diff --git a/include/util.hpp b/include/util.hpp
index 46ab672c..5abd7d57 100644
--- a/include/util.hpp
+++ b/include/util.hpp
@@ -4,7 +4,6 @@
 #define RGBDS_UTIL_HPP
 
 #include <algorithm>
-#include <ctype.h> // toupper
 #include <numeric>
 #include <optional>
 #include <stddef.h>
@@ -22,6 +21,7 @@ enum NumberBase {
 	BASE_16 = 16,
 };
 
+// Locale-independent character class functions
 bool isNewline(int c);
 bool isBlankSpace(int c);
 bool isWhitespace(int c);
@@ -35,6 +35,10 @@ bool isOctDigit(int c);
 bool isHexDigit(int c);
 bool isAlphanumeric(int c);
 
+// Locale-independent character transform functions
+char toLower(char c);
+char toUpper(char c);
+
 bool startsIdentifier(int c);
 bool continuesIdentifier(int c);
 
@@ -48,14 +52,14 @@ struct Uppercase {
 	// FNV-1a hash of an uppercased string
 	constexpr size_t operator()(std::string const &str) const {
 		return std::accumulate(RANGE(str), 0x811C9DC5, [](size_t hash, char c) {
-			return (hash ^ toupper(c)) * 16777619;
+			return (hash ^ toUpper(c)) * 16777619;
 		});
 	}
 
 	// Compare two strings without case-sensitivity (by converting to uppercase)
 	constexpr bool operator()(std::string const &str1, std::string const &str2) const {
 		return std::equal(RANGE(str1), RANGE(str2), [](char c1, char c2) {
-			return toupper(c1) == toupper(c2);
+			return toUpper(c1) == toUpper(c2);
 		});
 	}
 };
diff --git a/src/asm/parser.y b/src/asm/parser.y
index 8bf7ceef..1e3f509c 100644
--- a/src/asm/parser.y
+++ b/src/asm/parser.y
@@ -32,7 +32,6 @@
 
 %code {
 	#include <algorithm>
-	#include <ctype.h>
 	#include <inttypes.h>
 	#include <optional>
 	#include <stdio.h>
@@ -42,6 +41,7 @@
 
 	#include "extern/utf8decoder.hpp"
 	#include "helpers.hpp"
+	#include "util.hpp" // toLower, toUpper
 
 	#include "asm/charmap.hpp"
 	#include "asm/fixpoint.hpp"
@@ -1599,11 +1599,11 @@ string_literal:
 	}
 	| OP_STRUPR LPAREN string RPAREN {
 		$$ = std::move($3);
-		std::transform(RANGE($$), $$.begin(), [](char c) { return toupper(c); });
+		std::transform(RANGE($$), $$.begin(), toUpper);
 	}
 	| OP_STRLWR LPAREN string RPAREN {
 		$$ = std::move($3);
-		std::transform(RANGE($$), $$.begin(), [](char c) { return tolower(c); });
+		std::transform(RANGE($$), $$.begin(), toLower);
 	}
 	| OP_STRRPL LPAREN string COMMA string COMMA string RPAREN {
 		$$ = act_StringReplace($3, $5, $7);
diff --git a/src/fix/mbc.cpp b/src/fix/mbc.cpp
index b21ec94f..219be9c2 100644
--- a/src/fix/mbc.cpp
+++ b/src/fix/mbc.cpp
@@ -12,7 +12,7 @@
 
 #include "helpers.hpp"  // unreachable_
 #include "platform.hpp" // strcasecmp
-#include "util.hpp"     // isBlankSpace, isLower, isDigit
+#include "util.hpp"
 
 #include "fix/warning.hpp"
 
@@ -103,12 +103,10 @@ static void skipMBCSpace(char const *&ptr) {
 }
 
 static char normalizeMBCChar(char c) {
-	if (isLower(c)) {
-		c = c - 'a' + 'A'; // Uppercase for comparison with `mbc_Name`s
-	} else if (c == '_') {
-		c = ' '; // Treat underscores as spaces
+	if (c == '_') {
+		return ' '; // Treat underscores as spaces
 	}
-	return c;
+	return toUpper(c); // Uppercase for comparison with `mbc_Name`s
 }
 
 [[noreturn]]
diff --git a/src/util.cpp b/src/util.cpp
index 24bf6f5a..514c172f 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -58,6 +58,14 @@ bool isAlphanumeric(int c) {
 	return isLetter(c) || isDigit(c);
 }
 
+char toLower(char c) {
+	return isUpper(c) ? c - 'A' + 'a' : c;
+}
+
+char toUpper(char c) {
+	return isLower(c) ? c - 'a' + 'A' : c;
+}
+
 bool startsIdentifier(int c) {
 	// This returns false for anonymous labels, which internally start with a '!',
 	// and for section fragment literal labels, which internally start with a '$'.