diff --git a/bin/day2403.ml b/bin/day2403.ml
new file mode 100644
index 0000000..b82fb6d
--- /dev/null
+++ b/bin/day2403.ml
@@ -0,0 +1,23 @@
+let matched_group_opt n s = try Some (Str.matched_group n s) with _ -> None
+
+let find_nums s =
+  let r = Str.regexp {|mul(\([0-9][0-9]?[0-9]?\),\([0-9][0-9]?[0-9]?\))|} in
+  let rec impl acc pos =
+    try
+      let _ = Str.search_forward r s pos in
+      let p0 = Str.matched_group 0 s in
+      let p1 = matched_group_opt 1 s in
+      let p2 = matched_group_opt 2 s in
+      impl
+        ((p0, Option.map int_of_string p1, Option.map int_of_string p2) :: acc)
+        (Str.match_end ())
+    with Not_found -> acc
+  in
+  impl [] 0
+
+let nums_from_file fname =
+  Aoc.strings_from_file fname |> List.map find_nums |> List.concat
+
+let mac acc a b = acc + (Option.value a ~default:0 * Option.value b ~default:0)
+let day2403a = List.fold_left (fun acc (_, a, b) -> mac acc a b) 0
+let _ = Aoc.main nums_from_file [ (string_of_int, day2403a) ]
diff --git a/bin/dune b/bin/dune
index a3f35b9..4865e91 100644
--- a/bin/dune
+++ b/bin/dune
@@ -1,4 +1,4 @@
 (executables
- (public_names day2401 day2402)
- (names day2401 day2402)
+ (public_names day2401 day2402 day2403)
+ (names day2401 day2402 day2403)
  (libraries str aoc))