Validation on Menu Link Title

function node_form_submit($form, &$form_state) {
      $item = &$form_state['values']['menu'];
 if (preg_match('/[^a-z A-Z0-9-]/', $item['link_title'])) {
    form_set_error('link_title', t('The title name may only consist of lowercase letters, numbers, and hyphens.'));
  }
  else
  {

  global $user;

  $node = node_form_submit_build_node($form, $form_state);
  $insert = empty($node->nid);
  node_save($node);
  $node_link = l(t('view'), 'node/'. $node->nid);
  $watchdog_args = array('@type' => $node->type, '%title' => $node->title);
  $t_args = array('@type' => node_get_types('name', $node), '%title' => $node->title);

  if ($insert) {
    watchdog('content', '@type: added %title.', $watchdog_args, WATCHDOG_NOTICE, $node_link);
    drupal_set_message(t('@type %title has been created.', $t_args));
  }
  else {
    watchdog('content', '@type: updated %title.', $watchdog_args, WATCHDOG_NOTICE, $node_link);
    drupal_set_message(t('@type %title has been updated.', $t_args));
  }
  if ($node->nid) {
    unset($form_state['rebuild']);
    $form_state['nid'] = $node->nid;
    $form_state['redirect'] = 'node/'. $node->nid;
  }
  else {
    // In the unlikely case something went wrong on save, the node will be
    // rebuilt and node form redisplayed the same way as in preview.
    drupal_set_message(t('The post could not be saved.'), 'error');
  }
  }
}