diff --git a/src/s/osf5-0.h b/src/s/osf5-0.h
index b084ab8c887..54cfb16823f 100644
--- a/src/s/osf5-0.h
+++ b/src/s/osf5-0.h
@@ -2,8 +2,17 @@
 #include "osf1.h"
 
 /* It's enough to define _OSF_SOURCE instead of _BSD.  */
+
+/* -nointrinsics is required to avoid losing with definitions of POSIX
+   stuff in regex.c, for instance, where regex_t isn't consistent
+   with the system defintion.  This may be necessary also on OSF 4
+   systems with dtk 6.4 installed.  */
 #undef C_SWITCH_SYSTEM
+#ifdef __GNUC__
 #define C_SWITCH_SYSTEM	-D_OSF_SOURCE
+#else
+#define C_SWITCH_SYSTEM	-D_OSF_SOURCE -nointrinsics
+#endif
 
 #define WAIT_USE_INT
 #define SYS_SIGLIST_DECLARED