WordPress MU currently has a minimum limit of four characters set in its core. Unfortunately, this limit is still imposed in WordPress MS 3.0. The limit is probably there because usernames were used for the domain too and WP-Devs didn’t want to conflict with country codes. But that is not an issue for my client, so I wanted to kill the limit (without touching core).

Basically, I wrote a quick mu-plugin that unset the error message when someone tries to add a user with fewer than four characters. Doing this removes any halts that would stop processing the new user. Here is my code:

function remove_username_char_limit($result) {
  if ( is_wp_error( $result[ 'errors' ] ) && !empty( $result[ 'errors' ]->errors ) ) {

    // Get all the error messages from $result
    $messages = $result['errors']->get_error_messages();
    $i = 0;
    foreach ( $messages as $message ) {

      // Check if any message is the char limit message
      if ( 0 == strcasecmp("Username must be at least 4 characters", $message)) {
        // Unset whole 'user_name' error array if only 1 message exists
        // and that message is the char limit error
        if ( 1 == count($messages) ) {
          unset( $result['errors']->errors['user_name'] );
        } else {
          // Otherwise just unset the char limit message
          unset( $result['errors']->errors['user_name'][$i] );
        }
      }	

      $i++;
    }
  }

  return $result;
}
add_action('wpmu_validate_user_signup', 'remove_username_char_limit');

You can easily add this code into your functions.php. I was writing it for a WordPress MU site with multiple themes and needed to make sure it was enforced, so I stuck it in the mu-plugins directory, in a file named “extended-contact-info.php”.

This is the simple code block you need:

function extended_contact_info($user_contactmethods) {
	//Use this if you want to append to the default
	//contact methods (AIM, Yahoo, Jabber)
	//$user_contactmethods += array(
	//	'building' => __('Building'),
	//	'room' => __('Room'),
	//	'phone' => __('Phone')
	//);

	//Use this if you want to ignore the default
	//contact methods (AIM, Yahoo, Jabber)
	$user_contactmethods = array(
		'building' => __('Building'),
		'room' => __('Room'),
		'phone' => __('Phone')
	);

	return $user_contactmethods;
}

add_filter('user_contactmethods', 'extended_contact_info');