fix(utils): preserve trailing hash in ATX heading titles

packages/docusaurus-utils/src/__tests__/markdownUtils.test.ts

@@ -438,6 +438,62 @@ Lorem Ipsum
     });
   });
 
+  it('parses markdown h1 title preserving trailing hash in word (e.g. C#)', () => {
+    const markdown = dedent`
+
+          # Learn C#
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'Learn C#',
+    });
+  });
+
+  it('parses markdown h1 title preserving trailing hash in word with closing #', () => {
+    const markdown = dedent`
+
+          # Learn C# #
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'Learn C#',
+    });
+  });
+
+  it('parses markdown h1 title preserving trailing hash in word with anchor id', () => {
+    const markdown = dedent`
+
+          # Learn C# {#learn-csharp}
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'Learn C#',
+    });
+  });
+
+  it('parses markdown h1 title consisting only of closing hashes as empty', () => {
+    const markdown = dedent`
+
+          # ###
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: '',
+    });
+  });
+
   it('parses markdown h1 title at the top followed by h2 title', () => {
     const markdown = dedent`
 

packages/docusaurus-utils/src/markdownUtils.ts

@@ -334,7 +334,7 @@ export function parseMarkdownContentTitle(
       contentTitle: toTextContentTitle(
         regularTitleMatch
           .groups!.title!.trim()
-          .replace(/\s*(?:\{#*[\w-]+\}|#+)$/, ''),
+          .replace(/(?:\s*\{#*[\w-]+\}|\s+#+|^#+)$/, ''),
       ).trim(),
     };
   }