diff --git a/string.hpp b/string.hpp
index 301b027d..fc531b12 100644
--- a/string.hpp
+++ b/string.hpp
@@ -21,23 +21,26 @@ namespace cruft {
     std::string to_utf8 (const std::wstring&);
 
 
-    /// Convert the provided string to all upper case
-    inline std::string
-    to_upper (std::string &&val)
+    /// Apply a functor to each character of a string and return the result.
+    template <typename FunctionT>
+    std::string
+    transform (std::string &&val, FunctionT &&func)
     {
         std::transform (
             std::begin (val),
             std::end   (val),
             std::begin (val),
-            ::toupper
+            std::forward<FunctionT> (func)
         );
+
         return std::move (val);
     }
 
 
-    /// Convert the provided string to all upper case
-    inline std::string
-    to_upper (std::string const &val)
+    /// Apply a functor to each character of a string and return the result.
+    template <typename FunctionT>
+    std::string
+    transform (std::string const &val, FunctionT &&func)
     {
         std::string res;
         res.reserve (val.size ());
@@ -46,11 +49,43 @@ namespace cruft {
             std::begin (val),
             std::end   (val),
             std::back_inserter (res),
-            ::toupper
+            std::forward<FunctionT> (func)
         );
 
         return res;
     }
+
+
+    /// Convert the provided string to all upper case
+    inline std::string
+    to_upper (std::string &&val)
+    {
+        return transform (std::move (val), ::toupper);
+    }
+
+
+    /// Convert the provided string to all upper case
+    inline std::string
+    to_upper (std::string const &val)
+    {
+        return transform (val, ::toupper);
+    }
+
+
+    /// Convert the provided string to all upper case
+    inline std::string
+    to_lower (std::string &&val)
+    {
+        return transform (std::move (val), ::tolower);
+    }
+
+
+    /// Convert the provided string to all upper case
+    inline std::string
+    to_lower (std::string const &val)
+    {
+        return transform (val, ::tolower);
+    }
 }
 
 
diff --git a/test/string.cpp b/test/string.cpp
index ee510175..30c3443e 100644
--- a/test/string.cpp
+++ b/test/string.cpp
@@ -6,6 +6,32 @@
 #include <vector>
 
 
+///////////////////////////////////////////////////////////////////////////////
+void
+test_transforms (cruft::TAP::logger &tap)
+{
+    static struct {
+        std::string initial;
+        std::string upper;
+        std::string lower;
+
+        char const *message;
+    } const TESTS[] = {
+        { "a", "A", "a", "single lower character" },
+        { "A", "A", "a", "single upper character" },
+        { "asdf", "ASDF", "asdf", "four lower characters" },
+        { "ASDF", "ASDF", "asdf", "four upper characters" },
+
+        { "Don't.", "DON'T.", "don't.", "upper and lower and symbols" },
+    };
+
+    for (auto const [src, upper, lower, msg]: TESTS) {
+        tap.expect_eq (cruft::to_upper (src), upper, "to_upper, %!", msg);
+        tap.expect_eq (cruft::to_lower (src), lower, "to_lower, %!", msg);
+    }
+}
+
+
 ///////////////////////////////////////////////////////////////////////////////
 void
 test_position (cruft::TAP::logger &tap)
@@ -203,6 +229,7 @@ main (int, char**)
 {
     cruft::TAP::logger tap;
 
+    test_transforms (tap);
     test_tokeniser (tap);
     test_position (tap);
     test_contains (tap);