fix(utils): preserve trailing hash in markdown content title without preceding space

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

@@ -438,6 +438,74 @@ Lorem Ipsum
     });
   });
 
+  it('preserves trailing hash in h1 title when not preceded by whitespace (e.g. C#)', () => {
+    const markdown = dedent`
+
+          # Learn C#
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'Learn C#',
+    });
+  });
+
+  it('preserves trailing hash in h1 title with custom heading id', () => {
+    const markdown = dedent`
+
+          # Learn C# {#learn-csharp}
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'Learn C#',
+    });
+  });
+
+  it('removes atx closing # while preserving trailing hash in heading text', () => {
+    const markdown = dedent`
+
+          # Learn C# #
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdown)).toEqual({
+      content: markdown,
+      contentTitle: 'Learn C#',
+    });
+  });
+
+  it('removes atx closing # and custom heading id regardless of ordering', () => {
+    const markdownWithClosingAfter = dedent`
+
+          # Learn C# {#learn-csharp} #
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdownWithClosingAfter)).toEqual({
+      content: markdownWithClosingAfter,
+      contentTitle: 'Learn C#',
+    });
+
+    const markdownWithClosingBefore = dedent`
+
+          # Learn C# # {#learn-csharp}
+
+          Lorem Ipsum
+
+        `;
+    expect(parseMarkdownContentTitle(markdownWithClosingBefore)).toEqual({
+      content: markdownWithClosingBefore,
+      contentTitle: 'Learn C#',
+    });
+  });
+
   it('parses markdown h1 title at the top followed by h2 title', () => {
     const markdown = dedent`
 

packages/docusaurus-utils/src/markdownUtils.ts

@@ -334,7 +334,9 @@ export function parseMarkdownContentTitle(
       contentTitle: toTextContentTitle(
         regularTitleMatch
           .groups!.title!.trim()
-          .replace(/\s*(?:\{#*[\w-]+\}|#+)$/, ''),
+          .replace(/[ \t]+#+$/, '')
+          .replace(/\s*\{#*[\w-]+\}$/, '')
+          .replace(/[ \t]+#+$/, ''),
       ).trim(),
     };
   }