Index: lib/libc/regex/regcomp.c
===================================================================
--- lib/libc/regex/regcomp.c
+++ lib/libc/regex/regcomp.c
@@ -936,7 +936,7 @@
 	wint_t i;
 #ifndef LIBREGEX
 	struct xlocale_collate *table =
-		(struct xlocale_collate*)__get_locale()->components[XLC_COLLATE];
+	    (struct xlocale_collate*)__get_locale()->components[XLC_COLLATE];
 #endif
 	/* classify what we've got */
 	switch ((MORE()) ? PEEK() : '\0') {
@@ -984,15 +984,30 @@
 		if (start == finish)
 			CHadd(p, cs, start);
 		else {
+			/*
+			 * Below is the implementation choice we made based on
+			 * POSIX.2 defining range expressions outside of POSIX
+			 * locale to have unspecified behavior.
+			 *
+			 * 1. Do binary (non-collating) comparison in multibyte
+			 *    locales -- if character comes from POSIX locale
+			 *    range, it will be properly attributed as such by
+			 *    CHaddrange().
+			 * 2. Do collation-based comparison in single-byte
+			 *    locales as they do not seem to have the same
+			 *    problem as multi-byte ones mixing small/capital
+			 *    and other characters in the collation order.
+			 */
 #ifndef LIBREGEX
 			if (table->__collate_load_error || MB_CUR_MAX > 1) {
 #else
 			if (MB_CUR_MAX > 1) {
 #endif
-				(void)REQUIRE(start <= finish, REG_ERANGE);
+				(void) REQUIRE(start <= finish, REG_ERANGE);
 				CHaddrange(p, cs, start, finish);
 			} else {
-				(void)REQUIRE(p_range_cmp(start, finish) <= 0, REG_ERANGE);
+				(void) REQUIRE(p_range_cmp(start, finish) <= 0,
+				    REG_ERANGE);
 				for (i = 0; i <= UCHAR_MAX; i++) {
 					if (p_range_cmp(start, i) <= 0 &&
 					    p_range_cmp(i, finish) <= 0 )
Index: lib/libc/regex/regex.3
===================================================================
--- lib/libc/regex/regex.3
+++ lib/libc/regex/regex.3
@@ -32,7 +32,7 @@
 .\"	@(#)regex.3	8.4 (Berkeley) 3/20/94
 .\" $FreeBSD$
 .\"
-.Dd May 25, 2016
+.Dd January 23, 2018
 .Dt REGEX 3
 .Os
 .Sh NAME
@@ -612,6 +612,16 @@
 beginning and ending subexpressions in obsolete
 .Pq Dq basic
 REs are anchors, not ordinary characters.
+.Pp
+Range expressions outside of POSIX
+.Pq Dq C
+locale are defined to have unspecified behavior: conforming applications should
+not expect range expression to be valid, or have the well defined set of
+collating elements matched.
+Current implementation does collation-based comparison in singlebyte locales,
+and binary
+.Pq non-collating
+comparison in multibyte locales.
 .Sh DIAGNOSTICS
 Non-zero error codes from
 .Fn regcomp