New FAPI property #pattern is introduced for native HTML5 pattern attribute

textfield, tel, email, url and password now support '#pattern' FAPI property to provide easy validation for Form API fields. Browsers could also use this property to perform client-side validation, but you can ensure it is performed server-side as well.

 /**
 * Builds a simple form using the FAPI #pattern property.
 */
function form_test_pattern_form($form, &$form_state) {
  $form['textfield'] = array(
    '#type' => 'textfield',
    '#title' => 'One digit followed by lowercase letters',
    '#pattern' => '[0-9][a-z]+',
  );
  $form['tel'] = array(
    '#type' => 'tel',
    '#title' => 'Everything except numbers',
    '#pattern' => '[^\d]*',
  );
  $form['password'] = array(
    '#type' => 'password',
    '#title' => 'Password',
    '#pattern' => '[01]+',
  );
  $form['url'] = array(
    '#type' => 'url',
    '#title' => 'Client side validation',
    '#description' => 'Just client side validation, using the #pattern attribute.',
    '#attributes' => array(
      'pattern' => '.*foo.*',
    ),
    '#pattern' => 'ignored',
  );
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => 'Submit',
  );
  return $form;
}