Index: libcxx/include/regex
===================================================================
--- libcxx/include/regex
+++ libcxx/include/regex
@@ -169,15 +169,15 @@
     // assign:
     basic_regex& assign(const basic_regex& that);
     basic_regex& assign(basic_regex&& that) noexcept;
-    basic_regex& assign(const charT* ptr, flag_type f = regex_constants::ECMAScript);
-    basic_regex& assign(const charT* p, size_t len, flag_type f);
+    basic_regex& assign(const charT* ptr,           flag_type f = regex_constants::ECMAScript);
+    basic_regex& assign(const charT* p, size_t len, flag_type f = regex_constants::ECMAScript);
     template <class string_traits, class A>
         basic_regex& assign(const basic_string<charT, string_traits, A>& s,
-                            flag_type f = regex_constants::ECMAScript);
+                                                    flag_type f = regex_constants::ECMAScript);
     template <class InputIterator>
         basic_regex& assign(InputIterator first, InputIterator last,
-                            flag_type f = regex_constants::ECMAScript);
-    basic_regex& assign(initializer_list<charT>, flag_type = regex_constants::ECMAScript);
+                                                    flag_type f = regex_constants::ECMAScript);
+    basic_regex& assign(initializer_list<charT>,    flag_type = regex_constants::ECMAScript);
 
     // const operations:
     unsigned mark_count() const;
@@ -2617,7 +2617,7 @@
     basic_regex& assign(const value_type* __p, flag_type __f = regex_constants::ECMAScript)
         {return assign(__p, __p + __traits_.length(__p), __f);}
     _LIBCPP_INLINE_VISIBILITY
-    basic_regex& assign(const value_type* __p, size_t __len, flag_type __f)
+    basic_regex& assign(const value_type* __p, size_t __len, flag_type __f = regex_constants::ECMAScript)
         {return assign(__p, __p + __len, __f);}
     template <class _ST, class _SA>
         _LIBCPP_INLINE_VISIBILITY
Index: libcxx/test/std/re/re.regex/re.regex.assign/assign_ptr_size_flag.pass.cpp
===================================================================
--- libcxx/test/std/re/re.regex/re.regex.assign/assign_ptr_size_flag.pass.cpp
+++ libcxx/test/std/re/re.regex/re.regex.assign/assign_ptr_size_flag.pass.cpp
@@ -18,6 +18,16 @@
 
 int main(int, char**)
 {
+    std::regex r0;
+    r0.assign("(a([bc]))", 9);
+    assert(r0.flags() == std::regex::ECMAScript);
+    assert(r0.mark_count() == 2);
+
+    std::regex r1;
+    r1.assign("(a([bc]))", 9, std::regex::ECMAScript);
+    assert(r1.flags() == std::regex::ECMAScript);
+    assert(r1.mark_count() == 2);
+
     std::regex r2;
     r2.assign("(a([bc]))", 9, std::regex::extended);
     assert(r2.flags() == std::regex::extended);