diff --git a/modules/04_arrays_and_strings/include/std_string.hpp b/modules/04_arrays_and_strings/include/std_string.hpp
index 1b0fa9a..8377141 100644
--- a/modules/04_arrays_and_strings/include/std_string.hpp
+++ b/modules/04_arrays_and_strings/include/std_string.hpp
@@ -4,9 +4,9 @@
 
 namespace arrays {
 
-[[nodiscard]] auto to_uppercase(std::string text) -> std::string;
+[[nodiscard]] auto to_uppercase(const std::string &text) -> std::string;
 [[nodiscard]] auto trim(const std::string& text) -> std::string;
-[[nodiscard]] auto replace_all(std::string text, char from, char to) -> std::string;
+[[nodiscard]] auto replace_all(const std::string &text, char from, char to) -> std::string;
 [[nodiscard]] auto starts_with(const std::string& text, const std::string& prefix) -> bool;
 
 }  // namespace arrays
diff --git a/modules/04_arrays_and_strings/src/std_string.cpp b/modules/04_arrays_and_strings/src/std_string.cpp
index 9f1f729..e03de4a 100644
--- a/modules/04_arrays_and_strings/src/std_string.cpp
+++ b/modules/04_arrays_and_strings/src/std_string.cpp
@@ -1,10 +1,46 @@
 #include "std_string.hpp"
 
+#include <algorithm>
+#include <locale>
+#include <ranges>
+#include <sstream>
+
 namespace arrays {
 
-auto to_uppercase(std::string text) -> std::string { (void)text; return {}; }
-auto trim(const std::string& text) -> std::string { (void)text; return {}; }
-auto replace_all(std::string text, char from, char to) -> std::string { (void)text; (void)from; (void)to; return {}; }
-auto starts_with(const std::string& text, const std::string& prefix) -> bool { (void)text; (void)prefix; return false; }
+auto to_uppercase(const std::string &text) -> std::string {
+    // Returns copy of the string without mutating the original string
+    auto s = std::string{text};
+    auto c_to_upper = [](const unsigned char c) { return std::toupper(c); };
+    std::ranges::transform(s, s.begin(), c_to_upper);
+    return s;
+}
+
+auto trim(const std::string& text) -> std::string {
+    // Returns a trimmed copy of the string
+    auto s = std::string{text};
+    auto is_not_space = [](const unsigned char c) { return !std::isspace(c); };
+    const auto first = std::ranges::find_if(s, is_not_space);
+    const auto last = std::ranges::find_if(s.rbegin(), s.rend(), is_not_space).base();
+    s.erase(last, s.end());
+    s.erase(s.begin(), first);
+    return s;
+}
+
+auto replace_all(const std::string &text, const char from, const char to) -> std::string {
+    // This one also copies the string
+    auto s = std::string{text};
+    std::ranges::replace(s, from, to);
+    return s;
+}
+
+auto starts_with(const std::string& text, const std::string& prefix) -> bool {
+    // Since C++20 you can use std::string::starts_with, but anyways.
+    if (text.size() < prefix.size()) return false;
+    // uz -> unsigned size_t
+    for (auto idx = 0uz; idx < prefix.size(); ++idx) {
+        if (text[idx] != prefix[idx]) return false;
+    }
+    return true;
+}
 
 }  // namespace arrays
diff --git a/modules/04_arrays_and_strings/test/std_string_test.cpp b/modules/04_arrays_and_strings/test/std_string_test.cpp
index 4cd21d1..55e5c2f 100644
--- a/modules/04_arrays_and_strings/test/std_string_test.cpp
+++ b/modules/04_arrays_and_strings/test/std_string_test.cpp
@@ -7,6 +7,7 @@ TEST(StdString, ToUppercase) {
 
 TEST(StdString, Trim) {
     EXPECT_EQ(arrays::trim("  hi  "), "hi");
+    EXPECT_EQ(arrays::trim("  a b c  "), "a b c");
 }
 
 TEST(StdString, ReplaceAll) {