Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 45063)
+++ wp-includes/ms-functions.php	(working copy)
@@ -451,7 +451,7 @@
 	$user_name     = preg_replace( '/\s+/', '', sanitize_user( $user_name, true ) );
 
 	if ( $user_name != $orig_username || preg_match( '/[^a-z0-9]/', $user_name ) ) {
-		$errors->add( 'user_name', __( 'Usernames can only contain lowercase letters (a-z) and numbers.' ) );
+		$errors->add( 'user_name', __( 'Usernames can only contain lowercase letters (a-z) and numbers (0-9).' ) );
 		$user_name = $orig_username;
 	}
 
Index: wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php
===================================================================
--- wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php	(revision 45063)
+++ wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php	(working copy)
@@ -1130,7 +1130,7 @@
 		$username = (string) $value;
 
 		if ( ! validate_username( $username ) ) {
-			return new WP_Error( 'rest_user_invalid_username', __( 'Username contains invalid characters.' ), array( 'status' => 400 ) );
+			return new WP_Error( 'rest_user_invalid_username', __( 'Usernames can only contain lowercase letters (a-z) and numbers (0-9).' ), array( 'status' => 400 ) );
 		}
 
 		/** This filter is documented in wp-includes/user.php */
Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 45063)
+++ wp-includes/user.php	(working copy)
@@ -2437,7 +2437,7 @@
 	if ( $sanitized_user_login == '' ) {
 		$errors->add( 'empty_username', __( '<strong>ERROR</strong>: Please enter a username.' ) );
 	} elseif ( ! validate_username( $user_login ) ) {
-		$errors->add( 'invalid_username', __( '<strong>ERROR</strong>: This username is invalid because it uses illegal characters. Please enter a valid username.' ) );
+		$errors->add( 'invalid_username', __( 'Usernames can only contain lowercase letters (a-z) and numbers (0-9).' ) );
 		$sanitized_user_login = '';
 	} elseif ( username_exists( $sanitized_user_login ) ) {
 		$errors->add( 'username_exists', __( '<strong>ERROR</strong>: This username is already registered. Please choose another one.' ) );