diff --git a/include/regex.h b/include/regex.h
index 2c49a41d8097..21a7d303b087 100644
--- a/include/regex.h
+++ b/include/regex.h
@@ -75,6 +75,7 @@ typedef struct {
 #define	REG_DUMP	0200
 
 /* regerror() flags */
+#define REG_ENOSYS	-1	/* Reserved */
 #define	REG_NOMATCH	 1
 #define	REG_BADPAT	 2
 #define	REG_ECOLLATE	 3