diff --git a/re.h b/re.h
index 88a67705d5..e38dcb7f3b 100644
--- a/re.h
+++ b/re.h
@@ -36,4 +36,6 @@ void rb_match_busy _((VALUE));
 
 EXTERN int ruby_ignorecase;
 
+int rb_reg_mbclen2 _((unsigned int, VALUE));
+#define mbclen2(c,re) rb_reg_mbclen2((c),(re))
 #endif