From dedb9e22987c2a9510a09567f47872a1b667f97c Mon Sep 17 00:00:00 2001
From: Aryan Sachin <aryansachinixd@gmail.com>
Date: Fri, 6 Nov 2026 02:31:24 +0530
Subject: [PATCH] =?UTF-8?q?=EF=BB=BFfix(utils):=20preserve=20trailing=20ha?=
 =?UTF-8?q?sh=20in=20markdown=20heading=20titles=20(#12644)?=
MIME-Version: 1.0
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According to CommonMark 0.31.2 section 4.2, an optional closing sequence
of # characters on an ATX heading must be preceded by at least one space
or tab. Previously, parseMarkdownContentTitle used `\s*(?:\{#*[\w-]+\}|#+)$`
which matched trailing # characters even with zero preceding whitespace,
causing document titles such as `# Learn C#` or `# Learn F#` to drop their
trailing hash and become "Learn C" or "Learn F".

This separates custom anchor ID removal from ATX closing sequence
stripping and requires preceding whitespace (`\s+#+$`) for closing hashes,
preserving words and identifiers ending in # (such as C# or F#).

Closes #12644.

Signed-off-by: Aryan Sachin <aryansachinixd@gmail.com>
---
 .../src/__tests__/markdownUtils.test.ts       | 54 +++++++++++++++++++
 .../docusaurus-utils/src/markdownUtils.ts     |  3 +-
 2 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/packages/docusaurus-utils/src/__tests__/markdownUtils.test.ts b/packages/docusaurus-utils/src/__tests__/markdownUtils.test.ts
index 34648991ba35..a6c449851d3a 100644
--- a/packages/docusaurus-utils/src/__tests__/markdownUtils.test.ts
+++ b/packages/docusaurus-utils/src/__tests__/markdownUtils.test.ts
@@ -438,6 +438,60 @@ Lorem Ipsum
     });
   });
 
+  it('preserves trailing # when not preceded by space (e.g. C# or F#)', () => {
+    const markdownC = dedent`
+
+          # Learn C#
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdownC)).toEqual({
+      content: markdownC,
+      contentTitle: 'Learn C#',
+    });
+
+    const markdownF = dedent`
+
+          # Learn F#
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdownF)).toEqual({
+      content: markdownF,
+      contentTitle: 'Learn F#',
+    });
+  });
+
+  it('preserves trailing # in title with custom anchor id', () => {
+    const markdown = dedent`
+
+          # Learn C# {#learn-csharp}
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'Learn C#',
+    });
+  });
+
+  it('preserves trailing # in title with closing # sequence', () => {
+    const markdown = dedent`
+
+          # Learn C# #
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'Learn C#',
+    });
+  });
+
   it('parses markdown h1 title at the top followed by h2 title', () => {
     const markdown = dedent`
 
diff --git a/packages/docusaurus-utils/src/markdownUtils.ts b/packages/docusaurus-utils/src/markdownUtils.ts
index 3a82a52cbb6e..8206de260c6a 100644
--- a/packages/docusaurus-utils/src/markdownUtils.ts
+++ b/packages/docusaurus-utils/src/markdownUtils.ts
@@ -334,7 +334,8 @@ export function parseMarkdownContentTitle(
       contentTitle: toTextContentTitle(
         regularTitleMatch
           .groups!.title!.trim()
-          .replace(/\s*(?:\{#*[\w-]+\}|#+)$/, ''),
+          .replace(/\s*\{#*[\w-]+\}$/, '')
+          .replace(/\s+#+$/, ''),
       ).trim(),
     };
   }