diff --git a/include/boost/regex/v4/regex_traits_defaults.hpp b/include/boost/regex/v4/regex_traits_defaults.hpp
index 26b9252c..1dbb0bf9 100644
--- a/include/boost/regex/v4/regex_traits_defaults.hpp
+++ b/include/boost/regex/v4/regex_traits_defaults.hpp
@@ -33,6 +33,10 @@
 #include <boost/regex/config.hpp>
 #include <boost/cstdint.hpp>
 
+#include <cctype>
+#include <cwctype>
+#include <locale>
+
 #ifndef BOOST_REGEX_SYNTAX_TYPE_HPP
 #include <boost/regex/v4/syntax_type.hpp>
 #endif
diff --git a/include/boost/regex/v5/regex_traits_defaults.hpp b/include/boost/regex/v5/regex_traits_defaults.hpp
index bc6ea8d4..8d829f38 100644
--- a/include/boost/regex/v5/regex_traits_defaults.hpp
+++ b/include/boost/regex/v5/regex_traits_defaults.hpp
@@ -38,6 +38,7 @@
 #include <type_traits>
 #include <cstdint>
 #include <cctype>
+#include <locale>
 #include <cwctype>
 
 #ifdef BOOST_NO_STDC_NAMESPACE