Fix --parse-metadata when TO is a single field name (#14577)

Closes #14576
Authored by: clayote, bashonly

Co-authored-by: bashonly <88596187+bashonly@users.noreply.github.com>

README.md

@@ -1821,6 +1821,9 @@ $ yt-dlp --parse-metadata "title:%(artist)s - %(title)s"
 # Regex example
 $ yt-dlp --parse-metadata "description:Artist - (?P<artist>.+)"
 
+# Copy the episode field to the title field (with FROM and TO as single fields)
+$ yt-dlp --parse-metadata "episode:title"
+
 # Set title as "Series name S01E05"
 $ yt-dlp --parse-metadata "%(series)s S%(season_number)02dE%(episode_number)02d:%(title)s"
 

test/test_postprocessors.py

@@ -29,6 +29,11 @@ class TestMetadataFromField(unittest.TestCase):
             MetadataParserPP.format_to_regex('%(title)s - %(artist)s'),
             r'(?P<title>.+)\ \-\ (?P<artist>.+)')
         self.assertEqual(MetadataParserPP.format_to_regex(r'(?P<x>.+)'), r'(?P<x>.+)')
+        self.assertEqual(MetadataParserPP.format_to_regex(r'text (?P<x>.+)'), r'text (?P<x>.+)')
+        self.assertEqual(MetadataParserPP.format_to_regex('x'), r'(?s)(?P<x>.+)')
+        self.assertEqual(MetadataParserPP.format_to_regex('Field_Name1'), r'(?s)(?P<Field_Name1>.+)')
+        self.assertEqual(MetadataParserPP.format_to_regex('é'), r'(?s)(?P<é>.+)')
+        self.assertEqual(MetadataParserPP.format_to_regex('invalid '), 'invalid ')
 
     def test_field_to_template(self):
         self.assertEqual(MetadataParserPP.field_to_template('title'), '%(title)s')

yt_dlp/postprocessor/metadataparser.py

@@ -42,6 +42,9 @@ class MetadataParserPP(PostProcessor):
         to a regex like
            '(?P<title>.+)\ \-\ (?P<artist>.+)'
         """
+        if re.fullmatch(r'\w+', fmt):
+            # convert a single field name into regex pattern that matches the entire input
+            return rf'(?s)(?P<{fmt}>.+)'
         if not re.search(r'%\(\w+\)s', fmt):
             return fmt
         lastpos = 0