Fix quadratic processing time for consecutive abbreviation definitions

docs/changelog.md

@@ -36,6 +36,8 @@ See the [Contributing Guide](contributing.md) for details.
 * Stop definition list nesting short of the recursion limit, so deeply nested
   definitions are kept as text instead of raising an uncaught
   `RecursionError` (#1700).
+* Fix quadratic processing time for many consecutive abbreviation definitions
+  in the `abbr` extension, such as a glossary appended to every page (#1702).
 
 ## [3.11.3] - 2026-11-05
 

markdown/extensions/abbr.py

@@ -157,22 +157,31 @@ def run(self, parent: etree.Element, blocks: list[str]) -> bool:
 
         """
         block = blocks.pop(0)
-        m = self.RE.search(block)
-        if m:
+        start = end = -1
+        for m in self.RE.finditer(block):
+            # After the first definition, take only the definitions on the lines that
+            # directly follow it, so the rest of the block is parsed as before.
+            if end >= 0 and (m.start() != end + 1 or '\n' in m.group()):
+                break
             abbr = m.group('abbr').strip()
             title = m.group('title').strip()
-            if title and abbr:
-                if title == "''" or title == '""':
-                    self.abbrs.pop(abbr, None)
-                else:
-                    self.abbrs[abbr] = title
-                if block[m.end():].strip():
-                    # Add any content after match back to blocks as separate block
-                    blocks.insert(0, block[m.end():].lstrip('\n'))
-                if block[:m.start()].strip():
-                    # Add any content before match back to blocks as separate block
-                    blocks.insert(0, block[:m.start()].rstrip('\n'))
-                return True
+            if not (title and abbr):
+                break
+            if title == "''" or title == '""':
+                self.abbrs.pop(abbr, None)
+            else:
+                self.abbrs[abbr] = title
+            if start < 0:
+                start = m.start()
+            end = m.end()
+        if end >= 0:
+            if block[end:].strip():
+                # Add any content after match back to blocks as separate block
+                blocks.insert(0, block[end:].lstrip('\n'))
+            if block[:start].strip():
+                # Add any content before match back to blocks as separate block
+                blocks.insert(0, block[:start].rstrip('\n'))
+            return True
         # No match. Restore block.
         blocks.insert(0, block)
         return False

tests/test_syntax/extensions/test_abbr.py

@@ -589,6 +589,52 @@ def test_abbr_clear(self):
             )
         )
 
+    def test_many_abbr_definitions(self):
+        """
+        Consecutive definitions should be consumed together rather than returning
+        the remainder of the block to be parsed once per definition.
+        """
+        count = 10000
+        ext = AbbrExtension()
+        text = ''.join(f'*[ABBR{i}]: Abbreviation {i}\n' for i in range(count))
+        self.assertEqual(Markdown(extensions=[ext]).convert(text), '')
+        self.assertEqual(ext.abbrs, {f'ABBR{i}': f'Abbreviation {i}' for i in range(count)})
+
+    def test_abbr_definitions_before_setext_underline(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                *[ABBR]: Abbreviation
+                *[HTML]: Hyper Text Markup Language
+                ===
+                """
+            ),
+            self.dedent(
+                """
+                <p>===</p>
+                """
+            )
+        )
+
+    def test_three_abbr_definitions_before_setext_underline(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                *[ABBR]: Abbr
+                *[HTML]: Mark
+                *[CSS]: Styles
+                ===
+                ABBR, HTML and CSS
+                """
+            ),
+            self.dedent(
+                """
+                <p>===
+                <abbr title="Abbr">ABBR</abbr>, <abbr title="Mark">HTML</abbr> and <abbr title="Styles">CSS</abbr></p>
+                """
+            )
+        )
+
     def test_abbr_reset(self):
         ext = AbbrExtension()
         md = Markdown(extensions=[ext])