fix: prevent mixed =/- chars in Setext-style headings (+tests)

Closes #1604

Author: Jah-yee

docs/changelog.md

@@ -10,6 +10,13 @@ and this project adheres to the
 [Python Version Specification](https://packaging.python.org/en/latest/specifications/version-specifiers/).
 See the [Contributing Guide](contributing.md) for details.
 
+## [Unreleased]
+
+### Fixed
+
+* Fix `SetextHeaderProcessor` regex to prevent mixed `=` and `-` chars in setext-style headers (#1604).
+
+
 ## [3.10.2] - 2026-02-09
 
 ### Fixed

markdown/blockprocessors.py

@@ -494,7 +494,7 @@ class SetextHeaderProcessor(BlockProcessor):
     """ Process Setext-style Headers. """
 
     # Detect Setext-style header. Must be first 2 lines of block.
-    RE = re.compile(r'^.*?\n[=-]+[ ]*(\n|$)', re.MULTILINE)
+    RE = re.compile(r'^.*?\n(?:[=]+|[-]+)[ ]*(\n|$)', re.MULTILINE)
 
     def test(self, parent: etree.Element, block: str) -> bool:
         return bool(self.RE.match(block))

tests/test_syntax/blocks/test_headers.py

@@ -109,6 +109,58 @@ def test_setext_h2_followed_by_p(self):
 
     # TODO: fix this
     # see https://johnmacfarlane.net/babelmark2/?normalize=1&text=Paragraph%0AAn+H1%0A%3D%3D%3D%3D%3D
+
+    def test_setext_mixed_chars_not_h1(self):
+        """Mixed = and - chars should not form a valid H1."""
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                This is not an H1
+                =-
+                """
+            ),
+            self.dedent(
+                """
+                <p>This is not an H1
+                =-</p>
+                """
+            )
+        )
+
+    def test_setext_mixed_chars_not_h2(self):
+        """Mixed - and = chars should not form a valid H2."""
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                This is not an H2
+                -=
+                """
+            ),
+            self.dedent(
+                """
+                <p>This is not an H2
+                -=</p>
+                """
+            )
+        )
+
+    def test_setext_mixed_multiple_chars(self):
+        """Multiple mixed = and - chars should not form a valid H1/H2."""
+        self.assertMarkdownRenders(
+            self.dedent(
+                """
+                Not a Header
+                =-=-
+                """
+            ),
+            self.dedent(
+                """
+                <p>Not a Header
+                =-=-</p>
+                """
+            )
+        )
+
     @unittest.skip('This is broken in Python-Markdown')
     def test_p_followed_by_setext_h1(self):
         self.assertMarkdownRenders(