diff --git a/wp-admin/admin-functions.php b/wp-admin/admin-functions.php
index ab6be722e..0828d677f 100644
--- a/wp-admin/admin-functions.php
+++ b/wp-admin/admin-functions.php
@@ -329,13 +329,13 @@ function add_user() {
 }
 
 function edit_user($user_id = 0) {
-	global $current_user, $wp_roles;
+	global $current_user, $wp_roles, $wpdb;
 
 	if ($user_id != 0) {
 		$update = true;
 		$user->ID = $user_id;
 		$userdata = get_userdata($user_id);
-		$user->user_login = $userdata->user_login;
+		$user->user_login = $wpdb->escape($userdata->user_login);
 	} else {
 		$update = false;
 		$user = '';
@@ -406,6 +406,9 @@ function edit_user($user_id = 0) {
 	if (!empty ($pass1))
 		$user->user_pass = $pass1;
 
+	if ( !validate_username($user->user_login) )
+		$errors['user_login'] = __('<strong>ERROR</strong>: This username is invalid.  Please enter a valid username.');
+
 	if (!$update && username_exists($user->user_login))
 		$errors['user_login'] = __('<strong>ERROR</strong>: This username is already registered, please choose another one.');
 
diff --git a/wp-includes/functions-formatting.php b/wp-includes/functions-formatting.php
index 274d7e6db..6f955e8bc 100644
--- a/wp-includes/functions-formatting.php
+++ b/wp-includes/functions-formatting.php
@@ -265,13 +265,18 @@ function remove_accents($string) {
 	return $string;
 }
 
-function sanitize_user( $username ) {
+function sanitize_user( $username, $strict = false ) {
 	$raw_username = $username;
 	$username = strip_tags($username);
 	// Kill octets
 	$username = preg_replace('|%([a-fA-F0-9][a-fA-F0-9])|', '', $username);
 	$username = preg_replace('/&.+?;/', '', $username); // Kill entities
-	return apply_filters('sanitize_user', $username, $raw_username);
+
+	// If strict, reduce to ASCII for max portability.
+	if ( $strict )
+		$username = preg_replace('|[^a-z0-9 _.-@]|i', '', $username);
+
+	return apply_filters('sanitize_user', $username, $raw_username, $strict);
 }
 
 function sanitize_title($title, $fallback_title = '') {
diff --git a/wp-includes/registration-functions.php b/wp-includes/registration-functions.php
index a2313dcc7..ff9db3d22 100644
--- a/wp-includes/registration-functions.php
+++ b/wp-includes/registration-functions.php
@@ -10,6 +10,16 @@ function username_exists( $username ) {
 	return null;
 }
 
+function validate_username( $username ) {
+	$name = sanitize_user($username, true);
+	$valid = true;
+
+	if ( $name != $username )
+		$valid = false;	
+
+	return apply_filters('validate_username', $valid, $username);	
+}
+
 function wp_insert_user($userdata) {
 	global $wpdb;
 
@@ -24,6 +34,8 @@ function wp_insert_user($userdata) {
 		$user_pass = md5($user_pass);
 	}
 	
+	$user_login = sanitize_user($user_login, true);
+
 	if ( empty($user_nicename) )
 		$user_nicename = sanitize_title( $user_login );
 
diff --git a/wp-register.php b/wp-register.php
index 2176c280e..1177c1286 100644
--- a/wp-register.php
+++ b/wp-register.php
@@ -27,7 +27,10 @@ case 'register':
 		$errors['user_email'] = __('<strong>ERROR</strong>: The email address isn&#8217;t correct.');
 	}
 
-  if ( username_exists( $user_login ) )
+	if ( ! validate_username($user_login) )
+		$errors['user_login'] = __('<strong>ERROR</strong>: This username is invalid.  Please enter a valid username.');
+
+	if ( username_exists( $user_login ) )
 		$errors['user_login'] = __('<strong>ERROR</strong>: This username is already registered, please choose another one.');
 
 	/* checking the email isn't already used by another user */