diff --git a/llvm/lib/Support/regcomp.c b/llvm/lib/Support/regcomp.c
index 6bf89c9cf1b9..354e359f676b 100644
--- a/llvm/lib/Support/regcomp.c
+++ b/llvm/lib/Support/regcomp.c
@@ -1008,7 +1008,7 @@ p_b_coll_elem(struct parse *p,
 {
 	char *sp = p->next;
 	struct cname *cp;
-	int len;
+	size_t len;
 
 	while (MORE() && !SEETWO(endc, ']'))
 		NEXT();
@@ -1018,7 +1018,7 @@ p_b_coll_elem(struct parse *p,
 	}
 	len = p->next - sp;
 	for (cp = cnames; cp->name != NULL; cp++)
-		if (strncmp(cp->name, sp, len) == 0 && cp->name[len] == '\0')
+		if (strncmp(cp->name, sp, len) == 0 && strlen(cp->name) == len)
 			return(cp->code);	/* known name */
 	if (len == 1)
 		return(*sp);	/* single character */
diff --git a/llvm/unittests/Support/RegexTest.cpp b/llvm/unittests/Support/RegexTest.cpp
index 5e3ce39f0057..7e44a3c0614a 100644
--- a/llvm/unittests/Support/RegexTest.cpp
+++ b/llvm/unittests/Support/RegexTest.cpp
@@ -171,4 +171,12 @@ TEST_F(RegexTest, MatchInvalid) {
   EXPECT_FALSE(r1.match("X"));
 }
 
+// https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3727
+TEST_F(RegexTest, OssFuzz3727Regression) {
+  // Wrap in a StringRef so the NUL byte doesn't terminate the string
+  Regex r(StringRef("[[[=GS\x00[=][", 10));
+  std::string Error;
+  EXPECT_FALSE(r.isValid(Error));
+}
+
 }