diff --git a/docs/changelog.md b/docs/changelog.md
index f40a93e3..94b4a2f5 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -16,6 +16,9 @@ See the [Contributing Guide](contributing.md) for details.
 
 ### Fixed
 
+* Fix quadratic-time regex backtracking in `attr_list` when a block ends with an
+  unclosed attribute list followed by a long run of spaces, e.g. a line
+  consisting only of `{` and some text with many trailing spaces (#1656).
 * Fix regression in `BacktickInlineProcessor` where text could be lost when opening
   and closing backticks did not match (#1668).
 * Escape the language name in the `codehilite` extension's non-Pygments output to
diff --git a/markdown/extensions/attr_list.py b/markdown/extensions/attr_list.py
index c1643dd1..b3b8d951 100644
--- a/markdown/extensions/attr_list.py
+++ b/markdown/extensions/attr_list.py
@@ -105,7 +105,7 @@ def sanitize_name(name: str) -> str:
 
 class AttrListTreeprocessor(Treeprocessor):
 
-    BASE_RE = r'\{\:?[ ]*([^\}\n ][^\n]*)[ ]*\}'
+    BASE_RE = r'\{\:?[ ]*([^\}\n ][^\n]*)\}'
     HEADER_RE = re.compile(r'[ ]+{}[ ]*$'.format(BASE_RE))
     BLOCK_RE = re.compile(r'\n[ ]*{}[ ]*$'.format(BASE_RE))
     INLINE_RE = re.compile(r'^{}'.format(BASE_RE))
diff --git a/tests/test_syntax/extensions/test_attr_list.py b/tests/test_syntax/extensions/test_attr_list.py
index 1a0ae12c..dabac77d 100644
--- a/tests/test_syntax/extensions/test_attr_list.py
+++ b/tests/test_syntax/extensions/test_attr_list.py
@@ -78,6 +78,18 @@ def test_backslash_escape_value(self):
             '<h1 id="*Foo*"><code>*Foo*</code></h1>'
         )
 
+    def test_unclosed_curly_with_long_run_of_spaces(self):
+        """
+        An unclosed attribute list followed by a long run of spaces should be
+        left as plain text, no matter how many trailing spaces there are.
+
+        The content and the trailing spaces both match the same part of the
+        pattern, so without care the regex backtracks over every way of
+        splitting the spaces while failing to find the closing brace.
+        """
+        text = 'foo\n{a' + (' ' * 50000)
+        self.assertMarkdownRenders(text, f'<p>{text}</p>')
+
     def test_table_td(self):
         self.assertMarkdownRenders(
             self.dedent(