Note: The project used to handle syntax highlighting was recently changed and this FAQ is in the process of being updated
What is syntax highlighting?
Syntax highlighting allows code in posts to be highlighted based on the language it's written in, to make it easier to read.
How does it work?
Stack Exchange does not have its own syntax highlighting engine. It uses highlight.js, and is not necessarily using the latest release of that library. Therefore, any bugs and feature requests regarding syntax highlighting cannot be handled by Stack Exchange.
Syntax highlighting is assigned, based on tags, to the preview when creating or editing posts as soon as you stop typing for 5 seconds.
highlight.js supports a list of core languages that it can highlight (including C/C++, C#, Java, JavaScript/CoffeeScript, Perl, Python, Ruby, Regex, Bash, HTML, and XML), along with a default generic highlighter that works passably on most C-like languages and HTML-like markup languages.
Why isn't my code being highlighted correctly?
1. Check that the site you're using has syntax highlighting enabled
The full list is available on Meta. If syntax highlighting is not enabled on a site follow the instructions in this question.
2. Check that the language is supported by highlight.js
Look at the list of highlight.js supported languages. If your language is not on the list, it needs to be created within the highlight.js project before it can be deployed by Stack Exchange.
If a language is already on the list but not used on Stack Exchange, please raise a feature request here on Meta to have it deployed on the network.
3. Check that the language is used by Stack Exchange
Look at the highlight-loader.js file on the dev CDN, open in your favourite editor, and search for the language hint. For example, searching for "dart" brings you to a line consisting of hljs.registerLanguage('dart', .... This means that lang-dart is an available language hint. Searching for "psh" or "powershell" doesn't return any results, meaning that psh is not supported by the Stack Exchange version of highlight.js.
4. Check that the tags on the question have syntax highlighting enabled
Behind the scenes, Stack Exchanges uses the tags on the question to determine the language you are using. Each tag has a highlighting language specified in its properties, or no language. If there's more than one tag that has a highlighting language specified, or if none of the tags have one specified, it uses a default and lets Prettify infer what's the best language to use. The same highlighting language that is used on the question is also used on its answers.
Visit that tag's wiki page. Click a tag (such as "javascript"), then click "Learn more...", and the language hint (if any) that is currently being used for that tag will be displayed at the very bottom, below the buttons for the wiki:
If the tag doesn't have a highlighting language specified when it should, create a feature-request on the per-site meta for your site to request it. (Only moderators can change the highlighting language for a tag.)
How do I report a bug or request a new language?
highlight.js is actively maintained, please direct bugs and feature requests there. Check for existing reports first, and consider [joining the project][highlightjs-contribute]. For bug fixes and features that have already been implemented by highlight.js but aren't working here, you can raise a feature request on Meta to request that a new version of highlight.js be deployed.
Keep in mind that Stack Exchange does not maintain this syntax highlighter (aside from installing newer versions of it), and posting bug reports or feature requests concerning it here on Meta will not get them fixed or implemented.
Before you do anything, make sure that you've got the correct highlighting turned on.
How do I use syntax highlighting?
As long as all the conditions in the Why isn't my code being highlighted correctly? section above are satisfied, highlighting will kick in automatically on code blocks based on the tags on the question. (See 4. Check that the tags on your question have syntax highlighting enabled above for how this works.) Inline code (e.g. this) is not highlighted.
It is possible to explicitly override the default highlighting language in use on the post with your language of choice in a specific code block, by specifying a language hint above the code block. Note that this is only supported when using the code fence (```) method of code formatting; as of the implementation of CommonMark, doing so on code blocks using the four-space indent method is no longer supported*:
```lang-or-tag-here
code goes here
```
You may use either a language code or a tag name in the language hint to activate syntax highlighting. See below for the complete list of Prettify-supported language codes.
For example:
Here is a code block with language code (with "lang-" prefix) as a hint:
```lang-js
function greet(person) {
return "Hello " + person;
}
var user = "John Doe";
alert(greet(user));
```
Here is a code block with tag name (no "lang-" prefix) as a hint:
```typescript
var arr = [0, 1, 2]; // will highlight if typescript tag exists
```
If you don't want to have any syntax highlighting, you can use the lang-none language:
```lang-none
[code here]
```
You can also apply a language hint to all code blocks in your post, so you don't have to add a hint before each one, by adding an HTML comment at the top of your post. The hint will then be applied to all code blocks within your post, including those which use four-space indent or HTML <pre><code>:
<!-- language-all: lang-or-tag-here -->
Note that when used on a question, it does not override the highlighting language on its answers; those will still be highlighted by default as per the question's tags.
Language codes currently available on Stack Exchange
This is a complete list of every identifier that you can use in the language hint for syntax highlighting. Each language is grouped together based on how they appear in the Prettify JS file that Stack Exchange loads. All those language hints in each group point to the same highlighter.
- Default: interpret the code and guess
default - None: explicitly do not use any syntax highlighting
lang-none - Bash and other Shell scripting
lang-bash,lang-bsh,lang-csh,lang-sh - C, C++, Objective-C, et al.
lang-c,lang-cc,lang-cpp,lang-cxx,lang-cyc,lang-m - C#
lang-cs - Clojure
lang-clj - CoffeeScript
lang-coffee - CSS
lang-css - Dart
lang-dart - Erlang
lang-erl,lang-erlang - Go
lang-go - Haskell
lang-hs - HTML, XML, XSL, et al.
lang-html,lang-xml,lang-xsl - Java
lang-java - JavaScript
lang-js,lang-javascript - JSON
lang-json - Kotlin
lang-kotlin - LaTeX, TeX
lang-latex,lang-tex - Lisp, Scheme
lang-cl,lang-el,lang-lisp,lang-lsp,lang-scm,lang-ss,lang-rkt - Lua
lang-lua - MATLAB
lang-matlab - OCaml, SML, F#, et al.
lang-fs,lang-ml - Pascal, Delphi
lang-pascal - Perl
lang-pl,lang-perl - Protocol Buffers
lang-proto - Python
lang-py,lang-python,lang-cv - R, S
lang-r,lang-s - Regex
lang-regex - Ruby
lang-rb,lang-ruby - Rust
lang-rc,lang-rs,lang-rust - Scala
lang-scala - SQL
lang-sql - Swift
lang-swift - TypeScript
lang-typescript,lang-ts - VHDL
lang-vhdl,lang-vhd - Visual Basic, VBScript
lang-vb,lang-vbs - YAML
lang-yaml
Hinting: Tags
You can specify any tag that exists on the site, and it will use whatever language code is currently associated with that tag (which can be either null (no hint), default, or a specific language-code).
Keep in mind that by default all tags start off with none as their language code. Tags with none specified as their language code will be ignored and revert to default.
You can also use the plain none keyword to manually specify no syntax highlighting, similar to using the lang-none code above.
* The former method of specifying a highlighting language can still be used for HTML code blocks: place an HTML comment <!-- language: lang-or-tag-here --> before the <pre><code> tags and it will work.
Also, this former method hasn't been completely removed for four-space indented code blocks, but merely deprecated. While it will still work for the time being on four-space indented code blocks, it may/will be removed in the future.
Note to editors:
Please do not add to the above list unless you are 100% sure it exists.
Just because you type something in and it looks like it's highlighted correctly does not mean the identifier actually exists in the system. Keep in mind that invalid identifiers revert back to default.
Additionally, as newer versions of highlight.js have to be manually installed by Stack Exchange, languages that are indicated as supported in highlight.js's documentation may not actually be supported by the Stack Exchange version of highlight.js.
Because of these points, please link to the Meta question which confirms a hint's existence in your edit summary when adding a new hint to the list.
Note to commenters:
The comments on this FAQ are for requesting clarification of something you might not understand in the FAQ so that it can be fixed. Please DO NOT ask if certain languages will be supported in the future. That is not a question we can answer because Stack Exchange does not maintain this highlighter. Visit highlight.js for language support.
