wpmu-remove-username-limit.php

<?php
/**
 * @package WPMURemoveUsernameLimit
 * @version 1.0
 */
/*
Plugin Name: WPMU Remove Username Limit
Plugin URI: http://uppercase.no
Description: WPMU Plugin to Remove Username Limit. Based on blogpost: http://fullthrottledevelopment.com/remove-username-character-limit-from-wordpress-multi-site-multi-user
Author: Ole Christian Rynning
Version: 1.0
Author URI: http://uppercase.no
*/

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('user_name');
    $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');

?>