From 43f12ba95b3a7036d8a9bd82865d680bb955bfc0 Mon Sep 17 00:00:00 2001
From: Jah-yee <166608075+Jah-yee@users.noreply.github.com>
Date: Fri, 22 May 2026 23:43:14 +0800
Subject: [PATCH] fix: prevent mixed =/- chars in Setext-style headings

The regex [=-]+ incorrectly matches mixed runs like '=-' or '-='
which are not valid Setext headers (only = or - alone are valid).

Fix using (?:[=]+|[-]+) to match only homogeneous runs.

Fixes issue #1604
---
 markdown/blockprocessors.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/markdown/blockprocessors.py b/markdown/blockprocessors.py
index 3ed4cf07f..71d8854eb 100644
--- a/markdown/blockprocessors.py
+++ b/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))