diff --git a/src/libutil/string.hh b/src/libutil/string.hh
new file mode 100644
index 000000000..16ef75643
--- /dev/null
+++ b/src/libutil/string.hh
@@ -0,0 +1,17 @@
+#pragma once
+
+namespace nix {
+
+    /** Locale-independent version of std::islower(). */
+    inline bool isASCIILower(char c) { return c >= 'a' && c <= 'z'; };
+
+    /** Locale-independent version of std::isupper(). */
+    inline bool isASCIIUpper(char c) { return c >= 'A' && c <= 'Z'; };
+
+    /** Locale-independent version of std::isalpha(). */
+    inline bool isASCIIAlpha(char c) { return isASCIILower(c) || isASCIIUpper(c); };
+
+    /** Locale-independent version of std::isdigit(). */
+    inline bool isASCIIDigit(char c) { return c >= '0' && c <= '9'; };
+
+}
diff --git a/tests/unit/libutil/string.cc b/tests/unit/libutil/string.cc
new file mode 100644
index 000000000..381f2cc15
--- /dev/null
+++ b/tests/unit/libutil/string.cc
@@ -0,0 +1,59 @@
+#include <gtest/gtest.h>
+#include "string.hh"
+
+namespace nix {
+
+TEST(string, isASCIILower) {
+    ASSERT_TRUE(isASCIILower('a'));
+    ASSERT_TRUE(isASCIILower('z'));
+    ASSERT_FALSE(isASCIILower('A'));
+    ASSERT_FALSE(isASCIILower('Z'));
+    ASSERT_FALSE(isASCIILower('0'));
+    ASSERT_FALSE(isASCIILower('9'));
+    ASSERT_FALSE(isASCIILower(' '));
+    ASSERT_FALSE(isASCIILower('\n'));
+    ASSERT_FALSE(isASCIILower('\t'));
+    ASSERT_FALSE(isASCIILower(':'));
+}
+
+TEST(string, isASCIIUpper) {
+    ASSERT_FALSE(isASCIIUpper('a'));
+    ASSERT_FALSE(isASCIIUpper('z'));
+    ASSERT_TRUE(isASCIIUpper('A'));
+    ASSERT_TRUE(isASCIIUpper('Z'));
+    ASSERT_FALSE(isASCIIUpper('0'));
+    ASSERT_FALSE(isASCIIUpper('9'));
+    ASSERT_FALSE(isASCIIUpper(' '));
+    ASSERT_FALSE(isASCIIUpper('\n'));
+    ASSERT_FALSE(isASCIIUpper('\t'));
+    ASSERT_FALSE(isASCIIUpper(':'));
+}
+
+TEST(string, isASCIIAlpha) {
+    ASSERT_TRUE(isASCIIAlpha('a'));
+    ASSERT_TRUE(isASCIIAlpha('z'));
+    ASSERT_TRUE(isASCIIAlpha('A'));
+    ASSERT_TRUE(isASCIIAlpha('Z'));
+    ASSERT_FALSE(isASCIIAlpha('0'));
+    ASSERT_FALSE(isASCIIAlpha('9'));
+    ASSERT_FALSE(isASCIIAlpha(' '));
+    ASSERT_FALSE(isASCIIAlpha('\n'));
+    ASSERT_FALSE(isASCIIAlpha('\t'));
+    ASSERT_FALSE(isASCIIAlpha(':'));
+}
+
+TEST(string, isASCIIDigit) {
+    ASSERT_FALSE(isASCIIDigit('a'));
+    ASSERT_FALSE(isASCIIDigit('z'));
+    ASSERT_FALSE(isASCIIDigit('A'));
+    ASSERT_FALSE(isASCIIDigit('Z'));
+    ASSERT_TRUE(isASCIIDigit('0'));
+    ASSERT_TRUE(isASCIIDigit('1'));
+    ASSERT_TRUE(isASCIIDigit('9'));
+    ASSERT_FALSE(isASCIIDigit(' '));
+    ASSERT_FALSE(isASCIIDigit('\n'));
+    ASSERT_FALSE(isASCIIDigit('\t'));
+    ASSERT_FALSE(isASCIIDigit(':'));
+}
+
+}
\ No newline at end of file