Fix handling of vocabulary terms starting with an underscore

lib/rdf/vocabulary_namespace.ex

@@ -304,14 +304,18 @@ defmodule RDF.Vocabulary.Namespace do
 
   defp detect_case_violations(terms, data, base_uri) do
     aliased_terms = aliased_terms(terms)
-    Enum.filter terms, fn
-      {term, true} ->
-        if not term in aliased_terms do
-          proper_case?(term, base_uri, Atom.to_string(term), data)
-        end
-      {term, original_term} ->
-        proper_case?(term, base_uri, original_term, data)
-      end
+    terms
+    |> Enum.filter(fn {term, _} ->
+         not(Atom.to_string(term) |> String.starts_with?("_"))
+       end)
+    |> Enum.filter(fn
+         {term, true} ->
+           if not term in aliased_terms do
+             proper_case?(term, base_uri, Atom.to_string(term), data)
+           end
+         {term, original_term} ->
+           proper_case?(term, base_uri, original_term, data)
+       end)
   end
 
   defp proper_case?(term, base_uri, uri_suffix, data) do
@@ -502,7 +506,7 @@ defmodule RDF.Vocabulary.Namespace do
   defp lowercase?(term) when is_atom(term),
     do: Atom.to_string(term) |> lowercase?
   defp lowercase?(term),
-    do: term =~ ~r/^\p{Ll}/u
+    do: term =~ ~r/^(_|\p{Ll})/u
 
   defp strip_base_uri(uri, base_uri) do
     if String.starts_with?(uri, base_uri) do

test/unit/vocabulary_namespace_test.exs

@@ -337,6 +337,20 @@ defmodule RDF.Vocabulary.NamespaceTest do
         end
       end
     end
+
+    test "terms starting with an underscore are not checked" do
+      defmodule NSWithUnderscoreTerms do
+        use RDF.Vocabulary.Namespace
+
+        defvocab Example,
+          base_uri: "http://example.com/ex#",
+          case_violations: :fail,
+          data: RDF.Graph.new([
+            {~I<http://example.com/ex#_Foo>, ~I<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>, ~I<http://www.w3.org/1999/02/22-rdf-syntax-ns#Property>},
+            {~I<http://example.com/ex#_bar>, ~I<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>, ~I<http://www.w3.org/2000/01/rdf-schema#Resource>}
+          ])
+      end
+    end
   end
 
 
@@ -589,11 +603,26 @@ defmodule RDF.Vocabulary.NamespaceTest do
       assert RDF.uri(StrictExampleFromTerms.foo) == URI.parse("http://example.com/strict_from_terms#foo")
     end
 
-    test "captitalized terms" do
+    test "capitalized terms" do
       assert RDF.uri(ExampleFromGraph.Bar) == URI.parse("http://example.com/from_graph#Bar")
       assert RDF.uri(ExampleFromNTriplesFile.Bar) == URI.parse("http://example.com/from_ntriples/Bar")
       assert RDF.uri(StrictExampleFromTerms.Bar) == URI.parse("http://example.com/strict_from_terms#Bar")
     end
+
+    test "terms starting with an underscore" do
+      defmodule NSwithUnderscoreTerms do
+        use RDF.Vocabulary.Namespace
+
+        defvocab Example,
+          base_uri: "http://example.com/ex#",
+          terms: ~w[_foo]
+      end
+      alias NSwithUnderscoreTerms.Example
+      alias TestNS.EX
+
+      assert Example._foo == ~I<http://example.com/ex#_foo>
+      assert Example._foo(EX.S, 1) == RDF.description(EX.S, Example._foo, 1)
+    end
   end
 
 
@@ -607,6 +636,10 @@ defmodule RDF.Vocabulary.NamespaceTest do
       assert RDF.uri(NonStrictExampleFromTerms.Random) == URI.parse("http://example.com/non_strict_from_terms#Random")
     end
 
+    test "undefined terms starting with an underscore" do
+      assert NonStrictExampleFromTerms._random == URI.parse("http://example.com/non_strict_from_terms#_random")
+    end
+
     test "defined lowercase terms" do
       assert NonStrictExampleFromTerms.foo == URI.parse("http://example.com/non_strict_from_terms#foo")
     end