[Bug #1177831] Fix (?(id)yes|no) for a group other than the first one, and add a test case

Lib/sre_compile.py

@@ -156,7 +156,7 @@ def _compile(code, pattern, flags):
             emit(av-1)
         elif op is GROUPREF_EXISTS:
             emit(OPCODES[op])
-            emit((av[0]-1)*2)
+            emit(av[0]-1)
             skipyes = _len(code); emit(0)
             _compile(code, av[1], flags)
             if av[2]:

Lib/test/test_re.py

@@ -235,6 +235,16 @@ class ReTests(unittest.TestCase):
         self.assertEqual(re.match('^(?:(a)|c)((?(1)|d))$', 'a').groups(),
                          ('a', ''))
 
+	# Tests for bug #1177831: exercise groups other than the first group
+	p = re.compile('(?P<g1>a)(?P<g2>b)?((?(g2)c|d))')
+	self.assertEqual(p.match('abc').groups(),
+			 ('a', 'b', 'c'))
+	self.assertEqual(p.match('ad').groups(),
+			 ('a', None, 'd'))
+	self.assertEqual(p.match('abd'), None)
+	self.assertEqual(p.match('ac'), None)
+	
+
     def test_re_groupref(self):
         self.assertEqual(re.match(r'^(\|)?([^()]+)\1$', '|a|').groups(),
                          ('|', 'a'))