Support for custom grouping of XPath expressions.

This allows the use of expressions such as "(A or B) and C".

This fixes #59.

lib/oga/xpath/parser.y

@@ -35,6 +35,11 @@ rule
     ;
 
   expression
+    : expression_members
+    | T_LPAREN expression_members T_RPAREN { val[1] }
+    ;
+
+  expression_members
     : node_test_as_axis
     | operator
     | axis

spec/oga/xpath/parser/grouping_spec.rb

@@ -0,0 +1,41 @@
+require 'spec_helper'
+
+describe Oga::XPath::Parser do
+  context 'grouping of expressions' do
+    example 'parse "A + (B + C)"' do
+      parse_xpath('A + (B + C)').should == s(
+        :add,
+        s(:axis, 'child', s(:test, nil, 'A')),
+        s(
+          :add,
+          s(:axis, 'child', s(:test, nil, 'B')),
+          s(:axis, 'child', s(:test, nil, 'C'))
+        )
+      )
+    end
+
+    example 'parse "A or (B or C)"' do
+      parse_xpath('A or (B or C)').should == s(
+        :or,
+        s(:axis, 'child', s(:test, nil, 'A')),
+        s(
+          :or,
+          s(:axis, 'child', s(:test, nil, 'B')),
+          s(:axis, 'child', s(:test, nil, 'C'))
+        )
+      )
+    end
+
+    example 'parse "(A or B) and C"' do
+      parse_xpath('(A or B) and C').should == s(
+        :and,
+        s(
+          :or,
+          s(:axis, 'child', s(:test, nil, 'A')),
+          s(:axis, 'child', s(:test, nil, 'B'))
+        ),
+        s(:axis, 'child', s(:test, nil, 'C'))
+      )
+    end
+  end
+end