HTML

pattern input html

<form action="/action_page.php" enctype="multipart/form-data" >
  <label>Name</label>
  <input type="text" name="name" pattern="[A-Za-z0-9]{3}" ><br><br>
  <input type="submit">
</form>

html5 input pattern name

echo '<input type="text" maxlength="32" name="first_name" pattern="[A-Za-z]{1,32}" value="'.$_SESSION['user_first_name'].'" required>';

pattern input html

<form action="/action_page.php" enctype="multipart/form-data" >
  <label>Name</label>
  <input type="text" name="name" pattern="[A-Za-z]{3}" ><br><br>
  <input type="submit">
</form>