diff --git a/lib/regex.h b/lib/regex.h
index 6f3bae3..21fc00c 100644
--- a/lib/regex.h
+++ b/lib/regex.h
@@ -23,6 +23,12 @@
 
 #include <sys/types.h>
 
+/* IBM z/OS uses -D__string=1 as an inclusion guard.  */
+#if defined __MVS__ && defined(__string)
+# undef __string
+# define __string __string
+#endif
+
 /* Allow the use in C++ code.  */
 #ifdef __cplusplus
 extern "C" {
diff --git a/lib/string.in.h b/lib/string.in.h
index b3356bb..fa438a4 100644
--- a/lib/string.in.h
+++ b/lib/string.in.h
@@ -44,6 +44,12 @@
 #ifndef address@hidden@_STRING_H
 #define address@hidden@_STRING_H
 
+/* IBM z/OS uses -D__string=1 as an inclusion guard.  */
+#if defined __MVS__ && defined(__string)
+# undef __string
+# define __string __string
+#endif
+
 /* NetBSD 5.0 mis-defines NULL.  */
 #include <stddef.h>