step="10">
<!-- Snaps to: 0, 10, 20, ..., 100 -->

input:valid {
  border-color: #22c55e;   /* green when valid */
}

input:invalid {
  border-color: #ef4444;   /* red when invalid */
}

/* Avoid showing :invalid on page load (before user interaction): */
input:not(:placeholder-shown):invalid {
  border-color: #ef4444;
}
/* Only show invalid style after user has typed something */

<input
  type="text"
  pattern="[A-Za-z]{2,3}"
  title="Enter a 2 or 3 letter country code (e.g. US, GB, DEU)"
  name="country-code"
>
<!-- Browser shows the title text alongside the default error message -->

<input
  type="text"
  name="username"
  required
  minlength="3"
  maxlength="20"
  pattern="[a-z][a-z0-9_]*"
  title="Lowercase letters, numbers, underscores. Must start with a letter."
>
<!-- All constraints must pass for the field to be :valid -->

<input type="email" id="email">

<script>
const input = document.getElementById('email');
input.addEventListener('blur', () => {
  // Check validity on blur (when user leaves field):
  if (!input.validity.valid) {
    input.reportValidity();
  }
});
</script>