diff --git a/library/think/Lang.php b/library/think/Lang.php
index a7ac042c..23887a2c 100644
--- a/library/think/Lang.php
+++ b/library/think/Lang.php
@@ -211,7 +211,7 @@ class Lang
             }
         }
 
-        if (preg_match('/^([a-z\d\-]+)/i', $langSet, $matches)) {
+        if (preg_match('/^([a-z0-9\-_]{2,10})$/i', $langSet, $matches)) {
             $langSet = strtolower($matches[1]);
         } else {
             $langSet = self::$range;