test: write test to distinguish all colors (#518)

lib_test/expect/test_color.ml

@@ -0,0 +1,13 @@
+open Import
+
+let all_chars = String.init 256 Char.chr
+
+let%expect_test "match an re that distinguishes every single char" =
+  let re =
+    let open Re in
+    set all_chars |> whole_string |> compile
+  in
+  for i = 0 to String.length all_chars - 1 do
+    assert (Re.execp re (String.make 1 all_chars.[i]))
+  done
+;;