diff --git a/include/boost/regex/v4/icu.hpp b/include/boost/regex/v4/icu.hpp
index 9724b0f8..7e70f57e 100644
--- a/include/boost/regex/v4/icu.hpp
+++ b/include/boost/regex/v4/icu.hpp
@@ -187,7 +187,7 @@ namespace boost {
       }
       char_type translate_nocase(char_type c) const
       {
-         return ::u_tolower(c);
+         return ::u_foldCase(c, U_FOLD_CASE_DEFAULT);
       }
       char_type translate(char_type c, bool icase) const
       {
diff --git a/include/boost/regex/v5/icu.hpp b/include/boost/regex/v5/icu.hpp
index a9264965..f172553d 100644
--- a/include/boost/regex/v5/icu.hpp
+++ b/include/boost/regex/v5/icu.hpp
@@ -161,7 +161,7 @@ public:
    }
    char_type translate_nocase(char_type c) const
    {
-      return ::u_tolower(c);
+      return ::u_foldCase(c, U_FOLD_CASE_DEFAULT);
    }
    char_type translate(char_type c, bool icase) const
    {