diff --git a/string.hpp b/string.hpp
index 19177ece..70eadf06 100644
--- a/string.hpp
+++ b/string.hpp
@@ -8,6 +8,7 @@
 
 #pragma once
 
+#include "ascii.hpp"
 #include "debug.hpp"
 #include "view.hpp"
 
@@ -216,3 +217,57 @@ namespace cruft {
         }
     };
 }
+
+
+namespace cruft::string::compare {
+    ///////////////////////////////////////////////////////////////////////////
+    /// A case comparator that tests equality on a string after a
+    /// per-character transform is applied.
+    ///
+    /// Neither string will be modified.
+    ///
+    /// \tparam TransformV   A character transform function
+    template <char (*TransformV)(char) noexcept>
+    struct transform {
+        template <
+            typename CharT,
+            typename = std::void_t<
+                typename std::char_traits<CharT>::char_type
+            >
+        >
+        bool operator() (cruft::view<CharT const *> a, cruft::view<CharT const*> b)
+        {
+            if (a.size () != b.size ())
+                return false;
+
+            for (auto i = a.begin (), j = b.begin (); i != a.end (); ++i, ++j)
+                if (TransformV (*i) != TransformV (*j))
+                    return false;
+
+            return true;
+        }
+
+
+        template <
+            typename CharT,
+            typename = std::void_t<
+                typename std::char_traits<CharT>::char_type
+            >
+        >
+        bool operator() (CharT const *a, CharT const *b) const noexcept
+        {
+            auto const *i = a;
+            auto const *j = b;
+
+            for ( ; *i && *j; ++i, ++j)
+                if (TransformV (*i) != TransformV (*j))
+                    return false;
+
+            // Ensure we've reached the ends of both strings
+            return !*i && !*j;
+        }
+    };
+
+    using lower = transform<ascii::try_lower>;
+    using upper = transform<ascii::try_upper>;
+}
diff --git a/test/string.cpp b/test/string.cpp
index 529b2ded..ee510175 100644
--- a/test/string.cpp
+++ b/test/string.cpp
@@ -144,7 +144,7 @@ test_contains (cruft::TAP::logger &tap)
 
 
 ///////////////////////////////////////////////////////////////////////////////
-void test_comparator (cruft::TAP::logger &tap)
+void test_comparator_less (cruft::TAP::logger &tap)
 {
     cruft::string_less cmp;
 
@@ -164,6 +164,39 @@ void test_comparator (cruft::TAP::logger &tap)
 }
 
 
+///////////////////////////////////////////////////////////////////////////////
+void test_comparator_lower (cruft::TAP::logger &tap)
+{
+    struct {
+        char const *a;
+        char const *b;
+        bool equal;
+        char const *message;
+    } const TESTS[] = {
+        { "a", "a", true, "both single lower" },
+        { "a", "A", true, "lower, upper singles" },
+        { "A", "a", true, "upper, lower singles" },
+        { "AbcDEf", "aBcdEF", true, "mixed upper lower" },
+        { "A", "aa", false, "mixed case, different lengths" },
+    };
+
+    cruft::string::compare::lower cmp;
+
+    for (auto const &t: TESTS) {
+        tap.expect_eq (cmp (t.a, t.b), t.equal, "compare::lower, pointers, %!", t.message);
+        tap.expect_eq (
+            cmp (
+                cruft::view (t.a),
+                cruft::view (t.b)
+            ),
+            t.equal,
+            "compare::lower, views, %!",
+            t.message
+        );
+    }
+}
+
+
 ///////////////////////////////////////////////////////////////////////////////
 int
 main (int, char**)
@@ -173,7 +206,8 @@ main (int, char**)
     test_tokeniser (tap);
     test_position (tap);
     test_contains (tap);
-    test_comparator (tap);
+    test_comparator_less (tap);
+    test_comparator_lower (tap);
 
     return tap.status ();
 }
\ No newline at end of file