HTML5

Validation

Use the pattern attribute

<form action="/action_page.php">
  Country code: <input type="text" name="country_code" pattern="[A-Za-z]{3}" title="Three letter country code" required>
    <input pattern=".{3,}"   required title="3 characters minimum">
<input pattern=".{5,10}" required title="5 to 10 characters">
  <input type="submit">
</form>

https://stackoverflow.com/questions/17664537/html5-validation-easy-way-to-have-real-time-validation