diff --git a/lib/regex.c b/lib/regex.c
index eda11b535..247f5f1d7 100644
--- a/lib/regex.c
+++ b/lib/regex.c
@@ -127,7 +127,8 @@ init_syntax_once ()
 /* isalpha etc. are used for the character classes.  */
 #include <ctype.h>
 
-#ifndef isascii
+#if !defined (isascii) || defined (STDC_HEADERS)
+#undef isascii
 #define isascii(c) 1
 #endif