Index: lib/libc/regex/regcomp.c
===================================================================
--- lib/libc/regex/regcomp.c
+++ lib/libc/regex/regcomp.c
@@ -132,7 +132,7 @@
 static void p_b_eclass(struct parse *p, cset *cs);
 static wint_t p_b_symbol(struct parse *p);
 static wint_t p_b_coll_elem(struct parse *p, wint_t endc);
-static wint_t othercase(wint_t ch);
+static bool hasothercase(wint_t ch);
 static void bothcases(struct parse *p, wint_t ch);
 static void ordinary(struct parse *p, wint_t ch);
 static void nonnewline(struct parse *p);
@@ -1101,19 +1101,22 @@
 }
 
 /*
- - othercase - return the case counterpart of an alphabetic
- == static wint_t othercase(wint_t ch);
+ * Check if alphabetic character has the other case mapping.
  */
-static wint_t			/* if no counterpart, return ch */
-othercase(wint_t ch)
+static bool
+hasothercase(wint_t ch)
 {
+	wint_t other;
+
 	assert(iswalpha(ch));
+
+	other = ch;
 	if (iswupper(ch))
-		return(towlower(ch));
+		other = towlower(ch);
 	else if (iswlower(ch))
-		return(towupper(ch));
-	else			/* peculiar, but could happen */
-		return(ch);
+		other = towupper(ch);
+
+	return (other != ch);
 }
 
 /*
@@ -1131,7 +1134,7 @@
 	size_t n;
 	mbstate_t mbs;
 
-	assert(othercase(ch) != ch);	/* p_bracket() would recurse */
+	assert(hasothercase(ch));	/* p_bracket() would recurse */
 	p->next = bracket;
 	memset(&mbs, 0, sizeof(mbs));
 	n = wcrtomb(bracket, ch, &mbs);
@@ -1154,7 +1157,7 @@
 {
 	cset *cs;
 
-	if ((p->g->cflags&REG_ICASE) && iswalpha(ch) && othercase(ch) != ch)
+	if ((p->g->cflags&REG_ICASE) && iswalpha(ch) && hasothercase(ch))
 		bothcases(p, ch);
 	else if ((ch & OPDMASK) == ch)
 		EMIT(OCHAR, ch);
Index: lib/libc/regex/regex2.h
===================================================================
--- lib/libc/regex/regex2.h
+++ lib/libc/regex/regex2.h
@@ -113,7 +113,7 @@
 	wint_t		max;
 } crange;
 typedef struct {
-	unsigned char	bmp[NC / 8];
+	unsigned char	bmp[NC_MAX / 8];
 	wctype_t	*types;
 	unsigned int	ntypes;
 	wint_t		*wides;
@@ -133,9 +133,14 @@
 	if (ch < NC)
 		return (((cs->bmp[ch >> 3] & (1 << (ch & 7))) != 0) ^
 		    cs->invert);
-	for (i = 0; i < cs->nwides; i++)
-		if (ch == cs->wides[i])
+	for (i = 0; i < cs->nwides; i++) {
+		if (cs->icase) {
+			if (ch == towlower(cs->wides[i]) ||
+			    ch == towupper(cs->wides[i]))
+				return (!cs->invert);
+		} else if (ch == cs->wides[i])
 			return (!cs->invert);
+	}
 	for (i = 0; i < cs->nranges; i++)
 		if (cs->ranges[i].min <= ch && ch <= cs->ranges[i].max)
 			return (!cs->invert);
Index: lib/libc/regex/utils.h
===================================================================
--- lib/libc/regex/utils.h
+++ lib/libc/regex/utils.h
@@ -39,7 +39,9 @@
 /* utility definitions */
 #define	DUPMAX		_POSIX2_RE_DUP_MAX	/* xxx is this right? */
 #define	INFINITY	(DUPMAX + 1)
-#define	NC		(CHAR_MAX - CHAR_MIN + 1)
+#define	NC_MAX		(CHAR_MAX - CHAR_MIN + 1)
+#define	NC		((MB_CUR_MAX == 1) ? \
+			    (CHAR_MAX - CHAR_MIN + 1) : (CHAR_MAX + 1))
 typedef unsigned char uch;
 
 /* switch off assertions (if not already off) if no REDEBUG */