gistfile1.php

<?php

/**
 * Implements hook_form_alter().
 */
function MYMODULE_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'user_register_form') {
    $form['#validate'][] = 'MYMODULE_user_register_form_validate';
  }
}

/**
 * Additional handler for user_register_form form validation.
 */
function MYMODULE_user_register_form_validate($form, &$form_state) {
  if (!preg_match('/^.*(?=.{10,})(?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[@#$%^&+=]).*$/', $form_state['values']['pass'])) {
    form_set_error('pass', t('Password must be at least 10 characters and contain at least one lower case letter, one upper case letter, one digit and one special character.'));
  }
}