diff --git a/phpBB/includes/functions_user.php b/phpBB/includes/functions_user.php
index bf2eb2c4dd..7df2b0a60b 100644
--- a/phpBB/includes/functions_user.php
+++ b/phpBB/includes/functions_user.php
@@ -104,7 +104,7 @@ function validate_username($username)
 		return false;
 	}
 
-	if (!preg_match('#^' . $config['allow_name_chars'] . '$#i', $username))
+	if (!preg_match('#^' . str_replace('\\\\', '\\', $config['allow_name_chars']) . '$#i', $username))
 	{
 		return 'INVALID_CHARS';
 	}