diff --git a/src/V3Ast.cpp b/src/V3Ast.cpp
index 689c000d3..aea94b9aa 100644
--- a/src/V3Ast.cpp
+++ b/src/V3Ast.cpp
@@ -87,8 +87,8 @@ string AstNode::encodeName(const string& namein) {
     // Encode signal name raw from parser, then not called again on same signal
     string out;
     for (string::const_iterator pos = namein.begin(); pos != namein.end(); ++pos) {
-        if ((pos == namein.begin()) ? isalpha(pos[0])  // digits can't lead identifiers
-                                    : isalnum(pos[0])) {
+        if ((pos == namein.begin()) ? std::isalpha(pos[0])  // digits can't lead identifiers
+                                    : std::isalnum(pos[0])) {
             out += pos[0];
         } else if (pos[0] == '_') {
             if (pos[1] == '_') {
@@ -186,11 +186,14 @@ string AstNode::prettyName(const string& namein) {
                 pos += 7;
                 continue;
             }
-            if (pos[0] == '_' && pos[1] == '_' && pos[2] == '0' && isxdigit(pos[3])
-                && isxdigit(pos[4])) {
+            if (pos[0] == '_' && pos[1] == '_' && pos[2] == '0' && std::isxdigit(pos[3])
+                && std::isxdigit(pos[4])) {
                 char value = 0;
-                value += 16 * (isdigit(pos[3]) ? (pos[3] - '0') : (tolower(pos[3]) - 'a' + 10));
-                value += (isdigit(pos[4]) ? (pos[4] - '0') : (tolower(pos[4]) - 'a' + 10));
+                value += 16
+                         * (std::isdigit(pos[3]) ? (pos[3] - '0')
+                                                 : (std::tolower(pos[3]) - 'a' + 10));
+                value
+                    += (std::isdigit(pos[4]) ? (pos[4] - '0') : (std::tolower(pos[4]) - 'a' + 10));
                 pretty += value;
                 pos += 5;
                 continue;
@@ -232,11 +235,14 @@ string AstNode::vpiName(const string& namein) {
             } else if (0 == std::strncmp(pos, "__PVT__", 7)) {
                 pos += 7;
                 continue;
-            } else if (pos[0] == '_' && pos[1] == '_' && pos[2] == '0' && isxdigit(pos[3])
-                       && isxdigit(pos[4])) {
+            } else if (pos[0] == '_' && pos[1] == '_' && pos[2] == '0' && std::isxdigit(pos[3])
+                       && std::isxdigit(pos[4])) {
                 char value = 0;
-                value += 16 * (isdigit(pos[3]) ? (pos[3] - '0') : (tolower(pos[3]) - 'a' + 10));
-                value += (isdigit(pos[4]) ? (pos[4] - '0') : (tolower(pos[4]) - 'a' + 10));
+                value += 16
+                         * (std::isdigit(pos[3]) ? (pos[3] - '0')
+                                                 : (std::tolower(pos[3]) - 'a' + 10));
+                value
+                    += (std::isdigit(pos[4]) ? (pos[4] - '0') : (std::tolower(pos[4]) - 'a' + 10));
 
                 // __ doesn't always imply escaped ident
                 if (value != '_') inEscapedIdent = true;
diff --git a/src/V3Const.cpp b/src/V3Const.cpp
index c2f9b3d60..ed390e107 100644
--- a/src/V3Const.cpp
+++ b/src/V3Const.cpp
@@ -3094,12 +3094,12 @@ private:
                 if (!inPct && ch == '%') {
                     inPct = true;
                     fmt = ch;
-                } else if (inPct && (isdigit(ch) || ch == '.' || ch == '-')) {
+                } else if (inPct && (std::isdigit(ch) || ch == '.' || ch == '-')) {
                     fmt += ch;
                 } else if (inPct) {
                     inPct = false;
                     fmt += ch;
-                    switch (tolower(ch)) {
+                    switch (std::tolower(ch)) {
                     case '%': break;  // %% - just output a %
                     case 'm': break;  // %m - auto insert "name"
                     case 'l': break;  // %l - auto insert "library"
diff --git a/src/V3EmitCFunc.cpp b/src/V3EmitCFunc.cpp
index 29ec3e422..7f60b5371 100644
--- a/src/V3EmitCFunc.cpp
+++ b/src/V3EmitCFunc.cpp
@@ -155,7 +155,7 @@ void EmitCFunc::emitOpName(AstNode* nodep, const string& format, AstNode* lhsp,
             puts("(");
         } else {
             // Normal text
-            if (isalnum(pos[0])) needComma = true;
+            if (std::isalnum(pos[0])) needComma = true;
             COMMA;
             string s;
             s += pos[0];
@@ -329,7 +329,7 @@ void EmitCFunc::displayNode(AstNode* nodep, AstScopeName* scopenamep, const stri
             m_emitDispState.pushFormat(*pos);
         } else {  // Format character
             inPct = false;
-            switch (tolower(pos[0])) {
+            switch (std::tolower(pos[0])) {
             case '0':  // FALLTHRU
             case '1':  // FALLTHRU
             case '2':  // FALLTHRU
diff --git a/src/V3File.cpp b/src/V3File.cpp
index af4fa9162..0921f7956 100644
--- a/src/V3File.cpp
+++ b/src/V3File.cpp
@@ -633,7 +633,7 @@ bool V3OutFormatter::tokenMatch(const char* cp, const char* cmp) {
         ++cmp;
     }
     if (*cmp) return false;
-    if (*cp && !isspace(*cp)) return false;
+    if (*cp && !std::isspace(*cp)) return false;
     return true;
 }
 
@@ -659,7 +659,7 @@ int V3OutFormatter::endLevels(const char* strg) {
     int levels = m_indentLevel;
     {
         const char* cp = strg;
-        while (isspace(*cp)) ++cp;
+        while (std::isspace(*cp)) ++cp;
         switch (*cp) {
         case '\n':  // Newlines.. No need for whitespace before it
             return 0;
@@ -669,7 +669,7 @@ int V3OutFormatter::endLevels(const char* strg) {
         {
             // label/public/private:  Deindent by 2 spaces
             const char* mp = cp;
-            for (; isalnum(*mp); ++mp) {}
+            for (; std::isalnum(*mp); ++mp) {}
             if (mp[0] == ':' && mp[1] != ':') return (levels - m_blockIndent / 2);
         }
     }
@@ -708,7 +708,7 @@ void V3OutFormatter::puts(const char* strg) {
     bool equalsForBracket = false;  // Looking for "= {"
     for (const char* cp = strg; *cp; ++cp) {
         putcNoTracking(*cp);
-        if (isalpha(*cp)) {
+        if (std::isalpha(*cp)) {
             if (wordstart && m_lang == LA_VERILOG && tokenNotStart(cp)) notstart = true;
             if (wordstart && m_lang == LA_VERILOG && !notstart && tokenStart(cp)) indentInc();
             if (wordstart && m_lang == LA_VERILOG && tokenEnd(cp)) indentDec();
@@ -870,7 +870,7 @@ string V3OutFormatter::quoteNameControls(const string& namein, V3OutFormatter::L
                 out += std::string{">"};
             } else if (c == '&') {
                 out += std::string{"&"};
-            } else if (isprint(c)) {
+            } else if (std::isprint(c)) {
                 out += c;
             } else {
                 out += std::string{"&#"} + cvtToStr((unsigned int)(c & 0xff)) + ";";
@@ -887,7 +887,7 @@ string V3OutFormatter::quoteNameControls(const string& namein, V3OutFormatter::L
                 out += "\\r";
             } else if (c == '\t') {
                 out += "\\t";
-            } else if (isprint(c)) {
+            } else if (std::isprint(c)) {
                 out += c;
             } else {
                 // This will also cover \a etc
@@ -942,7 +942,7 @@ void V3OutCFile::putsGuard() {
     string var
         = VString::upcase(std::string{"VERILATED_"} + V3Os::filenameNonDir(filename()) + "_");
     for (char& c : var) {
-        if (!isalnum(c)) c = '_';
+        if (!std::isalnum(c)) c = '_';
     }
     puts("\n#ifndef " + var + "\n");
     puts("#define " + var + "  // guard\n");
diff --git a/src/V3FileLine.cpp b/src/V3FileLine.cpp
index 9bee6a86a..80c2d1a85 100644
--- a/src/V3FileLine.cpp
+++ b/src/V3FileLine.cpp
@@ -210,26 +210,26 @@ void FileLine::lineDirective(const char* textp, int& enterExitRef) {
     // will come from the same stream as the previous line.
 
     // Skip `line
-    while (*textp && isspace(*textp)) textp++;
-    while (*textp && !isspace(*textp)) textp++;
-    while (*textp && (isspace(*textp) || *textp == '"')) textp++;
+    while (*textp && std::isspace(*textp)) ++textp;
+    while (*textp && !std::isspace(*textp)) ++textp;
+    while (*textp && (std::isspace(*textp) || *textp == '"')) ++textp;
 
     // Grab linenumber
     bool fail = false;
     const char* const ln = textp;
-    while (*textp && !isspace(*textp)) textp++;
-    if (isdigit(*ln)) {
+    while (*textp && !std::isspace(*textp)) ++textp;
+    if (std::isdigit(*ln)) {
         lineno(std::atoi(ln));
     } else {
         fail = true;
     }
-    while (*textp && (isspace(*textp))) textp++;
+    while (*textp && (std::isspace(*textp))) ++textp;
     if (*textp != '"') fail = true;
-    while (*textp && (isspace(*textp) || *textp == '"')) textp++;
+    while (*textp && (std::isspace(*textp) || *textp == '"')) ++textp;
 
     // Grab filename
     const char* const fn = textp;
-    while (*textp && !(isspace(*textp) || *textp == '"')) textp++;
+    while (*textp && !(std::isspace(*textp) || *textp == '"')) ++textp;
     if (textp != fn) {
         string strfn = fn;
         strfn = strfn.substr(0, textp - fn);
@@ -239,8 +239,8 @@ void FileLine::lineDirective(const char* textp, int& enterExitRef) {
     }
 
     // Grab level
-    while (*textp && (isspace(*textp) || *textp == '"')) textp++;
-    if (isdigit(*textp)) {
+    while (*textp && (std::isspace(*textp) || *textp == '"')) ++textp;
+    if (std::isdigit(*textp)) {
         enterExitRef = std::atoi(textp);
         if (enterExitRef >= 3) fail = true;
     } else {
diff --git a/src/V3LinkResolve.cpp b/src/V3LinkResolve.cpp
index bdd81ed3f..6ecc90b2c 100644
--- a/src/V3LinkResolve.cpp
+++ b/src/V3LinkResolve.cpp
@@ -212,12 +212,12 @@ private:
                 inPct = true;
                 inIgnore = false;
                 fmt = ch;
-            } else if (inPct && (isdigit(ch) || ch == '.' || ch == '-')) {
+            } else if (inPct && (std::isdigit(ch) || ch == '.' || ch == '-')) {
                 fmt += ch;
             } else if (inPct) {
                 inPct = false;
                 fmt += ch;
-                switch (tolower(ch)) {
+                switch (std::tolower(ch)) {
                 case '%':  // %% - just output a %
                     break;
                 case '*':
diff --git a/src/V3Number.cpp b/src/V3Number.cpp
index 758805db1..c29829059 100644
--- a/src/V3Number.cpp
+++ b/src/V3Number.cpp
@@ -153,7 +153,7 @@ void V3Number::create(const char* sourcep) {
             if (*cp != '_') widthn += *cp;
         }
         while (*cp == '_') cp++;
-        if (*cp && tolower(*cp) == 's') {
+        if (*cp && std::tolower(*cp) == 's') {
             cp++;
             isSigned(true);
         }
@@ -191,11 +191,11 @@ void V3Number::create(const char* sourcep) {
         width(1, false);  // So we extend it
         setBit(0, 1);
         m_data.m_autoExtend = true;
-    } else if (tolower(base) == 'z') {
+    } else if (std::tolower(base) == 'z') {
         width(1, false);  // So we extend it
         setBit(0, 'z');
         m_data.m_autoExtend = true;
-    } else if (tolower(base) == 'x') {
+    } else if (std::tolower(base) == 'x') {
         width(1, false);  // So we extend it
         setBit(0, 'x');
         m_data.m_autoExtend = true;
@@ -207,7 +207,7 @@ void V3Number::create(const char* sourcep) {
     }
 
     // Ignore leading blanks
-    while (*value_startp == '_' || isspace(*value_startp)) value_startp++;
+    while (*value_startp == '_' || std::isspace(*value_startp)) ++value_startp;
     if (!*value_startp && !m_data.m_autoExtend) {
         v3error("Number is missing value digits: " << sourcep);
     }
@@ -217,7 +217,7 @@ void V3Number::create(const char* sourcep) {
     }
 
     int obit = 0;  // Start at LSB
-    if (tolower(base) == 'd') {
+    if (std::tolower(base) == 'd') {
         // Ignore leading zeros so we don't issue too many digit errors when lots of leading 0's
         while (*value_startp == '_' || *value_startp == '0') value_startp++;
         // Convert decimal number to hex
@@ -227,7 +227,7 @@ void V3Number::create(const char* sourcep) {
         int got_z = 0;
         int got_01 = 0;
         for (const char* cp = value_startp; *cp; cp++) {
-            switch (tolower(*cp)) {
+            switch (std::tolower(*cp)) {
             case '0':  // FALLTHRU
             case '1':  // FALLTHRU
             case '2':  // FALLTHRU
@@ -298,9 +298,9 @@ void V3Number::create(const char* sourcep) {
                 v3error("Too many digits for " << width() << " bit number: " << sourcep);
                 break;
             }
-            switch (tolower(base)) {
+            switch (std::tolower(base)) {
             case 'b': {
-                switch (tolower(*cp)) {
+                switch (std::tolower(*cp)) {
                 case '0': setBit(obit++, 0); break;
                 case '1': setBit(obit++, 1); break;
                 case 'z':
@@ -314,7 +314,7 @@ void V3Number::create(const char* sourcep) {
 
             case 'o':
             case 'c': {
-                switch (tolower(*cp)) {
+                switch (std::tolower(*cp)) {
                 // clang-format off
                 case '0': setBit(obit++, 0); setBit(obit++, 0);  setBit(obit++, 0);  break;
                 case '1': setBit(obit++, 1); setBit(obit++, 0);  setBit(obit++, 0);  break;
@@ -335,7 +335,7 @@ void V3Number::create(const char* sourcep) {
             }
 
             case 'h': {
-                switch (tolower(*cp)) {
+                switch (std::tolower(*cp)) {
                     // clang-format off
                 case '0': setBit(obit++,0); setBit(obit++,0); setBit(obit++,0); setBit(obit++,0); break;
                 case '1': setBit(obit++,1); setBit(obit++,0); setBit(obit++,0); setBit(obit++,0); break;
@@ -575,7 +575,7 @@ string V3Number::ascii(bool prefixed, bool cleanVerilog) const {
 
 bool V3Number::displayedFmtLegal(char format, bool isScan) {
     // Is this a valid format letter?
-    switch (tolower(format)) {
+    switch (std::tolower(format)) {
     case 'b': return true;
     case 'c': return true;
     case 'd': return true;  // Unsigned decimal
@@ -619,11 +619,11 @@ string V3Number::displayed(FileLine* fl, const string& vformat) const {
         ++pos;
     }
     string fmtsize;
-    for (; pos != vformat.cend() && (isdigit(pos[0]) || pos[0] == '.'); ++pos) {
+    for (; pos != vformat.cend() && (std::isdigit(pos[0]) || pos[0] == '.'); ++pos) {
         fmtsize += pos[0];
     }
     string str;
-    const char code = tolower(pos[0]);
+    const char code = std::tolower(pos[0]);
     switch (code) {
     case 'b':  // FALLTHRU
     case 'o':  // FALLTHRU
@@ -2468,15 +2468,13 @@ V3Number& V3Number::opReplN(const V3Number& lhs, uint32_t rhsval) {
 V3Number& V3Number::opToLowerN(const V3Number& lhs) {
     NUM_ASSERT_OP_ARGS1(lhs);
     NUM_ASSERT_STRING_ARGS1(lhs);
-    std::string out = lhs.toString();
-    for (auto& cr : out) cr = tolower(cr);
+    std::string out = VString::downcase(lhs.toString());
     return setString(out);
 }
 V3Number& V3Number::opToUpperN(const V3Number& lhs) {
     NUM_ASSERT_OP_ARGS1(lhs);
     NUM_ASSERT_STRING_ARGS1(lhs);
-    std::string out = lhs.toString();
-    for (auto& cr : out) cr = toupper(cr);
+    std::string out = VString::upcase(lhs.toString());
     return setString(out);
 }
 
diff --git a/src/V3Options.cpp b/src/V3Options.cpp
index 71097d95b..25dcdb317 100644
--- a/src/V3Options.cpp
+++ b/src/V3Options.cpp
@@ -237,20 +237,20 @@ void VTimescale::parseSlashed(FileLine* fl, const char* textp, VTimescale& unitr
     precr = VTimescale::NONE;
 
     const char* cp = textp;
-    for (; isspace(*cp); ++cp) {}
+    for (; std::isspace(*cp); ++cp) {}
     const char* const unitp = cp;
     for (; *cp && *cp != '/'; ++cp) {}
     const string unitStr(unitp, cp - unitp);
-    for (; isspace(*cp); ++cp) {}
+    for (; std::isspace(*cp); ++cp) {}
     string precStr;
     if (*cp == '/') {
         ++cp;
-        for (; isspace(*cp); ++cp) {}
+        for (; std::isspace(*cp); ++cp) {}
         const char* const precp = cp;
         for (; *cp && *cp != '/'; ++cp) {}
         precStr = string(precp, cp - precp);
     }
-    for (; isspace(*cp); ++cp) {}
+    for (; std::isspace(*cp); ++cp) {}
     if (*cp) {
         fl->v3error("`timescale syntax error: '" << textp << "'");
         return;
@@ -1295,8 +1295,8 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc, char
         fl->v3warn(DEPRECATED, "Option -O<letter> is deprecated. "
                                "Use -f<optimization> or -fno-<optimization> instead.");
         for (const char* cp = optp; *cp; ++cp) {
-            const bool flag = isupper(*cp);
-            switch (tolower(*cp)) {
+            const bool flag = std::isupper(*cp);
+            switch (std::tolower(*cp)) {
             case '0': optimize(0); break;
             case '1': optimize(1); break;
             case '2': optimize(2); break;
@@ -1648,8 +1648,8 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc, char
             || !std::strcmp(argv[i], "--j")) {  // Allow gnu -- switches
             ++i;
             int val = 0;
-            if (i < argc && isdigit(argv[i][0])) {
-                val = atoi(argv[i]);  // Can't be negative due to isdigit above
+            if (i < argc && std::isdigit(argv[i][0])) {
+                val = std::atoi(argv[i]);  // Can't be negative due to isdigit above
                 if (val == 0) val = std::thread::hardware_concurrency();
                 ++i;
             }
@@ -1718,7 +1718,8 @@ void V3Options::parseOptsFile(FileLine* fl, const string& filename, bool rel) {
                     ++pos;
                 }
             } else if (*pos == '/' && *(pos + 1) == '/'
-                       && (pos == line.begin() || isspace(lastch))) {  // But allow /file//path
+                       && (pos == line.begin()
+                           || std::isspace(lastch))) {  // But allow /file//path
                 break;  // Ignore to EOL
             } else if (*pos == '#' && space_begin) {  // Only # at [spaced] begin of line
                 break;  // Ignore to EOL
@@ -1728,7 +1729,7 @@ void V3Options::parseOptsFile(FileLine* fl, const string& filename, bool rel) {
                 // cppcheck-suppress StlMissingComparison
                 ++pos;
             } else {
-                if (!isspace(*pos)) space_begin = false;
+                if (!std::isspace(*pos)) space_begin = false;
                 oline += *pos;
             }
         }
@@ -1761,7 +1762,7 @@ void V3Options::parseOptsFile(FileLine* fl, const string& filename, bool rel) {
         char curr_char = whole_file[pos];
         switch (st) {
         case ST_IN_OPTION:  // Get all chars up to a white space or a "="
-            if (isspace(curr_char)) {  // End of option
+            if (std::isspace(curr_char)) {  // End of option
                 if (!arg.empty()) {  // End of word
                     args.push_back(arg);
                 }
diff --git a/src/V3Os.cpp b/src/V3Os.cpp
index 01dcfe8d3..a18dd6c2f 100644
--- a/src/V3Os.cpp
+++ b/src/V3Os.cpp
@@ -173,7 +173,7 @@ string V3Os::filenameSubstitute(const string& filename) {
             string::size_type endpos = pos + 1;
             while (((endpos + 1) < filename.length())
                    && (((brackets == NONE)
-                        && (isalnum(filename[endpos + 1]) || filename[endpos + 1] == '_'))
+                        && (std::isalnum(filename[endpos + 1]) || filename[endpos + 1] == '_'))
                        || ((brackets == CURLY) && (filename[endpos + 1] != '}'))
                        || ((brackets == PAREN) && (filename[endpos + 1] != ')'))))
                 ++endpos;
diff --git a/src/V3ParseGrammar.cpp b/src/V3ParseGrammar.cpp
index d77351c7a..b60e6bb07 100644
--- a/src/V3ParseGrammar.cpp
+++ b/src/V3ParseGrammar.cpp
@@ -257,7 +257,7 @@ string V3ParseGrammar::deQuote(FileLine* fileline, string text) {
     int octal_digits = 0;
     for (string::const_iterator cp = text.begin(); cp != text.end(); ++cp) {
         if (quoted) {
-            if (isdigit(*cp)) {
+            if (std::isdigit(*cp)) {
                 octal_val = octal_val * 8 + (*cp - '0');
                 if (++octal_digits == 3) {
                     octal_digits = 0;
@@ -286,13 +286,13 @@ string V3ParseGrammar::deQuote(FileLine* fileline, string text) {
                     newtext += '\t';
                 } else if (*cp == 'v') {
                     newtext += '\v';  // SystemVerilog 3.1
-                } else if (*cp == 'x' && isxdigit(cp[1])
-                           && isxdigit(cp[2])) {  // SystemVerilog 3.1
-#define vl_decodexdigit(c) ((isdigit(c) ? ((c) - '0') : (tolower((c)) - 'a' + 10)))
+                } else if (*cp == 'x' && std::isxdigit(cp[1])
+                           && std::isxdigit(cp[2])) {  // SystemVerilog 3.1
+#define vl_decodexdigit(c) ((std::isdigit(c) ? ((c) - '0') : (std::tolower((c)) - 'a' + 10)))
                     newtext
                         += static_cast<char>(16 * vl_decodexdigit(cp[1]) + vl_decodexdigit(cp[2]));
                     cp += 2;
-                } else if (isalnum(*cp)) {
+                } else if (std::isalnum(*cp)) {
                     fileline->v3error("Unknown escape sequence: \\" << *cp);
                     break;
                 } else {
diff --git a/src/V3ParseImp.cpp b/src/V3ParseImp.cpp
index a6e5cb7d3..1f264a38a 100644
--- a/src/V3ParseImp.cpp
+++ b/src/V3ParseImp.cpp
@@ -134,10 +134,10 @@ void V3ParseImp::lexVerilatorCmtLintRestore(FileLine* fl) {
 
 void V3ParseImp::lexVerilatorCmtLint(FileLine* fl, const char* textp, bool warnOff) {
     const char* sp = textp;
-    while (*sp && !isspace(*sp)) ++sp;
-    while (*sp && isspace(*sp)) ++sp;
-    while (*sp && !isspace(*sp)) ++sp;
-    while (*sp && isspace(*sp)) ++sp;
+    while (*sp && !std::isspace(*sp)) ++sp;
+    while (*sp && std::isspace(*sp)) ++sp;
+    while (*sp && !std::isspace(*sp)) ++sp;
+    while (*sp && std::isspace(*sp)) ++sp;
     string msg = sp;
     string::size_type pos;
     if ((pos = msg.find('*')) != string::npos) msg.erase(pos);
@@ -155,9 +155,9 @@ void V3ParseImp::lexVerilatorCmtBad(FileLine* fl, const char* textp) {
     if (cmtparse.substr(0, std::strlen("/*verilator")) == "/*verilator") {
         cmtparse.replace(0, std::strlen("/*verilator"), "");
     }
-    while (isspace(cmtparse[0])) cmtparse.replace(0, 1, "");
+    while (std::isspace(cmtparse[0])) cmtparse.replace(0, 1, "");
     string cmtname;
-    for (int i = 0; isalnum(cmtparse[i]); i++) { cmtname += cmtparse[i]; }
+    for (int i = 0; std::isalnum(cmtparse[i]); i++) cmtname += cmtparse[i];
     if (!v3Global.opt.isFuture(cmtname)) {
         fl->v3error("Unknown verilator comment: '" << textp << "'");
     }
@@ -191,7 +191,7 @@ double V3ParseImp::lexParseTimenum(const char* textp) {
     char* const strgp = new char[length + 1];
     char* dp = strgp;
     const char* sp = textp;
-    for (; isdigit(*sp) || *sp == '_' || *sp == '.'; ++sp) {
+    for (; std::isdigit(*sp) || *sp == '_' || *sp == '.'; ++sp) {
         if (*sp != '_') *dp++ = *sp;
     }
     *dp++ = '\0';
@@ -256,7 +256,7 @@ void V3ParseImp::preprocDumps(std::ostream& os) {
             if (noblanks) {
                 bool blank = true;
                 for (string::iterator its = buf.begin(); its != buf.end(); ++its) {
-                    if (!isspace(*its) && *its != '\n') {
+                    if (!std::isspace(*its) && *its != '\n') {
                         blank = false;
                         break;
                     }
diff --git a/src/V3PreProc.cpp b/src/V3PreProc.cpp
index ef662dc17..f59b5d1d3 100644
--- a/src/V3PreProc.cpp
+++ b/src/V3PreProc.cpp
@@ -389,14 +389,14 @@ string V3PreProcImp::commentCleanup(const string& text) {
     while ((pos = cmd.find('\"')) != string::npos) cmd.replace(pos, 1, " ");
     while ((pos = cmd.find('\t')) != string::npos) cmd.replace(pos, 1, " ");
     while ((pos = cmd.find("  ")) != string::npos) cmd.replace(pos, 2, " ");
-    while (!cmd.empty() && isspace(cmd[cmd.size() - 1])) cmd.erase(cmd.size() - 1);
+    while (!cmd.empty() && std::isspace(cmd[cmd.size() - 1])) cmd.erase(cmd.size() - 1);
     return cmd;
 }
 
 bool V3PreProcImp::commentTokenMatch(string& cmdr, const char* strg) {
     int len = std::strlen(strg);
-    if (VString::startsWith(cmdr, strg) && (cmdr[len] == '\0' || isspace(cmdr[len]))) {
-        if (isspace(cmdr[len])) len++;
+    if (VString::startsWith(cmdr, strg) && (cmdr[len] == '\0' || std::isspace(cmdr[len]))) {
+        if (std::isspace(cmdr[len])) len++;
         cmdr = cmdr.substr(len);
         return true;
     } else {
@@ -419,7 +419,7 @@ void V3PreProcImp::comment(const string& text) {
         return;
     }
 
-    while (isspace(*cp)) cp++;
+    while (std::isspace(*cp)) ++cp;
 
     bool synth = false;
     bool vlcomment = false;
@@ -452,7 +452,7 @@ void V3PreProcImp::comment(const string& text) {
 
     if (!vlcomment && !synth) return;  // Short-circuit
 
-    while (isspace(*cp)) cp++;
+    while (std::isspace(*cp)) ++cp;
     string cmd = commentCleanup(string(cp));
     // cmd now is comment without extra spaces and "verilator" prefix
 
@@ -480,7 +480,7 @@ void V3PreProcImp::comment(const string& text) {
             while (VString::isWordChar(cmd[endOfCmd])) ++endOfCmd;
             string baseCmd = cmd.substr(0, endOfCmd);
             string arg = cmd.substr(endOfCmd);
-            while (isspace(arg[0])) arg = arg.substr(1);
+            while (std::isspace(arg[0])) arg = arg.substr(1);
             if (arg.size() && baseCmd == "public_flat_rw_on")
                 baseCmd += "_sns";  // different cmd for applying sensitivity
             if (!printed) insertUnreadback("/*verilator " + baseCmd + "*/ " + arg + " /**/");
@@ -560,16 +560,16 @@ string V3PreProcImp::trimWhitespace(const string& strg, bool trailing) {
     // Remove leading whitespace
     string out = strg;
     string::size_type leadspace = 0;
-    while (out.length() > leadspace && isspace(out[leadspace])) leadspace++;
+    while (out.length() > leadspace && std::isspace(out[leadspace])) ++leadspace;
     if (leadspace) out.erase(0, leadspace);
     // Remove trailing whitespace
     if (trailing) {
         string::size_type trailspace = 0;
-        while (out.length() > trailspace && isspace(out[out.length() - 1 - trailspace]))
-            trailspace++;
+        while (out.length() > trailspace && std::isspace(out[out.length() - 1 - trailspace]))
+            ++trailspace;
         // Don't remove \{space_or_newline}
         if (trailspace && out.length() > trailspace && out[out.length() - 1 - trailspace] == '\\')
-            trailspace--;
+            --trailspace;
         if (trailspace) out.erase(out.length() - trailspace, trailspace);
     }
     return out;
@@ -681,13 +681,13 @@ string V3PreProcImp::defineSubst(VDefineRef* refp) {
             // UINFO(4, "CH "<<*cp<<"  an "<<argName<<endl);
             if (!quote && *cp == '\\') {
                 backslashesc = true;
-            } else if (isspace(*cp)) {
+            } else if (std::isspace(*cp)) {
                 backslashesc = false;
             }
             // We don't check for quotes; some simulators expand even inside quotes
-            if (isalpha(*cp) || *cp == '_'
+            if (std::isalpha(*cp) || *cp == '_'
                 || *cp == '$'  // Won't replace system functions, since no $ in argValueByName
-                || (argName != "" && (isdigit(*cp) || *cp == '$'))) {
+                || (argName != "" && (std::isdigit(*cp) || *cp == '$'))) {
                 argName += *cp;
                 continue;
             }
diff --git a/src/V3Simulate.h b/src/V3Simulate.h
index d6ea92e26..7668b453d 100644
--- a/src/V3Simulate.h
+++ b/src/V3Simulate.h
@@ -1099,7 +1099,7 @@ private:
 
                     inPct = false;
 
-                    if (V3Number::displayedFmtLegal(tolower(pos[0]), false)) {
+                    if (V3Number::displayedFmtLegal(std::tolower(pos[0]), false)) {
                         AstNode* const argp = nextArgp;
                         nextArgp = nextArgp->nextp();
                         AstConst* const constp = fetchConstNull(argp);
@@ -1111,7 +1111,7 @@ private:
                         const string pformat = std::string{"%"} + width + pos[0];
                         result += constp->num().displayed(nodep, pformat);
                     } else {
-                        switch (tolower(pos[0])) {
+                        switch (std::tolower(pos[0])) {
                         case '%': result += "%"; break;
                         case 'm':
                             // This happens prior to AstScope so we don't
diff --git a/src/V3String.cpp b/src/V3String.cpp
index 28f381444..f0aa6ecc8 100644
--- a/src/V3String.cpp
+++ b/src/V3String.cpp
@@ -80,13 +80,13 @@ string VString::dot(const string& a, const string& dot, const string& b) {
 
 string VString::downcase(const string& str) {
     string out = str;
-    for (auto& cr : out) cr = tolower(cr);
+    for (auto& cr : out) cr = std::tolower(cr);
     return out;
 }
 
 string VString::upcase(const string& str) {
     string out = str;
-    for (auto& cr : out) cr = toupper(cr);
+    for (auto& cr : out) cr = std::toupper(cr);
     return out;
 }
 
@@ -133,7 +133,7 @@ string VString::escapeStringForPath(const string& str) {
 string VString::spaceUnprintable(const string& str) {
     string out;
     for (const char c : str) {
-        if (isprint(c)) {
+        if (std::isprint(c)) {
             out += c;
         } else {
             out += ' ';
@@ -146,14 +146,14 @@ string VString::removeWhitespace(const string& str) {
     string out;
     out.reserve(str.size());
     for (const char c : str) {
-        if (!isspace(c)) out += c;
+        if (!std::isspace(c)) out += c;
     }
     return out;
 }
 
 bool VString::isWhitespace(const string& str) {
     for (const char c : str) {
-        if (!isspace(c)) return false;
+        if (!std::isspace(c)) return false;
     }
     return true;
 }
diff --git a/src/V3String.h b/src/V3String.h
index 85bdc06fd..bbd3ba948 100644
--- a/src/V3String.h
+++ b/src/V3String.h
@@ -70,7 +70,7 @@ inline uint32_t cvtToHash(const void* vp) {
 
 inline string ucfirst(const string& text) {
     string out = text;
-    out[0] = toupper(out[0]);
+    out[0] = std::toupper(out[0]);
     return out;
 }
 
diff --git a/src/V3Width.cpp b/src/V3Width.cpp
index 55ef8763c..194fb7cd9 100644
--- a/src/V3Width.cpp
+++ b/src/V3Width.cpp
@@ -4606,12 +4606,12 @@ private:
             if (!inPct && ch == '%') {
                 inPct = true;
                 fmt = ch;
-            } else if (inPct && (isdigit(ch) || ch == '.' || ch == '-')) {
+            } else if (inPct && (std::isdigit(ch) || ch == '.' || ch == '-')) {
                 fmt += ch;
             } else if (inPct) {
                 inPct = false;
                 bool added = false;
-                switch (tolower(ch)) {
+                switch (std::tolower(ch)) {
                 case '%': break;  // %% - just output a %
                 case 'm': break;  // %m - auto insert "name"
                 case 'l': break;  // %m - auto insert "library"
diff --git a/src/verilog.l b/src/verilog.l
index 1a25e7c42..a5bd2d1d7 100644
--- a/src/verilog.l
+++ b/src/verilog.l
@@ -910,7 +910,7 @@ vnum    {vnum1}|{vnum2}|{vnum3}|{vnum4}|{vnum5}
                           /* "# 1'b0" is a delay value so must lex as "#" "1" "'b0" */
                           if (PARSEP->lexPrevToken()=='#') {
                               int shortlen = 0;
-                              while (isdigit(yytext[shortlen])) ++shortlen;
+                              while (std::isdigit(yytext[shortlen])) ++shortlen;
                               if (shortlen) {
                                   // Push rest past numbers for later parse
                                   PARSEP->lexUnputString(yytext + shortlen, yyleng - shortlen);