diff --git a/markdown/blockprocessors.py b/markdown/blockprocessors.py
index 3ed4cf07..71d8854e 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))