diff --git a/src/keys.cc b/src/keys.cc
index 4b9a83ec..3e0277e0 100644
--- a/src/keys.cc
+++ b/src/keys.cc
@@ -78,7 +78,7 @@ KeyList parse_keys(StringView str)
         StringView desc{it.base()+1, end_it.base()};
         if (desc.length() > 2 and desc[1_byte] == '-')
         {
-            switch(tolower(desc[0_byte]))
+            switch(to_lower(desc[0_byte]))
             {
                 case 'c': modifier = Key::Modifiers::Control; break;
                 case 'a': modifier = Key::Modifiers::Alt; break;
@@ -94,7 +94,7 @@ KeyList parse_keys(StringView str)
             result.push_back(canonicalize_ifn({ modifier, name_it->key }));
         else if (desc.char_length() == 1)
             result.push_back(Key{ modifier, desc[0_char] });
-        else if (tolower(desc[0_byte]) == 'f' and desc.length() <= 3)
+        else if (to_lower(desc[0_byte]) == 'f' and desc.length() <= 3)
         {
             int val = str_to_int(desc.substr(1_byte));
             if (val >= 1 and val <= 12)
diff --git a/src/normal.cc b/src/normal.cc
index 9164342d..8d557dbf 100644
--- a/src/normal.cc
+++ b/src/normal.cc
@@ -114,7 +114,7 @@ void goto_commands(Context& context, NormalParams params)
             if (not cp)
                 return;
             auto& buffer = context.buffer();
-            switch (tolower(*cp))
+            switch (to_lower(*cp))
             {
             case 'g':
             case 'k':
@@ -246,7 +246,7 @@ void view_commands(Context& context, NormalParams params)
 
         const ByteCoord cursor = context.selections().main().cursor();
         Window& window = context.window();
-        switch (tolower(*cp))
+        switch (to_lower(*cp))
         {
         case 'v':
         case 'c':
@@ -306,26 +306,32 @@ void replace_with_char(Context& context, NormalParams)
     }, "replace with char", "enter char to replace with\n");
 }
 
-Codepoint to_lower(Codepoint cp) { return tolower(cp); }
-Codepoint to_upper(Codepoint cp) { return toupper(cp); }
-
 Codepoint swap_case(Codepoint cp)
 {
-    Codepoint res = std::tolower(cp);
-    return res == cp ? std::toupper(cp) : res;
+    Codepoint res = to_lower(cp);
+    return res == cp ? to_upper(cp) : res;
 }
 
 template<Codepoint (*func)(Codepoint)>
-void for_each_char(Context& context, NormalParams)
+void for_each_codepoint(Context& context, NormalParams)
 {
+    using Utf8It = utf8::iterator<BufferIterator, utf8::InvalidPolicy::Pass>;
+
     ScopedEdition edition(context);
-    Vector<String> sels = context.selections_content();
-    for (auto& sel : sels)
+    Buffer& buffer = context.buffer();
+    SelectionList& selections = context.selections();
+    Vector<String> strings;
+    for (auto& sel : selections)
     {
-        for (auto& c : sel)
-            c = func(c);
+        String str;
+        for (auto begin = Utf8It{buffer.iterator_at(sel.min()), buffer},
+                  end = Utf8It{buffer.iterator_at(sel.max()), buffer}+1;
+             begin != end; ++begin)
+            utf8::dump(std::back_inserter(str), func(*begin));
+
+        strings.push_back(std::move(str));
     }
-    context.selections().insert(sels, InsertMode::Replace);
+    selections.insert(strings, InsertMode::Replace);
 }
 
 void command(Context& context, NormalParams)
@@ -612,7 +618,7 @@ void regex_prompt(Context& context, const String prompt, T func)
 template<SelectMode mode, Direction direction>
 void search(Context& context, NormalParams params)
 {
-    const char reg = tolower(params.reg ? params.reg : '/');
+    const char reg = to_lower(params.reg ? params.reg : '/');
     regex_prompt(context, direction == Forward ? "search:" : "reverse search:",
                  [reg](Regex ex, PromptEvent event, Context& context) {
                      if (ex.empty())
@@ -627,7 +633,7 @@ void search(Context& context, NormalParams params)
 template<SelectMode mode, Direction direction>
 void search_next(Context& context, NormalParams params)
 {
-    const char reg = tolower(params.reg ? params.reg : '/');
+    const char reg = to_lower(params.reg ? params.reg : '/');
     StringView str = context.main_sel_register_value(reg);
     if (not str.empty())
     {
@@ -664,7 +670,7 @@ void use_selection_as_search_pattern(Context& context, NormalParams params)
         patterns.push_back(std::move(content));
     }
 
-    const char reg = tolower(params.reg ? params.reg : '/');
+    const char reg = to_lower(params.reg ? params.reg : '/');
 
     context.print_status({
         format("register '{}' set to '{}'", reg, patterns[sels.main_index()]),
@@ -679,7 +685,7 @@ void use_selection_as_search_pattern(Context& context, NormalParams params)
 
 void select_regex(Context& context, NormalParams params)
 {
-    const char reg = tolower(params.reg ? params.reg : '/');
+    const char reg = to_lower(params.reg ? params.reg : '/');
     regex_prompt(context, "select:", [reg](Regex ex, PromptEvent event, Context& context) {
         if (ex.empty())
             ex = Regex{context.main_sel_register_value(reg)};
@@ -692,7 +698,7 @@ void select_regex(Context& context, NormalParams params)
 
 void split_regex(Context& context, NormalParams params)
 {
-    const char reg = tolower(params.reg ? params.reg : '/');
+    const char reg = to_lower(params.reg ? params.reg : '/');
     regex_prompt(context, "split:", [reg](Regex ex, PromptEvent event, Context& context) {
         if (ex.empty())
             ex = Regex{context.main_sel_register_value(reg)};
@@ -1074,7 +1080,7 @@ void start_or_end_macro_recording(Context& context, NormalParams params)
         context.input_handler().stop_recording();
     else
     {
-        const char reg = tolower(params.reg ? params.reg : '@');
+        const char reg = to_lower(params.reg ? params.reg : '@');
         if (not is_basic_alpha(reg) and reg != '@')
             throw runtime_error("Macros can only use the '@' and alphabetic registers");
         context.input_handler().start_recording(reg);
@@ -1089,7 +1095,7 @@ void end_macro_recording(Context& context, NormalParams)
 
 void replay_macro(Context& context, NormalParams params)
 {
-    const char reg = tolower(params.reg ? params.reg : '@');
+    const char reg = to_lower(params.reg ? params.reg : '@');
     if (not is_basic_alpha(reg) and reg != '@')
         throw runtime_error("Macros can only use the '@' and alphabetic registers");
 
@@ -1280,7 +1286,7 @@ void spaces_to_tabs(Context& context, NormalParams params)
 
 void save_selections(Context& context, NormalParams params)
 {
-    const char reg = tolower(params.reg ? params.reg : '^');
+    const char reg = to_lower(params.reg ? params.reg : '^');
     if (not is_basic_alpha(reg) and reg != '^')
         throw runtime_error("selections can only be saved to the '^' and alphabetic registers");
 
@@ -1296,7 +1302,7 @@ void save_selections(Context& context, NormalParams params)
 
 void restore_selections(Context& context, NormalParams params)
 {
-    const char reg = tolower(params.reg ? params.reg : '^');
+    const char reg = to_lower(params.reg ? params.reg : '^');
     if (not is_basic_alpha(reg) and reg != '^')
         throw runtime_error("selections can only be saved to the '^' and alphabetic registers");
 
@@ -1617,9 +1623,9 @@ static NormalCmdDesc cmds[] =
 
     { Key::Escape, "end macro recording", end_macro_recording },
 
-    { '`', "convert to lower case in selections", for_each_char<to_lower> },
-    { '~', "convert to upper case in selections", for_each_char<to_upper> },
-    { alt('`'),  "swap case in selections", for_each_char<swap_case> },
+    { '`', "convert to lower case in selections", for_each_codepoint<to_lower> },
+    { '~', "convert to upper case in selections", for_each_codepoint<to_upper> },
+    { alt('`'),  "swap case in selections", for_each_codepoint<swap_case> },
 
     { '&', "align selection cursors", align },
     { alt('&'), "copy indentation", copy_indent },
diff --git a/src/ranked_match.cc b/src/ranked_match.cc
index a55df314..a7fd4fb3 100644
--- a/src/ranked_match.cc
+++ b/src/ranked_match.cc
@@ -24,7 +24,7 @@ static int count_word_boundaries_match(StringView candidate, StringView query)
         if (not is_word_boundary)
             continue;
 
-        const Codepoint lc = tolower(c);
+        const Codepoint lc = to_lower(c);
         for (; qit != query.end(); ++qit)
         {
             const Codepoint qc = *qit;
@@ -43,7 +43,7 @@ static int count_word_boundaries_match(StringView candidate, StringView query)
 
 static bool smartcase_eq(Codepoint query, Codepoint candidate)
 {
-    return query == (islower(query) ? tolower(candidate) : candidate);
+    return query == (islower(query) ? to_lower(candidate) : candidate);
 }
 
 static bool subsequence_match_smart_case(StringView str, StringView subseq)
diff --git a/src/register_manager.cc b/src/register_manager.cc
index e3f52bcb..64e56926 100644
--- a/src/register_manager.cc
+++ b/src/register_manager.cc
@@ -76,7 +76,7 @@ Register& RegisterManager::operator[](StringView reg)
 
 Register& RegisterManager::operator[](Codepoint c)
 {
-    c = tolower(c);
+    c = to_lower(c);
     if (c < 32 or c > 127)
         throw runtime_error(format("invalid register name: '{}'", c));
 
diff --git a/src/unicode.hh b/src/unicode.hh
index b74ebcfc..37f3541b 100644
--- a/src/unicode.hh
+++ b/src/unicode.hh
@@ -63,6 +63,12 @@ inline CharCategories categorize(Codepoint c)
                              : CharCategories::Punctuation;
 }
 
+inline Codepoint to_lower(Codepoint cp) { return towlower((wchar_t)cp); }
+inline Codepoint to_upper(Codepoint cp) { return towupper((wchar_t)cp); }
+
+inline char to_lower(char c) { return c >= 'A' and c <= 'Z' ? c - 'A' + 'a' : c; }
+inline char to_upper(char c) { return c >= 'a' and c <= 'z' ? c - 'a' + 'A' : c; }
+
 }
 
 #endif // unicode_hh_INCLUDED