Index: libcxx/include/regex
===================================================================
--- libcxx/include/regex
+++ libcxx/include/regex
@@ -1745,8 +1745,6 @@
 void
 __back_ref<_CharT>::__exec(__state& __s) const
 {
-    if (__mexp_ > __s.__sub_matches_.size())
-        __throw_regex_error<regex_constants::error_backref>();
     sub_match<const _CharT*>& __sm = __s.__sub_matches_[__mexp_-1];
     if (__sm.matched)
     {
@@ -4320,8 +4318,6 @@
             for (++__first;
                     __first != __last && '0' <= *__first && *__first <= '9'; ++__first)
                 __v = 10 * __v + *__first - '0';
-            if (__v > mark_count())
-                __throw_regex_error<regex_constants::error_backref>();
             __push_back_ref(__v);
         }
     }
@@ -4712,6 +4708,9 @@
 void
 basic_regex<_CharT, _Traits>::__push_back_ref(int __i)
 {
+    if (__i > static_cast<int>(mark_count()))
+        __throw_regex_error<regex_constants::error_backref>();
+
     if (flags() & icase)
         __end_->first() = new __back_ref_icase<_CharT, _Traits>
                                               (__traits_, __i, __end_->first());
Index: libcxx/test/std/re/re.regex/re.regex.construct/bad_backref.pass.cpp
===================================================================
--- libcxx/test/std/re/re.regex/re.regex.construct/bad_backref.pass.cpp
+++ libcxx/test/std/re/re.regex/re.regex.construct/bad_backref.pass.cpp
@@ -19,11 +19,14 @@
 #include <cassert>
 #include "test_macros.h"
 
-static bool error_badbackref_thrown(const char *pat)
+static bool error_badbackref_thrown(
+    const char *pat,
+    std::regex_constants::syntax_option_type flags =
+        std::regex_constants::ECMAScript)
 {
     bool result = false;
     try {
-        std::regex re(pat);
+        std::regex re(pat, flags);
     } catch (const std::regex_error &ex) {
         result = (ex.code() == std::regex_constants::error_backref);
     }
@@ -34,6 +37,7 @@
 {
     assert(error_badbackref_thrown("\\1abc"));      // no references
     assert(error_badbackref_thrown("ab(c)\\2def")); // only one reference
+    assert(error_badbackref_thrown("(cat)\\1", std::regex_constants::basic));
 
 //  this should NOT throw, because we only should look at the '1'
 //  See https://bugs.llvm.org/show_bug.cgi?id=31387