[outtmpl] Fix replacement/default when used with alternate

test/test_YoutubeDL.py

@@ -818,6 +818,8 @@ class TestYoutubeDL(unittest.TestCase):
         test('%(id&foo)s.bar', 'foo.bar')
         test('%(title&foo)s.bar', 'NA.bar')
         test('%(title&foo|baz)s.bar', 'baz.bar')
+        test('%(x,id&foo|baz)s.bar', 'foo.bar')
+        test('%(x,title&foo|baz)s.bar', 'baz.bar')
 
         # Laziness
         def gen():

yt_dlp/YoutubeDL.py

@@ -1099,10 +1099,11 @@ class YoutubeDL(object):
             (?P<fields>{field})
             (?P<maths>(?:{math_op}{math_field})*)
             (?:>(?P<strf_format>.+?))?
-            (?P<alternate>(?<!\\),[^|&)]+)?
-            (?:&(?P<replacement>.*?))?
-            (?:\|(?P<default>.*?))?
-            $'''.format(field=FIELD_RE, math_op=MATH_OPERATORS_RE, math_field=MATH_FIELD_RE))
+            (?P<remaining>
+                (?P<alternate>(?<!\\),[^|&)]+)?
+                (?:&(?P<replacement>.*?))?
+                (?:\|(?P<default>.*?))?
+            )$'''.format(field=FIELD_RE, math_op=MATH_OPERATORS_RE, math_field=MATH_FIELD_RE))
 
         def _traverse_infodict(k):
             k = k.split('.')
@@ -1173,7 +1174,7 @@ class YoutubeDL(object):
                 value = get_value(mobj)
                 replacement = mobj['replacement']
                 if value is None and mobj['alternate']:
-                    mobj = re.match(INTERNAL_FORMAT_RE, mobj['alternate'][1:])
+                    mobj = re.match(INTERNAL_FORMAT_RE, mobj['remaining'][1:])
                 else:
                     break