Fix UTF-16 character handling in regex patterns

lib/rdf/literal.ex

@@ -370,11 +370,18 @@ defmodule RDF.Literal do
   end
 
   defp xpath_regex_pattern(pattern, flags) do
-    with {:ok, regex} <- Regex.compile(pattern, xpath_regex_flags(flags)) do
+    with {:ok, regex} <-
+           pattern
+           |> convert_utf16_escaping()
+           |> Regex.compile(xpath_regex_flags(flags)) do
       {:regex, regex}
     end
   end
 
+  defp convert_utf16_escaping(pattern) do
+    String.replace(pattern, ~r/\\U(([0-9]|[A-F]|[a-f]){2})(([0-9]|[A-F]|[a-f]){6})/, "\\u{\\3}")
+  end
+
   defp xpath_regex_flags(flags) do
     String.replace(flags, "q", "") <> "u"
   end

test/unit/literal_test.exs

@@ -252,6 +252,7 @@ defmodule RDF.LiteralTest do
         {~L"abracadabra", ~L"^bra",   false},
         {@poem, ~L"Kaum.*krähen",     false},
         {@poem, ~L"^Kaum.*gesehen,$", false},
+        {~L"\u{01D4B8}", ~L"\\U0001D4B8", true},
 
         {~L"abracadabra"en, ~L"bra", true},
         {"abracadabra", "bra",       true},