diff --git a/system/core/Lang.php b/system/core/Lang.php
index c0cd34a..9aaf944 100644
--- a/system/core/Lang.php
+++ b/system/core/Lang.php
@@ -106,7 +106,7 @@
 
 		$langfile .= '.php';
 
-		if (empty($idiom) OR ! ctype_alpha($idiom))
+		if (empty($idiom) OR ! preg_match('/^[a-z_-]$/i', $idiom))
 		{
 			$config =& get_config();
 			$idiom = empty($config['language']) ? 'english' : $config['language'];