diff --git a/lib/libc/regex/regex.3 b/lib/libc/regex/regex.3
index 8959272e9891..d22dec1e87f7 100644
--- a/lib/libc/regex/regex.3
+++ b/lib/libc/regex/regex.3
@@ -32,7 +32,7 @@
 .\"	@(#)regex.3	8.4 (Berkeley) 3/20/94
 .\" $FreeBSD$
 .\"
-.Dd May 25, 2016
+.Dd April 15, 2017
 .Dt REGEX 3
 .Os
 .Sh NAME
@@ -183,6 +183,17 @@ compatible with but not specified by
 .St -p1003.2 ,
 and should be used with
 caution in software intended to be portable to other systems.
+.It Dv REG_POSIX
+Compile only
+.St -p1003.2
+compliant expressions.
+This flag has no effect unless linking against
+.Nm libregex .
+This is an extension,
+compatible with but not specified by
+.St -p1003.2 ,
+and should be used with
+caution in software intended to be portable to other systems.
 .El
 .Pp
 When successful,