Fixes #118 - decoding invalid entities.

Previous regular expression was too greedy in terms of matching
letters from outside of A-F hex scope, and matching letters when
not in hex mode.

lib/oga/xml/entities.rb

@@ -63,7 +63,7 @@ module Oga
       #
       # @return [Regexp]
       #
-      CODEPOINT_ENTITY = /&#(x)?([a-zA-Z0-9]+);/
+      CODEPOINT_ENTITY = /&#(x[a-fA-F0-9]+|\d+);/
 
       ##
       # @return [Regexp]
@@ -90,7 +90,7 @@ module Oga
 
         if input.include?(AMPERSAND)
           input = input.gsub(CODEPOINT_ENTITY) do |match|
-            [$1 ? Integer($2, 16) : Integer($2, 10)].pack('U*')
+            [$1.start_with?('x') ? Integer($1[1..-1], 16) : Integer($1, 10)].pack('U*')
           end
         end
 

spec/oga/xml/entities_spec.rb

@@ -73,6 +73,18 @@ describe Oga::XML::Entities do
     it 'decodes numeric entities starting with a 0' do
       described_class.decode('&').should == '&'
     end
+
+    it 'preserves entity-like tokens' do
+      described_class.decode('&#TAB;').should == '&#TAB;'
+    end
+
+    it 'preserves entity-like hex tokens' do
+      described_class.decode('&#x;').should == '&#x;'
+    end
+
+    it 'preserves entity-like letters in non-hex mode' do
+      described_class.decode('&#123A;').should == '&#123A;'
+    end
   end
 
   describe 'encode' do