diff --git a/Source/CPack/cmCPackInnoSetupGenerator.cxx b/Source/CPack/cmCPackInnoSetupGenerator.cxx
index 9522e86b6f..4b8533bb5e 100644
--- a/Source/CPack/cmCPackInnoSetupGenerator.cxx
+++ b/Source/CPack/cmCPackInnoSetupGenerator.cxx
@@ -4,12 +4,12 @@ file LICENSE.rst or https://cmake.org/licensing for details. */
 #include "cmCPackInnoSetupGenerator.h"
 
 #include <algorithm>
-#include <cctype>
 #include <cstdlib>
 #include <ostream>
 #include <utility>
 
 #include "cmsys/RegularExpression.hxx"
+#include "cmsys/String.h"
 
 #include "cmCPackComponentGroup.h"
 #include "cmCPackLog.h"
@@ -116,7 +116,7 @@ int cmCPackInnoSetupGenerator::PackageFiles()
     if (cmSystemTools::LowerCase(i) == "english") {
       params["MessagesFile"] = "\"compiler:Default.isl\"";
     } else {
-      i[0] = static_cast<char>(std::toupper(static_cast<unsigned char>(i[0])));
+      i[0] = static_cast<char>(cmsysString_toupper(i[0]));
       params["MessagesFile"] = cmStrCat("\"compiler:Languages\\", i, ".isl\"");
     }
 
diff --git a/Source/CPack/cmCPackRPMGenerator.cxx b/Source/CPack/cmCPackRPMGenerator.cxx
index 19cbee1159..2bc60c2746 100644
--- a/Source/CPack/cmCPackRPMGenerator.cxx
+++ b/Source/CPack/cmCPackRPMGenerator.cxx
@@ -3,12 +3,13 @@
 #include "cmCPackRPMGenerator.h"
 
 #include <algorithm>
-#include <cctype>
 #include <map>
 #include <ostream>
 #include <utility>
 #include <vector>
 
+#include "cmsys/String.h"
+
 #include "cmCPackComponentGroup.h"
 #include "cmCPackGenerator.h"
 #include "cmCPackLog.h"
@@ -143,7 +144,7 @@ int cmCPackRPMGenerator::PackageComponents(bool ignoreGroup)
            ++compIt) {
         std::string component(compIt->first);
         std::transform(component.begin(), component.end(), component.begin(),
-                       [](unsigned char c) { return std::toupper(c); });
+                       cmsysString_toupper);
 
         if (this->IsOn("CPACK_RPM_" + compIt->first + "_DEBUGINFO_PACKAGE") ||
             this->IsOn("CPACK_RPM_" + component + "_DEBUGINFO_PACKAGE")) {
@@ -157,7 +158,7 @@ int cmCPackRPMGenerator::PackageComponents(bool ignoreGroup)
            compGIt != this->ComponentGroups.end(); ++compGIt) {
         std::string component(compGIt->first);
         std::transform(component.begin(), component.end(), component.begin(),
-                       [](unsigned char c) { return std::toupper(c); });
+                       cmsysString_toupper);
 
         if (this->IsOn("CPACK_RPM_" + compGIt->first + "_DEBUGINFO_PACKAGE") ||
             this->IsOn("CPACK_RPM_" + component + "_DEBUGINFO_PACKAGE")) {
@@ -174,8 +175,7 @@ int cmCPackRPMGenerator::PackageComponents(bool ignoreGroup)
           if (!compIt->second.Group) {
             std::string component(compIt->first);
             std::transform(component.begin(), component.end(),
-                           component.begin(),
-                           [](unsigned char c) { return std::toupper(c); });
+                           component.begin(), cmsysString_toupper);
 
             if (this->IsOn("CPACK_RPM_" + compIt->first +
                            "_DEBUGINFO_PACKAGE") ||
@@ -207,8 +207,7 @@ int cmCPackRPMGenerator::PackageComponents(bool ignoreGroup)
 
     std::string mainComponentUpper(mainComponent);
     std::transform(mainComponentUpper.begin(), mainComponentUpper.end(),
-                   mainComponentUpper.begin(),
-                   [](unsigned char c) { return std::toupper(c); });
+                   mainComponentUpper.begin(), cmsysString_toupper);
 
     // The default behavior is to have one package by component group
     // unless CPACK_COMPONENTS_IGNORE_GROUP is specified.
@@ -220,7 +219,7 @@ int cmCPackRPMGenerator::PackageComponents(bool ignoreGroup)
            compGIt != this->ComponentGroups.end(); ++compGIt) {
         std::string component(compGIt->first);
         std::transform(component.begin(), component.end(), component.begin(),
-                       [](unsigned char c) { return std::toupper(c); });
+                       cmsysString_toupper);
 
         if (mainComponentUpper == component) {
           // main component will be handled last
@@ -242,7 +241,7 @@ int cmCPackRPMGenerator::PackageComponents(bool ignoreGroup)
         if (!compIt->second.Group) {
           std::string component(compIt->first);
           std::transform(component.begin(), component.end(), component.begin(),
-                         [](unsigned char c) { return std::toupper(c); });
+                         cmsysString_toupper);
 
           if (mainComponentUpper == component) {
             // main component will be handled last
@@ -285,7 +284,7 @@ int cmCPackRPMGenerator::PackageComponents(bool ignoreGroup)
            ++compIt) {
         std::string component(compIt->first);
         std::transform(component.begin(), component.end(), component.begin(),
-                       [](unsigned char c) { return std::toupper(c); });
+                       cmsysString_toupper);
 
         if (mainComponentUpper == component) {
           // main component will be handled last
diff --git a/Source/CursesDialog/cmCursesColor.cxx b/Source/CursesDialog/cmCursesColor.cxx
index c9add4f0e8..a9e5ff6fe0 100644
--- a/Source/CursesDialog/cmCursesColor.cxx
+++ b/Source/CursesDialog/cmCursesColor.cxx
@@ -2,12 +2,13 @@
    file LICENSE.rst or https://cmake.org/licensing for details.  */
 #include "cmCursesColor.h"
 
-#include <cctype>
 #include <cstdlib>
 #include <cstring>
 #include <unordered_map>
 #include <utility>
 
+#include "cmsys/String.h"
+
 #include "cmCursesStandardIncludes.h"
 
 bool cmCursesColor::HasColors()
@@ -42,7 +43,7 @@ short cmCursesColor::GetColor(char id, short fallback)
   if (!initialized) {
     if (auto* v = getenv("CCMAKE_COLORS")) {
       while (v[0] && v[1] && v[1] == '=') {
-        auto const n = std::toupper(static_cast<unsigned char>(*v));
+        char const n = cmsysString_toupper(*v);
 
         char buffer[12];
         memset(buffer, 0, sizeof(buffer));
diff --git a/Source/cmCTest.cxx b/Source/cmCTest.cxx
index f503e93039..3b8232561b 100644
--- a/Source/cmCTest.cxx
+++ b/Source/cmCTest.cxx
@@ -34,6 +34,7 @@
 #include "cmsys/Directory.hxx"
 #include "cmsys/FStream.hxx"
 #include "cmsys/RegularExpression.hxx"
+#include "cmsys/String.h"
 #include "cmsys/SystemInformation.hxx"
 #ifndef _WIN32
 #  include <unistd.h> // IWYU pragma: keep
@@ -3702,8 +3703,7 @@ bool cmCTest::ConvertInstrumentationJSONFileToXML(std::string const& fpath,
   bool generating_test_xml = root["role"] == "test";
   if (!generating_test_xml) {
     std::string element_name = root["role"].asString();
-    element_name[0] = static_cast<char>(
-      std::toupper(static_cast<unsigned char>(element_name[0])));
+    element_name[0] = static_cast<char>(cmsysString_toupper(element_name[0]));
     xml.StartElement(element_name);
     std::vector<std::string> keys = root.getMemberNames();
     for (auto const& key : keys) {
@@ -3735,8 +3735,8 @@ bool cmCTest::ConvertInstrumentationJSONFileToXML(std::string const& fpath,
   std::vector<std::string> keys = dynamic_information.getMemberNames();
   for (auto const& key : keys) {
     std::string measurement_name = key;
-    measurement_name[0] = static_cast<char>(
-      std::toupper(static_cast<unsigned char>(measurement_name[0])));
+    measurement_name[0] =
+      static_cast<char>(cmsysString_toupper(measurement_name[0]));
 
     xml.StartElement("NamedMeasurement");
     xml.Attribute("type", "numeric/double");
diff --git a/Source/cmComputeLinkInformation.cxx b/Source/cmComputeLinkInformation.cxx
index 324ff30a81..5ff8784f79 100644
--- a/Source/cmComputeLinkInformation.cxx
+++ b/Source/cmComputeLinkInformation.cxx
@@ -3,7 +3,6 @@
 #include "cmComputeLinkInformation.h"
 
 #include <algorithm>
-#include <cctype>
 #include <sstream>
 #include <utility>
 
@@ -13,6 +12,8 @@
 #include <cmext/algorithm>
 #include <cmext/string_view>
 
+#include "cmsys/String.h"
+
 #include "cmComputeLinkDepends.h"
 #include "cmGeneratorTarget.h"
 #include "cmGlobalGenerator.h"
@@ -1556,8 +1557,8 @@ std::string cmComputeLinkInformation::NoCaseExpression(std::string const& str)
       ret += c;
     } else {
       ret += '[';
-      ret += static_cast<char>(tolower(static_cast<unsigned char>(c)));
-      ret += static_cast<char>(toupper(static_cast<unsigned char>(c)));
+      ret += static_cast<char>(cmsysString_tolower(c));
+      ret += static_cast<char>(cmsysString_toupper(c));
       ret += ']';
     }
   }
diff --git a/Source/cmFindLibraryCommand.cxx b/Source/cmFindLibraryCommand.cxx
index ad36c4a469..57d2a41e2a 100644
--- a/Source/cmFindLibraryCommand.cxx
+++ b/Source/cmFindLibraryCommand.cxx
@@ -3,7 +3,6 @@
 #include "cmFindLibraryCommand.h"
 
 #include <algorithm>
-#include <cctype>
 #include <cstdio>
 #include <cstring>
 #include <set>
@@ -13,6 +12,7 @@
 #include <cm/optional>
 
 #include "cmsys/RegularExpression.hxx"
+#include "cmsys/String.h"
 
 #include "cmFindCommon.h"
 #include "cmGlobalGenerator.h"
@@ -349,7 +349,7 @@ void cmFindLibraryHelper::RegexFromLiteral(std::string& out,
       out += "\\";
     }
     if (dirCase == cmSystemTools::DirCase::Insensitive) {
-      out += static_cast<char>(tolower(static_cast<unsigned char>(ch)));
+      out += static_cast<char>(cmsysString_tolower(ch));
     } else {
       out += ch;
     }
diff --git a/Source/cmGccDepfileLexerHelper.cxx b/Source/cmGccDepfileLexerHelper.cxx
index 3136f69dd1..478a93d393 100644
--- a/Source/cmGccDepfileLexerHelper.cxx
+++ b/Source/cmGccDepfileLexerHelper.cxx
@@ -12,9 +12,8 @@
 #include "LexerParser/cmGccDepfileLexer.h"
 
 #ifdef _WIN32
-#  include <cctype>
-
 #  include "cmsys/Encoding.h"
+#  include "cmsys/String.h"
 #endif
 
 bool cmGccDepfileLexerHelper::readFile(char const* filePath)
@@ -128,8 +127,7 @@ void cmGccDepfileLexerHelper::sanitizeContent()
         // Some versions of GNU compilers can escape this character.
         // c\:\path must be transformed to c:\path
         if (pit->size() >= 3) {
-          auto pit0 = static_cast<char>(
-            std::toupper(static_cast<unsigned char>((*pit)[0])));
+          auto pit0 = static_cast<char>(cmsysString_toupper((*pit)[0]));
           if (pit0 >= 'A' && pit0 <= 'Z' && (*pit)[1] == '\\' &&
               (*pit)[2] == ':') {
             pit->erase(1, 1);
diff --git a/Source/cmPathResolver.cxx b/Source/cmPathResolver.cxx
index 67d63b94c5..f67dcbd396 100644
--- a/Source/cmPathResolver.cxx
+++ b/Source/cmPathResolver.cxx
@@ -16,6 +16,8 @@
 #  include <cctype>
 
 #  include <windows.h>
+
+#  include <cmsys/String.h>
 #endif
 
 #define MAX_SYMBOLIC_LINKS 32
@@ -171,8 +173,7 @@ std::string ImplBase::GetWorkingDirectoryOnDrive(char letter)
   std::string d = this->OS.GetWorkingDirectoryOnDrive(letter);
   std::replace(d.begin(), d.end(), '\\', '/');
   if (d.size() >= 3 &&
-      std::toupper(static_cast<unsigned char>(d[0])) ==
-        std::toupper(static_cast<unsigned char>(letter)) &&
+      cmsysString_toupper(d[0]) == cmsysString_toupper(letter) &&
       d[1] == ':' && d[2] == '/') {
     d[0] = letter;
     d.push_back('/');
@@ -182,8 +183,7 @@ std::string ImplBase::GetWorkingDirectoryOnDrive(char letter)
   // Use the current working directory if the drive matches.
   d = this->OS.GetWorkingDirectory();
   if (d.size() >= 3 &&
-      std::toupper(static_cast<unsigned char>(d[0])) ==
-        std::toupper(static_cast<unsigned char>(letter)) &&
+      cmsysString_toupper(d[0]) == cmsysString_toupper(letter) &&
       d[1] == ':' && d[2] == '/') {
     d[0] = letter;
     d.push_back('/');
@@ -284,7 +284,7 @@ Control Impl<Policy>::ResolveRoot(Root root)
 
     if (Policy::ActualCase == Options::ActualCase::Yes) {
       // Normalize the drive letter to upper-case.
-      P[0] = static_cast<char>(std::toupper(static_cast<unsigned char>(P[0])));
+      P[0] = static_cast<char>(cmsysString_toupper(P[0]));
     }
 
     // The root is a drive letter.  The root '/' immediately follows.
diff --git a/Source/cmSbomArguments.cxx b/Source/cmSbomArguments.cxx
index b9394488b6..76322ab494 100644
--- a/Source/cmSbomArguments.cxx
+++ b/Source/cmSbomArguments.cxx
@@ -3,10 +3,11 @@
 #include "cmSbomArguments.h"
 
 #include <algorithm>
-#include <cctype>
 
 #include <cm/string_view>
 
+#include "cmsys/String.h"
+
 #include "cmExecutionStatus.h"
 #include "cmGeneratorExpression.h"
 #include "cmStringAlgorithms.h"
@@ -99,6 +100,6 @@ std::string cmSbomArguments::GetPackageFileName() const
   std::string const pkgNameOnDisk = this->GetPackageDirName();
   std::string format = GetSbomFileExtension(this->GetFormat());
   std::transform(format.begin(), format.end(), format.begin(),
-                 [](unsigned char c) { return std::tolower(c); });
+                 cmsysString_tolower);
   return cmStrCat(pkgNameOnDisk, format);
 }
diff --git a/Source/cmStringAlgorithms.cxx b/Source/cmStringAlgorithms.cxx
index d46019bf7c..c2f3a55c57 100644
--- a/Source/cmStringAlgorithms.cxx
+++ b/Source/cmStringAlgorithms.cxx
@@ -8,15 +8,17 @@
 #include <cstdio>
 #include <cstdlib>
 
+#include "cmsys/String.h"
+
 bool cmStrCaseEq(cm::string_view s1, cm::string_view s2)
 {
   if (s1.size() != s2.size()) {
     return false;
   }
 
-  return std::equal(
-    s1.begin(), s1.end(), s2.begin(),
-    [](unsigned char a, unsigned char b) { return tolower(a) == tolower(b); });
+  return std::equal(s1.begin(), s1.end(), s2.begin(), [](char a, char b) {
+    return cmsysString_tolower(a) == cmsysString_tolower(b);
+  });
 }
 
 std::string cmTrimWhitespace(cm::string_view str)
diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index 1d15b55007..cb088e9e13 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -101,6 +101,7 @@
 #include "cmsys/Directory.hxx"
 #ifdef _WIN32
 #  include "cmsys/Encoding.hxx"
+#  include "cmsys/String.h"
 #endif
 #include "cmsys/FStream.hxx"
 #include "cmsys/RegularExpression.hxx"
@@ -1340,7 +1341,7 @@ std::string cmSystemTools::GetRealPathResolvingWindowsSubst(
   }
   // Normalize to upper-case drive letter as cm::PathResolver does.
   if (resolved_path.size() > 1 && resolved_path[1] == ':') {
-    resolved_path[0] = toupper(static_cast<unsigned char>(resolved_path[0]));
+    resolved_path[0] = cmsysString_toupper(resolved_path[0]);
   }
   return resolved_path;
 #else
@@ -1362,12 +1363,10 @@ std::string cmSystemTools::GetRealPath(std::string const& path,
   // limitation to otherwise preserve susbt drives.
   if (resolved_path.size() >= 2 && resolved_path[1] == ':' &&
       path.size() >= 2 && path[1] == ':' &&
-      toupper(static_cast<unsigned char>(resolved_path[0])) !=
-        toupper(static_cast<unsigned char>(path[0]))) {
+      cmsysString_toupper(resolved_path[0]) != cmsysString_toupper(path[0])) {
     // FIXME: Add thread_local or mutex if we use threads.
     static std::map<char, std::string> substMap;
-    char const drive =
-      static_cast<char>(toupper(static_cast<unsigned char>(path[0])));
+    char const drive = static_cast<char>(cmsysString_toupper(path[0]));
     std::string maybe_subst = cmStrCat(drive, ":/");
     auto smi = substMap.find(drive);
     if (smi == substMap.end()) {
diff --git a/Source/cmWindowsRegistry.cxx b/Source/cmWindowsRegistry.cxx
index 9d9de8c3a3..ecce2f55d3 100644
--- a/Source/cmWindowsRegistry.cxx
+++ b/Source/cmWindowsRegistry.cxx
@@ -4,7 +4,6 @@
 
 #include "cmWindowsRegistry.h"
 
-#include <cctype>
 #include <cstddef>
 #include <type_traits>
 #include <unordered_map>
@@ -13,6 +12,7 @@
 #include <cmext/string_view>
 
 #include "cmsys/RegularExpression.hxx"
+#include "cmsys/String.h"
 
 #if defined(_WIN32) && !defined(__CYGWIN__)
 #  include <algorithm>
@@ -47,8 +47,8 @@ int Strucmp(cm::string_view l, cm::string_view r)
   cm::string_view::size_type ri = 0;
 
   do {
-    lc = std::tolower(static_cast<unsigned char>(l[li++]));
-    rc = std::tolower(static_cast<unsigned char>(r[ri++]));
+    lc = cmsysString_tolower(l[li++]);
+    rc = cmsysString_tolower(r[ri++]);
   } while (lc == rc && li < l.size() && ri < r.size());
 
   return lc == rc ? static_cast<int>(l.size() - r.size()) : lc - rc;
diff --git a/Source/cmakemain.cxx b/Source/cmakemain.cxx
index 42fe88a8ce..35346afd38 100644
--- a/Source/cmakemain.cxx
+++ b/Source/cmakemain.cxx
@@ -5,7 +5,6 @@
 
 #include <algorithm>
 #include <cassert>
-#include <cctype>
 #include <climits>
 #include <cstring>
 #include <functional>
@@ -20,6 +19,8 @@
 
 #include <cm3p/uv.h>
 
+#include "cmsys/String.h"
+
 #include "cmBuildArgs.h"
 #include "cmBuildOptions.h"
 #include "cmCommandLineArgument.h"
@@ -500,8 +501,7 @@ int do_build(int ac, char const* const* av)
   };
   auto resolvePackagesLambda = [&](std::string const& value) -> bool {
     std::string v = value;
-    std::transform(v.begin(), v.end(), v.begin(),
-                   [](unsigned char c) { return std::tolower(c); });
+    std::transform(v.begin(), v.end(), v.begin(), cmsysString_tolower);
     if (v == "on") {
       resolveMode = PackageResolveMode::Force;
     } else if (v == "only") {
diff --git a/Tests/CMakeLib/testPathResolver.cxx b/Tests/CMakeLib/testPathResolver.cxx
index 6a38c3148c..ce759e50d5 100644
--- a/Tests/CMakeLib/testPathResolver.cxx
+++ b/Tests/CMakeLib/testPathResolver.cxx
@@ -9,7 +9,7 @@
 #include <utility>
 
 #ifdef _WIN32
-#  include <cctype>
+#  include <cmsys/String.h>
 #endif
 
 #include <cmsys/Status.hxx>
@@ -92,8 +92,7 @@ public:
   std::string GetWorkingDirectoryOnDrive(char letter) override
   {
     std::string result;
-    auto i = this->WorkDirOnDrive.find(
-      std::tolower(static_cast<unsigned char>(letter)));
+    auto i = this->WorkDirOnDrive.find(cmsysString_tolower(letter));
     if (i != this->WorkDirOnDrive.end()) {
       result = i->second;
     }
diff --git a/Tests/CMakeLib/testUVProcessChainHelper.cxx b/Tests/CMakeLib/testUVProcessChainHelper.cxx
index ea843d9922..50ad761b54 100644
--- a/Tests/CMakeLib/testUVProcessChainHelper.cxx
+++ b/Tests/CMakeLib/testUVProcessChainHelper.cxx
@@ -1,4 +1,3 @@
-#include <cctype>
 #include <chrono>
 #include <cstdlib>
 #include <iostream>
@@ -7,6 +6,7 @@
 #include <string>
 #include <thread>
 
+#include "cmsys/String.h"
 #include "cmsys/SystemTools.hxx"
 
 #ifdef _WIN32
@@ -41,7 +41,7 @@ int main(int argc, char** argv)
     std::this_thread::sleep_for(std::chrono::milliseconds(12000));
     std::string input = getStdin();
     for (auto& c : input) {
-      c = static_cast<char>(std::toupper(static_cast<unsigned char>(c)));
+      c = static_cast<char>(cmsysString_toupper(c));
     }
     std::cout << input << std::flush;
     std::cerr << "2" << std::flush;