Fix quadratic processing time in the tables extension for long blocks

docs/changelog.md

@@ -36,6 +36,9 @@ 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 in the `tables` extension for long blocks which
+  are split into many smaller blocks, such as many headers with no blank lines
+  between them (#1704).
 
 ## [3.11.3] - 2026-11-05
 

markdown/extensions/tables.py

@@ -55,7 +55,9 @@ def test(self, parent: etree.Element, block: str) -> bool:
         Keep border check and separator row do avoid repeating the work.
         """
         is_table = False
-        rows = [row.strip(' ') for row in block.split('\n')]
+        # Only the first two rows are needed in most cases. Splitting the whole
+        # block here would make every test linear in the size of the block.
+        rows = [row.strip(' ') for row in block.split('\n', 2)[:2]]
         if len(rows) > 1:
             header0 = rows[0]
             self.border = PIPE_NONE
@@ -69,10 +71,11 @@ def test(self, parent: etree.Element, block: str) -> bool:
 
             # Each row in a single column table needs at least one pipe.
             if not is_table and row0_len == 1 and self.border:
-                for index in range(1, len(rows)):
-                    is_table = rows[index].startswith('|')
+                for row in block.split('\n')[1:]:
+                    row = row.strip(' ')
+                    is_table = row.startswith('|')
                     if not is_table:
-                        is_table = self.RE_END_BORDER.search(rows[index]) is not None
+                        is_table = self.RE_END_BORDER.search(row) is not None
                     if not is_table:
                         break
 

tests/test_syntax/extensions/test_tables.py

@@ -843,6 +843,122 @@ def test_single_column_tables(self):
             extensions=['tables']
         )
 
+    def test_many_lines_in_one_block(self):
+        """
+        Testing a block for a table should only look at the first rows, as the
+        remainder of a long block can be tested once per line.
+        """
+        count = 5000
+        self.assertMarkdownRenders(
+            '# Header\n' * count,
+            '<h1>Header</h1>\n' * (count - 1) + '<h1>Header</h1>',
+            extensions=['tables']
+        )
+
+    def test_table_after_header_in_same_block(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                # Table
+                | A | B |
+                |---|---|
+                | a | b |
+                """
+            ),
+            self.dedent(
+                """
+                <h1>Table</h1>
+                <table>
+                <thead>
+                <tr>
+                <th>A</th>
+                <th>B</th>
+                </tr>
+                </thead>
+                <tbody>
+                <tr>
+                <td>a</td>
+                <td>b</td>
+                </tr>
+                </tbody>
+                </table>
+                """
+            ),
+            extensions=['tables']
+        )
+
+    def test_no_table_after_paragraph_line_in_same_block(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                Not a table
+                | A | B |
+                |---|---|
+                | a | b |
+                """
+            ),
+            self.dedent(
+                """
+                <p>Not a table
+                | A | B |
+                |---|---|
+                | a | b |</p>
+                """
+            ),
+            extensions=['tables']
+        )
+
+    def test_single_column_table_after_header_in_same_block(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                # Table
+                | A |
+                |---|
+                | a |
+                """
+            ),
+            self.dedent(
+                """
+                <h1>Table</h1>
+                <table>
+                <thead>
+                <tr>
+                <th>A</th>
+                </tr>
+                </thead>
+                <tbody>
+                <tr>
+                <td>a</td>
+                </tr>
+                </tbody>
+                </table>
+                """
+            ),
+            extensions=['tables']
+        )
+
+    def test_no_single_column_table_with_row_without_pipe(self):
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                | A |
+                |---|
+                | a |
+                not a row
+                """
+            ),
+            self.dedent(
+                """
+                <p>| A |
+                |---|
+                | a |
+                not a row</p>
+                """
+            ),
+            extensions=['tables']
+        )
+
     def test_align_columns_legacy(self):
         self.assertMarkdownRenders(
             self.dedent(