[utils] Correctly match class names in `get_element[s]_by_class()`

* reproduce CSS .classname behaviour ("bar" matches "bar", "foo bar baz", etc)
* add tests

test/test_utils.py

@@ -1603,11 +1603,14 @@ Line 1
 
     def test_get_element_by_class(self):
         html = '''
-            <span class="foo bar">nice</span>
+            <span class="foo bar baz-bam">nice</span>
         '''
 
         self.assertEqual(get_element_by_class('foo', html), 'nice')
+        self.assertEqual(get_element_by_class('bar', html), 'nice')
         self.assertEqual(get_element_by_class('no-such-class', html), None)
+        self.assertEqual(get_element_by_class('baz', html), None)
+        self.assertEqual(get_element_by_class('bam', html), None)
 
     def test_get_element_by_attribute(self):
         html = '''
@@ -1626,10 +1629,13 @@ Line 1
 
     def test_get_elements_by_class(self):
         html = '''
-            <span class="foo bar">nice</span><span class="foo bar">also nice</span>
+            <span class="foo bar baz-bam">nice</span><span class="foo bar">also nice</span>
         '''
 
         self.assertEqual(get_elements_by_class('foo', html), ['nice', 'also nice'])
+        self.assertEqual(get_elements_by_class('bar', html), ['nice', 'also nice'])
+        self.assertEqual(get_elements_by_class('baz', html), [])
+        self.assertEqual(get_elements_by_class('bam', html), [])
         self.assertEqual(get_elements_by_class('no-such-class', html), [])
 
     def test_get_elements_by_attribute(self):

youtube_dl/utils.py

@@ -1960,7 +1960,7 @@ def get_element_by_attribute(attribute, value, html, escape_value=True):
 def get_elements_by_class(class_name, html):
     """Return the content of all tags with the specified class in the passed HTML document as a list"""
     return get_elements_by_attribute(
-        'class', r'[^\'"]*\b%s\b[^\'"]*' % re.escape(class_name),
+        'class', r'[^\'"]*(?<!-)\b%s\b(?!-)[^\'"]*' % re.escape(class_name),
         html, escape_value=False)