Append .md to internal links

Author: duncanmcclean

app/Http/Controllers/DocsMarkdownController.php

@@ -22,8 +22,41 @@ public function __invoke(string $uri)
             ])->filter()->implode("\n\n");
         });
 
+        $markdown = $this->appendMdExtensionToInternalLinks($markdown);
+
         return response($markdown, 200, [
             'Content-Type' => 'text/markdown; charset=UTF-8',
         ]);
     }
+
+    private function appendMdExtensionToInternalLinks(string $markdown): string
+    {
+        return preg_replace_callback(
+            '/(?<!!)\[([^\]]+)\]\(([^)]+)\)/',
+            function ($matches) {
+                $text = $matches[1];
+                $url = $matches[2];
+
+                if ($this->shouldAppendMdExtension($url)) {
+                    $url .= '.md';
+                }
+
+                return "[$text]($url)";
+            },
+            $markdown
+        );
+    }
+
+    private function shouldAppendMdExtension(string $url): bool
+    {
+        if (preg_match('/^https?:\/\//', $url)) {
+            return false;
+        }
+
+        if (preg_match('/\.[a-z0-9]{2,4}$/i', $url)) {
+            return false;
+        }
+
+        return true;
+    }
 }