pattern attribute

• Specifies a regular expression the input value must match

• Applies to text, password, email, and url types of the input element, and the textarea element

• Regular expression dialect is same as JavaScript's (ECMA 262)

<form>
<p>
  <label>Card number: 
  <input type="text" name="cardnumber" 
         pattern='\d\d\d\d ?\d\d\d\d ?\d\d\d\d ?\d(\d\d\d)?'/>
  </label>
</p>
<p>
  <label>
  <input type="submit" value="Send data" />
  </label>
</p>
</form>