Add locale-aware internal link rewriter for Markdown content (#363)

src/lib/markdown.ts

@@ -398,6 +398,59 @@ function decorateExternalLinks(contentHtml: string): string {
   );
 }
 
+function rewriteInternalLinks(contentHtml: string, locale: string): string {
+  return contentHtml.replace(
+    /<a\b([^>]*?)href=(["'])(.*?)\2([^>]*)>([\s\S]*?)<\/a>/gi,
+    (
+      fullMatch,
+      attributesBeforeHref: string,
+      quote: string,
+      href: string,
+      attributesAfterHref: string,
+      linkContent: string,
+    ) => {
+      const trimmedHref = href.trim();
+
+      // Only process root-relative paths (starting with '/' but not '//')
+      if (!trimmedHref.startsWith('/') || trimmedHref.startsWith('//')) {
+        return fullMatch;
+      }
+
+      // Leave asset paths untouched (path segment containing a file extension)
+      const pathPart = trimmedHref.split(/[?#]/)[0];
+      const lastSegment = pathPart.split('/').pop() || '';
+      if (lastSegment.includes('.')) {
+        return fullMatch;
+      }
+
+      // Check if it already has a locale prefix (e.g. /de/ or /de)
+      const hasLocalePrefix = /^\/(de|en)(?:[\/?#]|$)/.test(trimmedHref);
+
+      // Normalize trailing slash
+      let normalizedHref = trimmedHref;
+      const match = trimmedHref.match(/^([^?#]*)(.*)$/);
+      if (match) {
+        let pPart = match[1];
+        const queryAndHash = match[2];
+        if (pPart !== '/' && pPart.endsWith('/')) {
+          pPart = pPart.slice(0, -1);
+        }
+        normalizedHref = pPart + queryAndHash;
+      }
+
+      // Prepend locale prefix if needed
+      let newHref = normalizedHref;
+      if (!hasLocalePrefix && locale !== 'en') {
+        const prefix = `/${locale}`;
+        newHref =
+          normalizedHref === '/' ? prefix : `${prefix}${normalizedHref}`;
+      }
+
+      return `<a${attributesBeforeHref}href=${quote}${newHref}${quote}${attributesAfterHref}>${linkContent}</a>`;
+    },
+  );
+}
+
 const STANDALONE_IMAGE_STYLE =
   'display: block; max-width: 100%; height: auto; margin-left: auto; margin-right: auto;';
 
@@ -722,8 +775,11 @@ export async function getPostBySlug(
       .process(transformedContent);
     const contentHtml = highlightCodeBlocks(
       decorateHeadlinesWithAnchors(
-        decorateExternalLinks(
-          centerStandaloneHtmlImages(processedContent.toString()),
+        rewriteInternalLinks(
+          decorateExternalLinks(
+            centerStandaloneHtmlImages(processedContent.toString()),
+          ),
+          locale,
         ),
       ),
     );

tests/e2e/internal-links.spec.ts

@@ -0,0 +1,45 @@
+import { test, expect } from '@playwright/test';
+
+test.describe('Locale-aware internal link rewriter', () => {
+  test('rewrites internal links in a German post to include /de prefix and normalizes trailing slashes', async ({ page }) => {
+    // Navigate to a German post
+    await page.goto('/de/posts/2025-12-15-cra');
+
+    const articleBody = page.locator('main .prose').first();
+
+    // /contact should be rewritten to /de/contact
+    const contactLink = articleBody.locator('a[href="/de/contact"]').first();
+    await expect(contactLink).toBeVisible();
+
+    // /newsletter should be rewritten to /de/newsletter
+    const newsletterLink = articleBody.locator('a[href="/de/newsletter"]').first();
+    await expect(newsletterLink).toBeVisible();
+
+    // External links should NOT be rewritten
+    const externalLink = articleBody.locator('a[href^="https://eur-lex.europa.eu"]');
+    await expect(externalLink).toBeVisible();
+    await expect(externalLink).not.toHaveAttribute('href', /^\/de/);
+  });
+
+  test('rewrites shortcode relref links in a German post to include /de prefix', async ({ page }) => {
+    // Navigate to the German review 2025 post which contains a relref to the 2024 post
+    await page.goto('/de/posts/2026-02-10-review-2025');
+
+    const articleBody = page.locator('main .prose').first();
+
+    // The relref posts/2025-01-16-open-elements-in-2024 should be rewritten to /de/posts/2025-01-16-open-elements-in-2024
+    const relrefLink = articleBody.locator('a[href="/de/posts/2025-01-16-open-elements-in-2024"]');
+    await expect(relrefLink).toBeVisible();
+  });
+
+  test('does not prefix internal links in an English post', async ({ page }) => {
+    // Navigate to the English review 2025 post
+    await page.goto('/posts/2026-02-10-review-2025');
+
+    const articleBody = page.locator('main .prose').first();
+
+    // In English post, relref should remain unprefixed /posts/2025-01-16-open-elements-in-2024
+    const relrefLink = articleBody.locator('a[href="/posts/2025-01-16-open-elements-in-2024"]');
+    await expect(relrefLink).toBeVisible();
+  });
+});