diff --git a/Source/CPack/cmCPackAppImageGenerator.cxx b/Source/CPack/cmCPackAppImageGenerator.cxx
index c6d4d8a919..03dffab464 100644
--- a/Source/CPack/cmCPackAppImageGenerator.cxx
+++ b/Source/CPack/cmCPackAppImageGenerator.cxx
@@ -4,7 +4,6 @@
 #include "cmCPackAppImageGenerator.h"
 
 #include <algorithm>
-#include <cctype>
 #include <cstddef>
 #include <utility>
 #include <vector>
@@ -14,6 +13,7 @@
 #include <sys/types.h>
 
 #include "cmsys/FStream.hxx"
+#include "cmsys/String.h"
 
 #include "cmCPackLog.h"
 #include "cmELF.h"
@@ -340,11 +340,9 @@ namespace {
 // Trim leading and trailing whitespace from a string
 std::string trim(std::string const& str)
 {
-  auto start = std::find_if_not(
-    str.begin(), str.end(), [](unsigned char c) { return std::isspace(c); });
-  auto end = std::find_if_not(str.rbegin(), str.rend(), [](unsigned char c) {
-               return std::isspace(c);
-             }).base();
+  auto start = std::find_if_not(str.begin(), str.end(), cmsysString_isspace);
+  auto end =
+    std::find_if_not(str.rbegin(), str.rend(), cmsysString_isspace).base();
   return (start < end) ? std::string(start, end) : std::string();
 }
 } // namespace
diff --git a/Source/CTest/cmCTestGIT.cxx b/Source/CTest/cmCTestGIT.cxx
index 52adedd925..fdb0bb74ec 100644
--- a/Source/CTest/cmCTestGIT.cxx
+++ b/Source/CTest/cmCTestGIT.cxx
@@ -11,6 +11,7 @@
 #include <cmext/algorithm>
 
 #include "cmsys/FStream.hxx"
+#include "cmsys/String.h"
 
 #include "cmCTest.h"
 #include "cmCTestVC.h"
@@ -412,14 +413,14 @@ protected:
 
   char const* ConsumeSpace(char const* c)
   {
-    while (*c && cmIsSpace(*c)) {
+    while (*c && cmsysString_isspace(*c)) {
       ++c;
     }
     return c;
   }
   char const* ConsumeField(char const* c)
   {
-    while (*c && !cmIsSpace(*c)) {
+    while (*c && !cmsysString_isspace(*c)) {
       ++c;
     }
     return c;
@@ -479,7 +480,7 @@ private:
   {
     // Person Name <person@domain.com> 1234567890 +0000
     char const* c = str;
-    while (*c && cmIsSpace(*c)) {
+    while (*c && cmsysString_isspace(*c)) {
       ++c;
     }
 
@@ -488,7 +489,7 @@ private:
       ++c;
     }
     char const* name_last = c;
-    while (name_last != name_first && cmIsSpace(*(name_last - 1))) {
+    while (name_last != name_first && cmsysString_isspace(*(name_last - 1))) {
       --name_last;
     }
     person.Name.assign(name_first, name_last - name_first);
diff --git a/Source/QtDialog/RegexExplorer.cxx b/Source/QtDialog/RegexExplorer.cxx
index 9065f66d2f..b0333cf490 100644
--- a/Source/QtDialog/RegexExplorer.cxx
+++ b/Source/QtDialog/RegexExplorer.cxx
@@ -2,6 +2,8 @@
    file LICENSE.rst or https://cmake.org/licensing for details.  */
 #include "RegexExplorer.h"
 
+#include "cmsys/String.h"
+
 RegexExplorer::RegexExplorer(QWidget* p)
   : QDialog(p)
   , m_matched(false)
@@ -147,7 +149,7 @@ bool RegexExplorer::stripEscapes(std::string& source)
       } else if (nextc == 'n') {
         result.append(1, '\n');
         in++;
-      } else if (isalnum(nextc) || nextc == '\0') {
+      } else if (cmsysString_isalnum(nextc) || nextc == '\0') {
         return false;
       } else {
         result.append(1, nextc);
diff --git a/Source/bindexplib.cxx b/Source/bindexplib.cxx
index bbcb0bccc9..59680aa4fd 100644
--- a/Source/bindexplib.cxx
+++ b/Source/bindexplib.cxx
@@ -72,6 +72,7 @@
 #  include <windows.h>
 
 #  include "cmsys/Encoding.hxx"
+#  include "cmsys/String.h"
 #endif
 
 #include "cmsys/FStream.hxx"
@@ -261,7 +262,7 @@ public:
           }
 
           // clear out any leading spaces
-          while (isspace(symbol[0]))
+          while (cmsysString_isspace(symbol[0]))
             symbol.erase(0, 1);
           // if it starts with _ and has an @ then it is a __cdecl
           // so remove the @ stuff for the export
diff --git a/Source/cmCMakeHostSystemInformationCommand.cxx b/Source/cmCMakeHostSystemInformationCommand.cxx
index 9674c520be..514f00c25c 100644
--- a/Source/cmCMakeHostSystemInformationCommand.cxx
+++ b/Source/cmCMakeHostSystemInformationCommand.cxx
@@ -4,7 +4,6 @@
 
 #include <algorithm>
 #include <cassert>
-#include <cctype>
 #include <cstddef>
 #include <initializer_list>
 #include <map>
@@ -18,6 +17,7 @@
 
 #include "cmsys/FStream.hxx"
 #include "cmsys/Glob.hxx"
+#include "cmsys/String.h"
 #include "cmsys/SystemInformation.hxx"
 
 #include "cmArgumentParser.h"
@@ -218,10 +218,10 @@ cm::optional<std::pair<std::string, std::string>> ParseOSReleaseLine(
   for (auto ch : line) {
     switch (state) {
       case PARSE_KEY_1ST:
-        if (std::isalpha(ch) || ch == '_') {
+        if (cmsysString_isalpha(ch) || ch == '_') {
           key += ch;
           state = PARSE_KEY;
-        } else if (!cmIsSpace(ch)) {
+        } else if (!cmsysString_isspace(ch)) {
           state = IGNORE_REST;
         }
         break;
@@ -229,7 +229,7 @@ cm::optional<std::pair<std::string, std::string>> ParseOSReleaseLine(
       case PARSE_KEY:
         if (ch == '=') {
           state = FOUND_EQ;
-        } else if (std::isalnum(ch) || ch == '_') {
+        } else if (cmsysString_isalnum(ch) || ch == '_') {
           key += ch;
         } else {
           state = IGNORE_REST;
@@ -281,7 +281,7 @@ cm::optional<std::pair<std::string, std::string>> ParseOSReleaseLine(
         break;
 
       case PARSE_VALUE:
-        if (ch == '#' || cmIsSpace(ch)) {
+        if (ch == '#' || cmsysString_isspace(ch)) {
           state = IGNORE_REST;
         } else {
           value += ch;
@@ -355,8 +355,8 @@ std::map<std::string, std::string> GetOSReleaseVariables(
     auto const& filename = cmSystemTools::GetFilenameName(filepath);
     // NOTE Minimum filename length expected:
     //   NNN-<at-least-one-char-name>.cmake  --> 11
-    return (filename.size() < 11) || !std::isdigit(filename[0]) ||
-      !std::isdigit(filename[1]) || !std::isdigit(filename[2]) ||
+    return (filename.size() < 11) || !cmsysString_isdigit(filename[0]) ||
+      !cmsysString_isdigit(filename[1]) || !cmsysString_isdigit(filename[2]) ||
       filename[3] != '-';
   };
   scripts.erase(std::remove_if(scripts.begin(), scripts.end(), checkName),
diff --git a/Source/cmCTest.cxx b/Source/cmCTest.cxx
index 3b8232561b..ed1516f9a1 100644
--- a/Source/cmCTest.cxx
+++ b/Source/cmCTest.cxx
@@ -3,7 +3,6 @@
 #include "cmCTest.h"
 
 #include <algorithm>
-#include <cctype>
 #include <chrono>
 #include <cstdint>
 #include <cstdio>
@@ -300,7 +299,8 @@ std::string cmCTest::DecodeURL(std::string const& in)
 {
   std::string out;
   for (char const* c = in.c_str(); *c; ++c) {
-    if (*c == '%' && isxdigit(*(c + 1)) && isxdigit(*(c + 2))) {
+    if (*c == '%' && cmsysString_isxdigit(*(c + 1)) &&
+        cmsysString_isxdigit(*(c + 2))) {
       char buf[3] = { *(c + 1), *(c + 2), 0 };
       out.append(1, static_cast<char>(strtoul(buf, nullptr, 16)));
       c += 2;
diff --git a/Source/cmCommandLineArgument.h b/Source/cmCommandLineArgument.h
index f11d1fb1a1..4c88f23491 100644
--- a/Source/cmCommandLineArgument.h
+++ b/Source/cmCommandLineArgument.h
@@ -2,11 +2,11 @@
    file LICENSE.rst or https://cmake.org/licensing for details.  */
 #pragma once
 
-#include <cctype>
-
 #include <cm/optional>
 #include <cm/string_view>
 
+#include "cmsys/String.h"
+
 #include "cmStringAlgorithms.h"
 #include "cmSystemTools.h"
 
@@ -288,6 +288,6 @@ private:
   static bool IsFlag(cm::string_view arg)
   {
     return !arg.empty() && arg[0] == '-' &&
-      !(arg.size() >= 2 && std::isdigit(arg[1]));
+      !(arg.size() >= 2 && cmsysString_isdigit(arg[1]));
   }
 };
diff --git a/Source/cmDocumentation.cxx b/Source/cmDocumentation.cxx
index 35cf9e4bd5..66e3730dcf 100644
--- a/Source/cmDocumentation.cxx
+++ b/Source/cmDocumentation.cxx
@@ -3,13 +3,13 @@
 #include "cmDocumentation.h"
 
 #include <algorithm>
-#include <cctype>
 #include <cstring>
 #include <utility>
 
 #include "cmsys/FStream.hxx"
 #include "cmsys/Glob.hxx"
 #include "cmsys/RegularExpression.hxx"
+#include "cmsys/String.h"
 
 #if !defined(CMAKE_BOOTSTRAP)
 #  include <memory>
@@ -516,7 +516,7 @@ void cmDocumentation::PrintNames(std::ostream& os, std::string const& pattern)
     std::string line;
     cmsys::ifstream fin(f.c_str());
     while (fin && cmSystemTools::GetLineFromStream(fin, line)) {
-      if (!line.empty() && (isalnum(line[0]) || line[0] == '<')) {
+      if (!line.empty() && (cmsysString_isalnum(line[0]) || line[0] == '<')) {
         names.push_back(line);
         break;
       }
diff --git a/Source/cmExecuteProcessCommand.cxx b/Source/cmExecuteProcessCommand.cxx
index 7050f0db9a..63dcb6079a 100644
--- a/Source/cmExecuteProcessCommand.cxx
+++ b/Source/cmExecuteProcessCommand.cxx
@@ -24,6 +24,8 @@
 #  include "cm_fileno.hxx"
 #endif
 
+#include "cmsys/String.h"
+
 #include "cmArgumentParser.h"
 #include "cmExecutionStatus.h"
 #include "cmList.h"
@@ -40,7 +42,7 @@
 namespace {
 bool cmExecuteProcessCommandIsWhitespace(char c)
 {
-  return (cmIsSpace(c) || c == '\n' || c == '\r');
+  return (cmsysString_isspace(c) || c == '\n' || c == '\r');
 }
 
 FILE* FopenCLOEXEC(std::string const& path, char const* mode)
diff --git a/Source/cmFileAPICommand.cxx b/Source/cmFileAPICommand.cxx
index dbd8f05199..091fbc8c8a 100644
--- a/Source/cmFileAPICommand.cxx
+++ b/Source/cmFileAPICommand.cxx
@@ -4,13 +4,14 @@ file LICENSE.rst or https://cmake.org/licensing for details.  */
 
 #include <algorithm>
 #include <array>
-#include <cctype>
 #include <cstdlib>
 #include <utility>
 
 #include <cm/string_view>
 #include <cmext/string_view>
 
+#include "cmsys/String.h"
+
 #include "cmArgumentParser.h"
 #include "cmArgumentParserTypes.h"
 #include "cmExecutionStatus.h"
@@ -23,11 +24,6 @@ file LICENSE.rst or https://cmake.org/licensing for details.  */
 
 namespace {
 
-bool isCharDigit(char ch)
-{
-  return std::isdigit(static_cast<unsigned char>(ch));
-}
-
 std::string processObjectKindVersions(cmFileAPI& fileApi,
                                       cmFileAPI::ObjectKind objectKind,
                                       cm::string_view keyword,
@@ -103,7 +99,7 @@ bool handleQueryCommand(std::vector<std::string> const& args,
   }
 
   if (!std::all_of(arguments.ApiVersion.begin(), arguments.ApiVersion.end(),
-                   isCharDigit)) {
+                   cmsysString_isdigit)) {
     status.SetError("QUERY given non-integer API_VERSION.");
     return false;
   }
diff --git a/Source/cmFileCommand.cxx b/Source/cmFileCommand.cxx
index 72a62b2078..16cd19bf8b 100644
--- a/Source/cmFileCommand.cxx
+++ b/Source/cmFileCommand.cxx
@@ -4,7 +4,6 @@
 
 #include <algorithm>
 #include <cassert>
-#include <cctype>
 #include <cmath>
 #include <cstdio>
 #include <cstdlib>
@@ -26,6 +25,7 @@
 #include "cmsys/FStream.hxx"
 #include "cmsys/Glob.hxx"
 #include "cmsys/RegularExpression.hxx"
+#include "cmsys/String.h"
 
 #include "cm_sys_stat.h"
 
@@ -519,7 +519,8 @@ bool HandleStringsCommand(std::vector<std::string> const& args,
     }
 
     if (c >= 0 && c <= 0xFF &&
-        (isprint(c) || c == '\t' || (c == '\n' && newline_consume))) {
+        (cmsysString_isprint(static_cast<char>(c)) || c == '\t' ||
+         (c == '\n' && newline_consume))) {
       // This is an ASCII character that may be part of a string.
       // Cast added to avoid compiler warning. Cast is ok because
       // c is guaranteed to fit in char by the above if...
@@ -3767,7 +3768,7 @@ bool HandleArchiveCreateCommand(std::vector<std::string> const& args,
 
   if (!parsedArgs.CompressionLevel.empty()) {
     if (parsedArgs.CompressionLevel.size() != 1 &&
-        !std::isdigit(parsedArgs.CompressionLevel[0])) {
+        !cmsysString_isdigit(parsedArgs.CompressionLevel[0])) {
       status.SetError(
         cmStrCat("compression level ", parsedArgs.CompressionLevel, " for ",
                  parsedArgs.Compression, " should be in range ",
@@ -3800,7 +3801,7 @@ bool HandleArchiveCreateCommand(std::vector<std::string> const& args,
   constexpr int minThreads = 0;
   if (!parsedArgs.Threads.empty()) {
     if (parsedArgs.Threads.size() != 1 &&
-        !std::isdigit(parsedArgs.Threads[0])) {
+        !cmsysString_isdigit(parsedArgs.Threads[0])) {
       status.SetError(cmStrCat("number of threads ", parsedArgs.Threads,
                                " should be at least ", minThreads));
       cmSystemTools::SetFatalErrorOccurred();
diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index c5f4e35aad..c94f025ec9 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -5,7 +5,6 @@
 #include <algorithm>
 #include <array>
 #include <cassert>
-#include <cctype>
 #include <cstddef>
 #include <cstdio>
 #include <cstring>
@@ -20,6 +19,8 @@
 #include <cmext/algorithm>
 #include <cmext/string_view>
 
+#include "cmsys/String.h"
+
 #include "cmAlgorithms.h"
 #include "cmComputeLinkInformation.h" // IWYU pragma: keep
 #include "cmCryptoHash.h"
@@ -3953,8 +3954,7 @@ std::string cmGeneratorTarget::GetLinkerTool(std::string const& lang,
 
     if (linkerType != "DEFAULT"_s) {
       auto isCMakeLinkerType = [](std::string const& type) -> bool {
-        return std::all_of(type.cbegin(), type.cend(),
-                           [](char c) { return std::isupper(c); });
+        return std::all_of(type.cbegin(), type.cend(), cmsysString_isupper);
       };
       if (isCMakeLinkerType(linkerType)) {
         this->LocalGenerator->IssueMessage(
diff --git a/Source/cmGlobalNinjaGenerator.cxx b/Source/cmGlobalNinjaGenerator.cxx
index 46bb45efdd..b14564174d 100644
--- a/Source/cmGlobalNinjaGenerator.cxx
+++ b/Source/cmGlobalNinjaGenerator.cxx
@@ -4,7 +4,6 @@
 
 #include <algorithm>
 #include <cassert>
-#include <cctype>
 #include <cstdio>
 #include <functional>
 #include <iterator>
@@ -23,6 +22,7 @@
 #include <cm3p/json/writer.h>
 
 #include "cmsys/FStream.hxx"
+#include "cmsys/String.h"
 
 #include "cmCustomCommand.h"
 #include "cmCxxModuleMapper.h"
@@ -172,7 +172,7 @@ std::string cmGlobalNinjaGenerator::EncodeRuleName(std::string const& name)
   // "." and all invalid characters as hexadecimal.
   std::string encoded;
   for (char i : name) {
-    if (isalnum(i) || i == '_' || i == '-') {
+    if (cmsysString_isalnum(i) || i == '_' || i == '-') {
       encoded += i;
     } else {
       char buf[16];
diff --git a/Source/cmGraphVizWriter.cxx b/Source/cmGraphVizWriter.cxx
index 53c9659670..baba225b2a 100644
--- a/Source/cmGraphVizWriter.cxx
+++ b/Source/cmGraphVizWriter.cxx
@@ -3,7 +3,6 @@
 #include "cmGraphVizWriter.h"
 
 #include <algorithm>
-#include <cctype>
 #include <iostream>
 #include <memory>
 #include <set>
@@ -13,6 +12,7 @@
 #include <cm/memory>
 
 #include "cmsys/RegularExpression.hxx"
+#include "cmsys/String.h"
 
 #include "cmGeneratedFileStream.h"
 #include "cmGeneratorTarget.h"
@@ -602,7 +602,7 @@ std::string cmGraphVizWriter::PathSafeString(std::string const& str)
   auto const extra_chars = std::set<char>{ '.', '-', '_' };
 
   for (char c : str) {
-    if (std::isalnum(c) || extra_chars.find(c) != extra_chars.cend()) {
+    if (cmsysString_isalnum(c) || extra_chars.find(c) != extra_chars.cend()) {
       pathSafeStr += c;
     }
   }
diff --git a/Source/cmHexFileConverter.cxx b/Source/cmHexFileConverter.cxx
index a028bb45e7..3036473e01 100644
--- a/Source/cmHexFileConverter.cxx
+++ b/Source/cmHexFileConverter.cxx
@@ -2,10 +2,11 @@
    file LICENSE.rst or https://cmake.org/licensing for details.  */
 #include "cmHexFileConverter.h"
 
-#include <cctype>
 #include <cstdio>
 #include <cstring>
 
+#include "cmsys/String.h"
+
 #include "cmSystemTools.h"
 
 #define INTEL_HEX_MIN_LINE_LENGTH (1 + 8 + 2)
@@ -163,7 +164,7 @@ cmHexFileConverter::FileType cmHexFileConverter::DetermineFileType(
   }
 
   for (unsigned int i = 1; i < slen; i++) {
-    if (!isxdigit(buf[i])) {
+    if (!cmsysString_isxdigit(buf[i])) {
       return Binary;
     }
   }
diff --git a/Source/cmInstrumentationCommand.cxx b/Source/cmInstrumentationCommand.cxx
index 182deca708..0b0d0f8c9c 100644
--- a/Source/cmInstrumentationCommand.cxx
+++ b/Source/cmInstrumentationCommand.cxx
@@ -3,7 +3,6 @@ file LICENSE.rst or https://cmake.org/licensing for details.  */
 #include "cmInstrumentationCommand.h"
 
 #include <algorithm>
-#include <cctype>
 #include <cstdlib>
 #include <functional>
 #include <set>
@@ -14,6 +13,8 @@ file LICENSE.rst or https://cmake.org/licensing for details.  */
 #include <cm3p/json/reader.h>
 #include <cm3p/json/value.h>
 
+#include "cmsys/String.h"
+
 #include "cmArgumentParser.h"
 #include "cmArgumentParserTypes.h"
 #include "cmExecutionStatus.h"
@@ -27,14 +28,11 @@ file LICENSE.rst or https://cmake.org/licensing for details.  */
 
 namespace {
 
-bool isCharDigit(char ch)
-{
-  return std::isdigit(static_cast<unsigned char>(ch));
-}
 bool validateVersion(std::string const& key, std::string const& versionString,
                      int& version, cmExecutionStatus& status)
 {
-  if (!std::all_of(versionString.begin(), versionString.end(), isCharDigit)) {
+  if (!std::all_of(versionString.begin(), versionString.end(),
+                   cmsysString_isdigit)) {
     status.SetError(cmStrCat("given a non-integer ", key, '.'));
     return false;
   }
diff --git a/Source/cmLocalGenerator.cxx b/Source/cmLocalGenerator.cxx
index 5ce085125a..4a8c3d2ace 100644
--- a/Source/cmLocalGenerator.cxx
+++ b/Source/cmLocalGenerator.cxx
@@ -5,7 +5,6 @@
 #include <algorithm>
 #include <array>
 #include <cassert>
-#include <cctype>
 #include <cstdio>
 #include <cstdlib>
 #include <initializer_list>
@@ -23,6 +22,7 @@
 #include <cmext/string_view>
 
 #include "cmsys/RegularExpression.hxx"
+#include "cmsys/String.h"
 
 #include "cmAlgorithms.h"
 #include "cmCMakePath.h"
@@ -2556,7 +2556,8 @@ void cmLocalGenerator::AddConfigVariableFlags(std::string& flags,
 void cmLocalGenerator::AppendFlags(std::string& flags,
                                    std::string const& newFlags) const
 {
-  bool allSpaces = std::all_of(newFlags.begin(), newFlags.end(), cmIsSpace);
+  bool allSpaces =
+    std::all_of(newFlags.begin(), newFlags.end(), cmsysString_isspace);
 
   if (!newFlags.empty() && !allSpaces) {
     if (!flags.empty()) {
@@ -3489,8 +3490,7 @@ void cmLocalGenerator::AppendLinkerTypeFlags(std::string& flags,
     }
   } else if (linkerType != "DEFAULT"_s) {
     auto isCMakeLinkerType = [](std::string const& type) -> bool {
-      return std::all_of(type.cbegin(), type.cend(),
-                         [](char c) { return std::isupper(c); });
+      return std::all_of(type.cbegin(), type.cend(), cmsysString_isupper);
     };
     if (isCMakeLinkerType(linkerType)) {
       this->IssueMessage(
diff --git a/Source/cmMakefile.cxx b/Source/cmMakefile.cxx
index a78fc1e74f..6ff3489e9c 100644
--- a/Source/cmMakefile.cxx
+++ b/Source/cmMakefile.cxx
@@ -6,7 +6,6 @@
 
 #include <algorithm>
 #include <cassert>
-#include <cctype>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
@@ -28,6 +27,7 @@
 
 #include "cmsys/FStream.hxx"
 #include "cmsys/RegularExpression.hxx"
+#include "cmsys/String.h"
 
 #include "cmCustomCommand.h"
 #include "cmCustomCommandLines.h"
@@ -1299,8 +1299,8 @@ static void s_RemoveDefineFlag(std::string const& flag, std::string& dflags)
   for (std::string::size_type lpos = dflags.find(flag, 0);
        lpos != std::string::npos; lpos = dflags.find(flag, lpos)) {
     std::string::size_type rpos = lpos + len;
-    if ((lpos <= 0 || cmIsSpace(dflags[lpos - 1])) &&
-        (rpos >= dflags.size() || cmIsSpace(dflags[rpos]))) {
+    if ((lpos <= 0 || cmsysString_isspace(dflags[lpos - 1])) &&
+        (rpos >= dflags.size() || cmsysString_isspace(dflags[rpos]))) {
       dflags.erase(lpos, len);
     } else {
       ++lpos;
@@ -2705,7 +2705,7 @@ MessageType cmMakefile::ExpandVariablesInStringImpl(
             last = next + 1;
           } else if (nextc == ';' && openstack.empty()) {
             // Handled in ExpandListArgument; pass the backslash literally.
-          } else if (isalnum(nextc) || nextc == '\0') {
+          } else if (cmsysString_isalnum(nextc) || nextc == '\0') {
             errorstr += "Invalid character escape '\\";
             if (nextc) {
               errorstr += nextc;
@@ -2773,8 +2773,8 @@ MessageType cmMakefile::ExpandVariablesInStringImpl(
         CM_FALLTHROUGH;
       default: {
         if (!openstack.empty() &&
-            !(isalnum(inc) || inc == '_' || inc == '/' || inc == '.' ||
-              inc == '+' || inc == '-')) {
+            !(cmsysString_isalnum(inc) || inc == '_' || inc == '/' ||
+              inc == '.' || inc == '+' || inc == '-')) {
           errorstr += cmStrCat("Invalid character ('", inc);
           result.append(last, in - last);
           errorstr += cmStrCat("') in a variable name: '",
diff --git a/Source/cmOutputConverter.cxx b/Source/cmOutputConverter.cxx
index ddae68c92a..c72dbe45f0 100644
--- a/Source/cmOutputConverter.cxx
+++ b/Source/cmOutputConverter.cxx
@@ -4,7 +4,6 @@
 
 #include <algorithm>
 #include <cassert>
-#include <cctype>
 #include <set>
 #include <vector>
 
@@ -13,6 +12,8 @@
 #  include <utility>
 #endif
 
+#include "cmsys/String.h"
+
 #include "cmList.h"
 #include "cmState.h"
 #include "cmStateDirectory.h"
@@ -425,7 +426,7 @@ static bool Shell_CharNeedsQuotesOnWindows(char c)
 
 static bool Shell_CharIsMakeVariableName(char c)
 {
-  return c && (c == '_' || isalpha((static_cast<int>(c))));
+  return c && (c == '_' || cmsysString_isalpha((static_cast<int>(c))));
 }
 
 bool cmOutputConverter::Shell_CharNeedsQuotes(char c, int flags)
diff --git a/Source/cmPathResolver.cxx b/Source/cmPathResolver.cxx
index f67dcbd396..f384dbfe02 100644
--- a/Source/cmPathResolver.cxx
+++ b/Source/cmPathResolver.cxx
@@ -13,8 +13,6 @@
 #include <cmext/string_view>
 
 #ifdef _WIN32
-#  include <cctype>
-
 #  include <windows.h>
 
 #  include <cmsys/String.h>
@@ -97,7 +95,7 @@ private:
 Root ClassifyRoot(cm::string_view p)
 {
 #ifdef _WIN32
-  if (p.size() >= 2 && std::isalpha(p[0]) && p[1] == ':') {
+  if (p.size() >= 2 && cmsysString_isalpha(p[0]) && p[1] == ':') {
     return Root::Drive;
   }
   if (p.size() >= 3 && p[0] == '/' && p[1] == '/' && p[2] != '/') {
diff --git a/Source/cmPkgConfigResolver.cxx b/Source/cmPkgConfigResolver.cxx
index 8be5f3408a..54598c80b3 100644
--- a/Source/cmPkgConfigResolver.cxx
+++ b/Source/cmPkgConfigResolver.cxx
@@ -4,7 +4,6 @@
 #include "cmPkgConfigResolver.h"
 
 #include <algorithm>
-#include <cctype>
 #include <cstring>
 #include <iterator>
 #include <string>
@@ -15,6 +14,8 @@
 #include <cm/optional>
 #include <cm/string_view>
 
+#include "cmsys/String.h"
+
 #include "cmPkgConfigParser.h"
 #include "cmStringAlgorithms.h"
 
@@ -24,7 +25,7 @@ void TrimBack(std::string& str)
 {
   if (!str.empty()) {
     auto it = str.end() - 1;
-    for (; std::isspace(*it); --it) {
+    for (; cmsysString_isspace(*it); --it) {
       if (it == str.begin()) {
         str.clear();
         return;
@@ -45,11 +46,11 @@ std::string AppendAndTrim(std::string& str, cm::string_view sv)
   auto begin = str.begin() + size;
   auto cur = str.end() - 1;
 
-  while (cur != begin && std::isspace(*cur)) {
+  while (cur != begin && cmsysString_isspace(*cur)) {
     --cur;
   }
 
-  if (std::isspace(*cur)) {
+  if (cmsysString_isspace(*cur)) {
     return {};
   }
 
@@ -59,7 +60,8 @@ std::string AppendAndTrim(std::string& str, cm::string_view sv)
 cm::string_view TrimFlag(cm::string_view flag)
 {
   std::size_t trim_size = 2;
-  for (auto c = flag.rbegin(); c != flag.rend() && std::isspace(*c); ++c) {
+  for (auto c = flag.rbegin(); c != flag.rend() && cmsysString_isspace(*c);
+       ++c) {
     ++trim_size;
   }
   return { flag.data() + 2, flag.size() - trim_size };
@@ -406,7 +408,7 @@ std::vector<cm::string_view> cmPkgConfigResolver::TokenizeFlags(
   std::vector<cm::string_view> result;
 
   auto it = flagline.begin();
-  while (it != flagline.end() && std::isspace(*it)) {
+  while (it != flagline.end() && cmsysString_isspace(*it)) {
     ++it;
   }
 
@@ -414,11 +416,11 @@ std::vector<cm::string_view> cmPkgConfigResolver::TokenizeFlags(
     char const* start = &(*it);
     std::size_t len = 0;
 
-    for (; it != flagline.end() && !std::isspace(*it); ++it) {
+    for (; it != flagline.end() && !cmsysString_isspace(*it); ++it) {
       ++len;
     }
 
-    for (; it != flagline.end() && std::isspace(*it); ++it) {
+    for (; it != flagline.end() && cmsysString_isspace(*it); ++it) {
       ++len;
     }
 
@@ -665,12 +667,12 @@ cmPkgConfigVersionReq cmPkgConfigResolver::ParseVersion(
       return result;
     }
 
-    if (!std::isspace(*cur)) {
+    if (!cmsysString_isspace(*cur)) {
       break;
     }
   }
 
-  for (; cur != end && !std::isspace(*cur) && *cur != ','; ++cur) {
+  for (; cur != end && !cmsysString_isspace(*cur) && *cur != ','; ++cur) {
     result.Version += *cur;
   }
 
@@ -687,7 +689,7 @@ std::vector<cmPkgConfigDependency> cmPkgConfigResolver::ParseDependencies(
   auto end = deps.end();
 
   while (cur != end) {
-    while ((std::isspace(*cur) || *cur == ',')) {
+    while ((cmsysString_isspace(*cur) || *cur == ',')) {
       if (++cur == end) {
         return result;
       }
@@ -696,7 +698,7 @@ std::vector<cmPkgConfigDependency> cmPkgConfigResolver::ParseDependencies(
     result.emplace_back();
     auto& dep = result.back();
 
-    while (!std::isspace(*cur) && *cur != ',') {
+    while (!cmsysString_isspace(*cur) && *cur != ',') {
       dep.Name += *cur;
       if (++cur == end) {
         return result;
@@ -713,7 +715,7 @@ std::vector<cmPkgConfigDependency> cmPkgConfigResolver::ParseDependencies(
           return true;
         }
 
-        if (!std::isspace(*cur)) {
+        if (!cmsysString_isspace(*cur)) {
           return false;
         }
       }
@@ -769,11 +771,11 @@ bool cmPkgConfigResolver::CheckVersion(cmPkgConfigVersionReq const& desired,
   auto b_end = provided.end();
 
   while (a_cur != a_end && b_cur != b_end) {
-    while (a_cur != a_end && !std::isalnum(*a_cur) && *a_cur != '~') {
+    while (a_cur != a_end && !cmsysString_isalnum(*a_cur) && *a_cur != '~') {
       ++a_cur;
     }
 
-    while (b_cur != b_end && !std::isalnum(*b_cur) && *b_cur != '~') {
+    while (b_cur != b_end && !cmsysString_isalnum(*b_cur) && *b_cur != '~') {
       ++b_cur;
     }
 
@@ -799,23 +801,23 @@ bool cmPkgConfigResolver::CheckVersion(cmPkgConfigVersionReq const& desired,
     auto b_seg = b_cur;
     bool is_num;
 
-    if (std::isdigit(*a_cur)) {
+    if (cmsysString_isdigit(*a_cur)) {
       is_num = true;
-      while (a_cur != a_end && std::isdigit(*a_cur)) {
+      while (a_cur != a_end && cmsysString_isdigit(*a_cur)) {
         ++a_cur;
       }
 
-      while (b_cur != b_end && std::isdigit(*b_cur)) {
+      while (b_cur != b_end && cmsysString_isdigit(*b_cur)) {
         ++b_cur;
       }
 
     } else {
       is_num = false;
-      while (a_cur != a_end && std::isalpha(*a_cur)) {
+      while (a_cur != a_end && cmsysString_isalpha(*a_cur)) {
         ++a_cur;
       }
 
-      while (b_cur != b_end && std::isalpha(*b_cur)) {
+      while (b_cur != b_end && cmsysString_isalpha(*b_cur)) {
         ++b_cur;
       }
     }
diff --git a/Source/cmPlaceholderExpander.cxx b/Source/cmPlaceholderExpander.cxx
index f5dc544373..aea427b681 100644
--- a/Source/cmPlaceholderExpander.cxx
+++ b/Source/cmPlaceholderExpander.cxx
@@ -2,7 +2,7 @@
    file LICENSE.rst or https://cmake.org/licensing for details.  */
 #include "cmPlaceholderExpander.h"
 
-#include <cctype>
+#include "cmsys/String.h"
 
 std::string& cmPlaceholderExpander::ExpandVariables(std::string& s)
 {
@@ -23,7 +23,7 @@ std::string& cmPlaceholderExpander::ExpandVariables(std::string& s)
     char c = s[start + 1];
     // if the next char after the < is not A-Za-z then
     // skip it and try to find the next < in the string
-    if (!isalpha(c)) {
+    if (!cmsysString_isalpha(c)) {
       start = s.find('<', start + 1);
     } else {
       // extract the var
diff --git a/Source/cmPolicies.cxx b/Source/cmPolicies.cxx
index 45838630b3..337f220059 100644
--- a/Source/cmPolicies.cxx
+++ b/Source/cmPolicies.cxx
@@ -3,12 +3,13 @@
 #include "cmPolicies.h"
 
 #include <cassert>
-#include <cctype>
 #include <cstdio>
 #include <cstring>
 #include <sstream>
 #include <vector>
 
+#include "cmsys/String.h"
+
 #include "cmListFileCache.h"
 #include "cmMakefile.h"
 #include "cmMessageType.h"
@@ -32,7 +33,7 @@ static bool stringToId(char const* input, cmPolicies::PolicyID& pid)
     return true;
   }
   for (int i = 3; i < 7; ++i) {
-    if (!isdigit(*(input + i))) {
+    if (!cmsysString_isdigit(*(input + i))) {
       return false;
     }
   }
diff --git a/Source/cmRST.cxx b/Source/cmRST.cxx
index 51245d67bb..e49037099e 100644
--- a/Source/cmRST.cxx
+++ b/Source/cmRST.cxx
@@ -8,6 +8,7 @@
 #include <utility>
 
 #include "cmsys/FStream.hxx"
+#include "cmsys/String.h"
 
 #include "cmAlgorithms.h"
 #include "cmRange.h"
@@ -158,7 +159,7 @@ void cmRST::ProcessLine(std::string const& line)
   // A line starting in .. is an explicit markup start.
   if (line == ".." ||
       (line.size() >= 3 && line[0] == '.' && line[1] == '.' &&
-       cmIsSpace(line[2]))) {
+       cmsysString_isspace(line[2]))) {
     this->Reset();
     this->MarkupType =
       (line.find_first_not_of(" \t", 2) == std::string::npos ? Markup::Empty
@@ -218,7 +219,7 @@ void cmRST::ProcessLine(std::string const& line)
   }
   // Indented lines following an explicit markup start are explicit markup.
   else if (this->MarkupType != Markup::None &&
-           (line.empty() || cmIsSpace(line[0]))) {
+           (line.empty() || cmsysString_isspace(line[0]))) {
     this->MarkupType = Markup::Normal;
     // Record markup lines if the start line was recorded.
     if (!this->MarkupLines.empty()) {
diff --git a/Source/cmScanDepFormat.cxx b/Source/cmScanDepFormat.cxx
index bd3827388f..00165447e9 100644
--- a/Source/cmScanDepFormat.cxx
+++ b/Source/cmScanDepFormat.cxx
@@ -3,7 +3,6 @@
 
 #include "cmScanDepFormat.h"
 
-#include <cctype>
 #include <cstdio>
 #include <utility>
 
@@ -16,6 +15,7 @@
 #include <cm3p/json/writer.h>
 
 #include "cmsys/FStream.hxx"
+#include "cmsys/String.h"
 
 #include "cmGeneratedFileStream.h"
 #include "cmStringAlgorithms.h"
@@ -38,7 +38,7 @@ static Json::Value EncodeFilename(std::string const& path)
   data.reserve(path.size());
 
   for (auto const& byte : path) {
-    if (std::iscntrl(byte)) {
+    if (cmsysString_iscntrl(byte)) {
       // Control characters.
       data.append("\\u");
       char buf[5];
diff --git a/Source/cmStringAlgorithms.cxx b/Source/cmStringAlgorithms.cxx
index c2f3a55c57..e13c22691b 100644
--- a/Source/cmStringAlgorithms.cxx
+++ b/Source/cmStringAlgorithms.cxx
@@ -27,7 +27,7 @@ std::string cmTrimWhitespace(cm::string_view str)
   // because the qualification of `auto` is platform-dependent.
   // NOLINTNEXTLINE(readability-qualified-auto)
   auto start = str.begin();
-  while (start != str.end() && cmIsSpace(*start)) {
+  while (start != str.end() && cmsysString_isspace(*start)) {
     ++start;
   }
   if (start == str.end()) {
@@ -35,7 +35,7 @@ std::string cmTrimWhitespace(cm::string_view str)
   }
   // NOLINTNEXTLINE(readability-qualified-auto)
   auto stop = str.end() - 1;
-  while (cmIsSpace(*stop)) {
+  while (cmsysString_isspace(*stop)) {
     --stop;
   }
   return std::string(start, stop + 1);
@@ -46,14 +46,14 @@ cm::string_view cmStripWhitespace(cm::string_view str)
   std::string::size_type const l = str.size();
 
   std::string::size_type s = 0;
-  while (s < l && cmIsSpace(str[s])) {
+  while (s < l && cmsysString_isspace(str[s])) {
     ++s;
   }
   if (s == l) {
     return cm::string_view{};
   }
   std::string::size_type e = l - 1;
-  while (cmIsSpace(str[e])) {
+  while (cmsysString_isspace(str[e])) {
     --e;
   }
   return str.substr(s, e + 1 - s);
@@ -198,7 +198,7 @@ bool cmStrToULong(char const* str, unsigned long* value)
 {
   errno = 0;
   char* endp;
-  while (cmIsSpace(*str)) {
+  while (cmsysString_isspace(*str)) {
     ++str;
   }
   if (*str == '-') {
@@ -230,7 +230,7 @@ bool cmStrToULongLong(char const* str, unsigned long long* value)
 {
   errno = 0;
   char* endp;
-  while (cmIsSpace(*str)) {
+  while (cmsysString_isspace(*str)) {
     ++str;
   }
   if (*str == '-') {
diff --git a/Source/cmStringAlgorithms.h b/Source/cmStringAlgorithms.h
index d847ece5bd..966091afd3 100644
--- a/Source/cmStringAlgorithms.h
+++ b/Source/cmStringAlgorithms.h
@@ -4,7 +4,6 @@
 
 #include "cmConfigure.h" // IWYU pragma: keep
 
-#include <cctype>
 #include <cstring>
 #include <initializer_list>
 #include <iterator>
@@ -50,15 +49,6 @@ private:
  */
 bool cmStrCaseEq(cm::string_view a, cm::string_view b);
 
-/** Returns true if the character @a ch is a whitespace character.  **/
-inline bool cmIsSpace(char ch)
-{
-  // isspace takes 'int' but documents that the value must be representable
-  // by 'unsigned char', or be EOF.  Cast to 'unsigned char' to avoid sign
-  // extension while converting to 'int'.
-  return std::isspace(static_cast<unsigned char>(ch));
-}
-
 /** Returns a string that has whitespace removed from the start and the end. */
 std::string cmTrimWhitespace(cm::string_view str);
 
diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index cb088e9e13..780c83a186 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -77,7 +77,6 @@
 
 #include <algorithm>
 #include <cassert>
-#include <cctype>
 #include <cerrno>
 #include <cstdint>
 #include <cstdio>
@@ -101,10 +100,10 @@
 #include "cmsys/Directory.hxx"
 #ifdef _WIN32
 #  include "cmsys/Encoding.hxx"
-#  include "cmsys/String.h"
 #endif
 #include "cmsys/FStream.hxx"
 #include "cmsys/RegularExpression.hxx"
+#include "cmsys/String.h"
 #include "cmsys/System.h"
 
 #if defined(_WIN32)
@@ -575,7 +574,7 @@ void cmSystemTools::ParseWindowsCommandLine(char const* command,
     } else {
       arg.append(backslashes, '\\');
       backslashes = 0;
-      if (cmIsSpace(*c)) {
+      if (cmsysString_isspace(*c)) {
         if (in_quotes) {
           arg.append(1, *c);
         } else if (in_argument) {
@@ -734,7 +733,7 @@ bool cmSystemTools::SplitProgramFromArgs(std::string const& command,
   char const* c = command.c_str();
 
   // Skip leading whitespace.
-  while (cmIsSpace(*c)) {
+  while (cmsysString_isspace(*c)) {
     ++c;
   }
 
@@ -764,7 +763,7 @@ bool cmSystemTools::SplitProgramFromArgs(std::string const& command,
       in_double = true;
     } else if (*c == '\'') {
       in_single = true;
-    } else if (cmIsSpace(*c)) {
+    } else if (cmsysString_isspace(*c)) {
       break;
     } else {
       program += *c;
@@ -3865,7 +3864,7 @@ static size_t cm_strverscmp_find_first_difference_or_end(char const* lhs,
 static size_t cm_strverscmp_find_digits_begin(char const* s, size_t i)
 {
   /* Step back until we are not preceded by a digit.  */
-  while (i > 0 && isdigit(s[i - 1])) {
+  while (i > 0 && cmsysString_isdigit(s[i - 1])) {
     --i;
   }
   return i;
@@ -3874,7 +3873,7 @@ static size_t cm_strverscmp_find_digits_begin(char const* s, size_t i)
 static size_t cm_strverscmp_find_digits_end(char const* s, size_t i)
 {
   /* Step forward over digits.  */
-  while (isdigit(s[i])) {
+  while (cmsysString_isdigit(s[i])) {
     ++i;
   }
   return i;
@@ -3884,7 +3883,7 @@ static size_t cm_strverscmp_count_leading_zeros(char const* s, size_t b)
 {
   size_t i = b;
   /* Step forward over zeros that are followed by another digit.  */
-  while (s[i] == '0' && isdigit(s[i + 1])) {
+  while (s[i] == '0' && cmsysString_isdigit(s[i + 1])) {
     ++i;
   }
   return i - b;
@@ -3896,7 +3895,8 @@ static int cm_strverscmp(char const* lhs, char const* rhs)
   if (lhs[i] != rhs[i]) {
     /* The strings differ starting at 'i'.  Check for a digit sequence.  */
     size_t const b = cm_strverscmp_find_digits_begin(lhs, i);
-    if (b != i || (isdigit(lhs[i]) && isdigit(rhs[i]))) {
+    if (b != i ||
+        (cmsysString_isdigit(lhs[i]) && cmsysString_isdigit(rhs[i]))) {
       /* A digit sequence starts at 'b', preceding or at 'i'.  */
 
       /* Look for leading zeros, implying a leading decimal point.  */