From 30cb7e140145406527af5c51aeb03d04e1325f2c Mon Sep 17 00:00:00 2001
From: hong4rc <hong4rc@gmail.com>
Date: Sun, 12 Jul 2026 11:38:08 +0700
Subject: [PATCH] fix: Recognize setext headings whose first line starts with #

A line like `#1 Goals` or `#hashtag` followed by a setext underline was
rendered as a paragraph plus `<hr>` instead of a heading. A line only
starts an ATX heading when the `#`s are followed by whitespace or EOL, so
these are paragraph text and the underline makes a setext heading. The
lheading ATX-interrupt check was bare ` {0,3}#{1,6}`, missing the
`(?:\s|$)` that the paragraph and ATX heading rules already require.
---
 src/rules.ts                             | 4 ++--
 test/specs/new/lheading_hash_prefix.html | 3 +++
 test/specs/new/lheading_hash_prefix.md   | 8 ++++++++
 3 files changed, 13 insertions(+), 2 deletions(-)
 create mode 100644 test/specs/new/lheading_hash_prefix.html
 create mode 100644 test/specs/new/lheading_hash_prefix.md

diff --git a/src/rules.ts b/src/rules.ts
index cdbbeadb93..8fe4e24316 100644
--- a/src/rules.ts
+++ b/src/rules.ts
@@ -115,7 +115,7 @@ const lheading = edit(lheadingCore)
   .replace(/blockCode/g, /(?: {4}| {0,3}\t)/) // indented code blocks can interrupt
   .replace(/fences/g, / {0,3}(?:`{3,}|~{3,})/) // fenced code blocks can interrupt
   .replace(/blockquote/g, / {0,3}>/) // blockquote can interrupt
-  .replace(/heading/g, / {0,3}#{1,6}/) // ATX heading can interrupt
+  .replace(/heading/g, / {0,3}#{1,6}(?:\s|$)/) // ATX heading can interrupt
   .replace(/html/g, / {0,3}<[^\n>]+>\n/) // block html can interrupt
   .replace(/\|table/g, '') // table not in commonmark
   .getRegex();
@@ -124,7 +124,7 @@ const lheadingGfm = edit(lheadingCore)
   .replace(/blockCode/g, /(?: {4}| {0,3}\t)/) // indented code blocks can interrupt
   .replace(/fences/g, / {0,3}(?:`{3,}|~{3,})/) // fenced code blocks can interrupt
   .replace(/blockquote/g, / {0,3}>/) // blockquote can interrupt
-  .replace(/heading/g, / {0,3}#{1,6}/) // ATX heading can interrupt
+  .replace(/heading/g, / {0,3}#{1,6}(?:\s|$)/) // ATX heading can interrupt
   .replace(/html/g, / {0,3}<[^\n>]+>\n/) // block html can interrupt
   .replace(/table/g, / {0,3}\|?(?:[:\- ]*\|)+[\:\- ]*\n/) // table can interrupt
   .getRegex();
diff --git a/test/specs/new/lheading_hash_prefix.html b/test/specs/new/lheading_hash_prefix.html
new file mode 100644
index 0000000000..b5c5ec34aa
--- /dev/null
+++ b/test/specs/new/lheading_hash_prefix.html
@@ -0,0 +1,3 @@
+<h2>#1 Goals</h2>
+<h1>#hashtag</h1>
+<h1>####### seven</h1>
diff --git a/test/specs/new/lheading_hash_prefix.md b/test/specs/new/lheading_hash_prefix.md
new file mode 100644
index 0000000000..4d88c47bf3
--- /dev/null
+++ b/test/specs/new/lheading_hash_prefix.md
@@ -0,0 +1,8 @@
+#1 Goals
+---------
+
+#hashtag
+===
+
+####### seven
+===
\ No newline at end of file