1 files changed, 7 insertions, 3 deletions
diff --git a/src/koi8.c b/src/koi8.c
index 37023c6..90a04f9 100644
--- a/src/koi8.c
+++ b/src/koi8.c
@@ -2,7 +2,7 @@
   koi8.c -  Oniguruma (regular expression library)
 **********************************************************************/
 /*-
- * Copyright (c) 2002-2019  K.Kosako
+ * Copyright (c) 2002-2020  K.Kosako
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -105,12 +105,16 @@ static const unsigned short EncKOI8_CtypeTable[256] = {
 
 
 static int
-koi8_mbc_case_fold(OnigCaseFoldType flag ARG_UNUSED,
+koi8_mbc_case_fold(OnigCaseFoldType flag,
                    const UChar** pp, const UChar* end ARG_UNUSED, UChar* lower)
 {
   const UChar* p = *pp;
 
-  *lower = ENC_KOI8_TO_LOWER_CASE(*p);
+  if (CASE_FOLD_IS_NOT_ASCII_ONLY(flag) || ONIGENC_IS_ASCII_CODE(*p))
+    *lower = ENC_KOI8_TO_LOWER_CASE(*p);
+  else
+    *lower = *p;
+
   (*pp)++;
   return 1;
 }