there are WordPress plugins that automatically add syntax highlighting to existing code blocks, including those created with the default WordPress “Code” block. These plugins do not require you to use a special block or shortcode—they work with the standard code blocks already present in your content.

Top Plugins That Highlight Existing Code Blocks by Default

Highlight: Syntax-highlighting Code Block

• Automatic highlighting for all standard WordPress code blocks—no need to change your existing content.
• Server-side rendering means no JavaScript is needed on the frontend, improving performance and compatibility (including AMP support).
• Language auto-detection is built in, but you can override it per block if needed.
• Line numbers and highlighted lines are supported.
• Works with all languages supported by highlight.php (over 185).

“Pre-existing Code blocks on a site are automatically extended to include syntax highlighting.”

Other Notable Plugins

• SyntaxHighlighter Evolved: Works with its own block or shortcodes, but is very popular and easy to use for both new and existing code.
• WP-Syntax: Simple, reliable, and works with existing code snippets.

How to Use

• Install and activate one of these plugins (e.g., Syntax-highlighting Code Block).
• All your existing “Code” blocks will be automatically highlighted—no need to edit or re-insert them.
• For advanced features (like specifying language or highlighting lines), use the block sidebar options.

Summary

Syntax-highlighting Code Block is the most recommended if you want true “set and forget” syntax highlighting for all existing code blocks, with no need to modify your current content. Other plugins like SyntaxHighlighter Evolved and WP-Syntax also support highlighting for existing code, but may require shortcodes or specific markup in some cases.

If you need further customization or have a unique workflow, there are many other plugins available that can be explored in the WordPress plugin repository.

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SyntaxHighlighter Evolved does require you to use its dedicated block or shortcode for highlighting; it does not automatically highlight all existing WordPress “Code” blocks by default.

If you want a plugin that automatically highlights all existing code blocks (including those created with the default WordPress “Code” block, without needing to switch blocks or add special markup), use Syntax-highlighting Code Block (with Server-side Rendering).

Why Syntax-highlighting Code Block Is the Solution

• Automatic Highlighting: It extends the core WordPress “Code” block and automatically applies syntax highlighting to all pre-existing and new Code blocks—no manual conversion or dedicated block needed.
• No JavaScript Needed: Highlighting is rendered on the server, so there’s no frontend JavaScript and no flash of unstyled code.
• Language Detection: The plugin tries to auto-detect the language, but you can specify it in the block sidebar if you want.
• Performance: Because everything is rendered server-side, it’s fast and works even with AMP or JavaScript disabled.
• No Content Changes Required: You do not need to edit your existing posts or change your code blocks; just install and activate the plugin.

How to Use

1. Install and activate the “Syntax-highlighting Code Block” plugin.
2. All your existing and future WordPress “Code” blocks will be automatically highlighted on the frontend.
3. You can optionally set the language or highlight lines via the block sidebar when editing a code block.

In summary:
If you want syntax highlighting on all standard WordPress code blocks by default, use the Syntax-highlighting Code Block plugin. It is designed specifically for this purpose and requires no changes to your existing content.