diff --git a/src/common/utils/string.cpp b/src/common/utils/string.cpp
index 80e4610..b263025 100644
--- a/src/common/utils/string.cpp
+++ b/src/common/utils/string.cpp
@@ -31,20 +31,24 @@ std::vector<std::string> split(const std::string& s, const char delim) {
   return elems;
 }
 
-std::string to_lower(std::string text) {
-  std::transform(text.begin(), text.end(), text.begin(), [](const char input) {
-    return static_cast<char>(tolower(input));
-  });
+std::string to_lower(const std::string text) {
+  std::string result;
+  std::ranges::transform(text, std::back_inserter(result),
+                         [](const unsigned char input) {
+                           return static_cast<char>(std::tolower(input));
+                         });
 
   return text;
 }
 
-std::string to_upper(std::string text) {
-  std::transform(text.begin(), text.end(), text.begin(), [](const char input) {
-    return static_cast<char>(toupper(input));
-  });
+std::string to_upper(const std::string text) {
+  std::string result;
+  std::ranges::transform(text, std::back_inserter(result),
+                         [](const unsigned char input) {
+                           return static_cast<char>(std::toupper(input));
+                         });
 
-  return text;
+  return result;
 }
 
 bool starts_with(const std::string& text, const std::string& substring) {
diff --git a/src/common/utils/string.hpp b/src/common/utils/string.hpp
index 00426c3..a3de846 100644
--- a/src/common/utils/string.hpp
+++ b/src/common/utils/string.hpp
@@ -76,8 +76,8 @@ const char* va(const char* fmt, ...);
 
 std::vector<std::string> split(const std::string& s, char delim);
 
-std::string to_lower(std::string text);
-std::string to_upper(std::string text);
+std::string to_lower(const std::string& text);
+std::string to_upper(const std::string& text);
 bool starts_with(const std::string& text, const std::string& substring);
 bool ends_with(const std::string& text, const std::string& substring);