Ability to set a specific field in the file's metadata

Eg: `--parse-metadata "description:(?s)(?P<meta_comment>.+)"`
sets the "comment" field using `description`

test/test_postprocessors.py

@@ -14,10 +14,10 @@
 class TestMetadataFromField(unittest.TestCase):
     def test_format_to_regex(self):
         pp = MetadataFromFieldPP(None, ['title:%(title)s - %(artist)s'])
-        self.assertEqual(pp._data[0]['regex'], r'(?P<title>[^\r\n]+)\ \-\ (?P<artist>[^\r\n]+)')
+        self.assertEqual(pp._data[0]['regex'], r'(?P<title>.+)\ \-\ (?P<artist>.+)')
 
 
 class TestMetadataFromTitle(unittest.TestCase):
     def test_format_to_regex(self):
         pp = MetadataFromTitlePP(None, '%(title)s - %(artist)s')
-        self.assertEqual(pp._titleregex, r'(?P<title>[^\r\n]+)\ \-\ (?P<artist>[^\r\n]+)')
+        self.assertEqual(pp._titleregex, r'(?P<title>.+)\ \-\ (?P<artist>.+)')

yt_dlp/postprocessor/ffmpeg.py

@@ -10,7 +10,7 @@
 
 from .common import AudioConversionError, PostProcessor
 
-from ..compat import compat_str
+from ..compat import compat_str, compat_numeric_types
 from ..utils import (
     encodeArgument,
     encodeFilename,
@@ -530,6 +530,8 @@ def run(self, info):
         metadata = {}
 
         def add(meta_list, info_list=None):
+            if not meta_list:
+                return
             if not info_list:
                 info_list = meta_list
             if not isinstance(meta_list, (list, tuple)):
@@ -537,7 +539,7 @@ def add(meta_list, info_list=None):
             if not isinstance(info_list, (list, tuple)):
                 info_list = (info_list,)
             for info_f in info_list:
-                if info.get(info_f) is not None:
+                if isinstance(info.get(info_f), (compat_str, compat_numeric_types)):
                     for meta_f in meta_list:
                         metadata[meta_f] = info[info_f]
                     break
@@ -563,6 +565,10 @@ def add(meta_list, info_list=None):
         add('episode_id', ('episode', 'episode_id'))
         add('episode_sort', 'episode_number')
 
+        prefix = 'meta_'
+        for key in filter(lambda k: k.startswith(prefix), info.keys()):
+            add(key[len(prefix):], key)
+
         if not metadata:
             self.to_screen('There isn\'t any metadata to add')
             return [], info
@@ -577,7 +583,7 @@ def add(meta_list, info_list=None):
         else:
             options.extend(['-c', 'copy'])
 
-        for (name, value) in metadata.items():
+        for name, value in metadata.items():
             options.extend(['-metadata', '%s=%s' % (name, value)])
 
         chapters = info.get('chapters', [])

yt_dlp/postprocessor/metadatafromfield.py

@@ -45,7 +45,7 @@ def format_to_regex(fmt):
         # replace %(..)s with regex group and escape other string parts
         for match in re.finditer(r'%\((\w+)\)s', fmt):
             regex += re.escape(fmt[lastpos:match.start()])
-            regex += r'(?P<%s>[^\r\n]+)' % match.group(1)
+            regex += r'(?P<%s>.+)' % match.group(1)
             lastpos = match.end()
         if lastpos < len(fmt):
             regex += re.escape(fmt[lastpos:])