fix(utils): do not drop trailing hash in heading title unless preceded by space

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

@@ -438,6 +438,76 @@ Lorem Ipsum
     });
   });
 
+  it('parses markdown h1 title ending with # when not preceded by space (e.g. C#)', () => {
+    const markdown = dedent`
+
+          # Learn C#
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'Learn C#',
+    });
+  });
+
+  it('parses markdown h1 title ending with # and trailing ATX closing marker', () => {
+    const markdown = dedent`
+
+          # Learn C# #
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'Learn C#',
+    });
+  });
+
+  it('parses markdown h1 title ending with # and custom anchor-id', () => {
+    const markdown = dedent`
+
+          # Learn C# {#learn-c-sharp}
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'Learn C#',
+    });
+  });
+
+  it('parses markdown h1 title ending with attached # (e.g. foo#)', () => {
+    const markdown = dedent`
+
+          # foo#
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'foo#',
+    });
+  });
+
+  it('parses empty markdown h1 title with closing # marker only', () => {
+    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 +334,8 @@ export function parseMarkdownContentTitle(
       contentTitle: toTextContentTitle(
         regularTitleMatch
           .groups!.title!.trim()
-          .replace(/\s*(?:\{#*[\w-]+\}|#+)$/, ''),
+          .replace(/\s*\{#*[\w-]+\}$/, '')
+          .replace(/(?:^|\s+)#+$/, ''),
       ).trim(),
     };
   }