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Markdown implementation

Zulip has a special flavor of Markdown, currently called 'bugdown' after Zulip's original name of "humbug". End users are using Bugdown within the client, not original Markdown.

Zulip has two implementations of Bugdown. The backend implementation at zerver/lib/bugdown/ is based on Python-Markdown and is used to authoritatively render messages to HTML (and implements slow/expensive/complex features like querying the Twitter API to render tweets nicely). The frontend implementation is in JavaScript, based on marked.js (static/js/echo.js), and is used to preview and locally echo messages the moment the sender hits enter, without waiting for round trip from the server.

The JavaScript markdown implementation has a function, echo.contains_bugdown, that is used to check whether a message contains any syntax that needs to be rendered to HTML on the backend. If echo.contains_bugdown returns true, the frontend simply won't echo the message for the sender until it receives the rendered HTML from the backend. If there is a bug where echo.contains_bugdown returns false incorrectly, the frontend will discover this when the backend returns the newly sent message, and will update the HTML based on the authoritative backend rendering (which would cause a change in the rendering that is visible only to the sender shortly after a message is sent). As a result, we try to make sure that echo.contains_bugdown is always correct.

Testing

The Python-Markdown implementation is tested by zerver/tests/test_bugdown.py, and the marked.js implementation and echo.contains_bugdown are tested by frontend_tests/node_tests/echo.js. A shared set of fixed test data ("test fixtures") is present in zerver/fixtures/bugdown-data.json, and is automatically used by both test suites; as a result, it the preferred place to add new tests for Zulip's markdown system.

Changing Zulip's markdown processor

When changing Zulip's markdown syntax, you need to update several places:

The backend markdown processor (zerver/lib/bugdown/__init__.py).
The frontend markdown processor (static/js/echo.js and sometimes static/third/marked/lib/marked.js), or echo.contains_bugdown if your changes won't be supported in the frontend processor.
If desired, the typeahead logic in static/js/composebox_typeahead.js.
The test suite, probably via adding entries to zerver/fixtures/bugdown-data.json.
The in-app markdown documentation (templates/zerver/markdown_help.html).
The list of changes to markdown at the end of this document.

Important considerations for any changes are:

Security: A bug in the markdown processor can lead to XSS issues. For example, we should not insert unsanitized HTML from a third-party web application into a Zulip message.
Uniqueness: We want to avoid users having a bad experience due to accidentally triggering markdown syntax or typeahead that isn't related to what they are trying to express.
Performance: Zulip can render a lot of messages very quickly, and we'd like to keep it that way. New regular expressions similar to the ones already present are unlikely to be a problem, but we need to be thoughtful about expensive computations or third-party API requests.
Database: The backend markdown processor runs inside a Python thread (as part of how we implement timeouts for third-party API queries), and for that reason we currently should avoid making database queries inside the markdown processor. This is a technical implementation detail that could be changed with a few days of work, but is important detail to know about until we do that work.
Testing: Every new feature should have both positive and negative tests; they're easy to write and give us the flexibility to refactor frequently.

Zulip's Markdown philosophy

Note that this discussion is based on a comparison with the original Markdown, not newer Markdown variants like CommonMark.

Markdown is great for group chat for the same reason it's been successful in products ranging from blogs to wikis to bug trackers: it's close enough to how people try to express themselves when writing plain text (e.g. emails) that it helps more than getting in the way.

The main issue for using Markdown in instant messaging is that the Markdown standard syntax used in a lot of wikis/blogs has nontrivial error rates, where the author needs to go back and edit the post to fix the formatting after typing it the first time. While that's basically fine when writing a blog, it gets annoying very fast in a chat product; even though you can edit messages to fix formatting mistakes, you don't want to be doing that often. There are basically 2 types of error rates that are important for a product like Zulip:

What fraction of the time, if you pasted a short technical email that you wrote to your team and passed it through your Markdown implementation, would you need to change the text of your email for it to render in a reasonable way? This is the "accidental Markdown syntax" problem, common with Markdown syntax like the italics syntax interacting with talking about char *s.
What fraction of the time do users attempting to use a particular Markdown syntax actually succeed at doing so correctly? Syntax like required a blank line between text and the start of a bulleted list raise this figure substantially.

Both of these are minor issues for most products using Markdown, but they are major problems in the instant messaging context, because one can't edit a message that has already been sent and users are generally writing quickly. Zulip's Markdown strategy is based on the principles of giving users the power they need to express complicated ideas in a chat context while minimizing those two error rates.

Zulip's Changes to Markdown

Below, we document the changes that Zulip has against stock Python-Markdown; some of the features we modify / disable may already be non-standard.

Basic syntax

Enable `nl2br extension: this means one newline creates a line break (not paragraph break).
Disable italics entirely. This resolves an issue where people were using * and _ and hitting it by mistake too often. E.g. with stock Markdown You should use char * instead of void * there would trigger italics.
Allow only ** syntax for bold, not __ (easy to hit by mistake if discussing Python __init__ or something)
Add ~~ syntax for strikethrough.
Disable special use of \ to escape other syntax. Rendering \\ as \ was hugely controversial, but having no escape syntax is also controversial. We may revisit this. For now you can always put things in code blocks.

Lists

Allow tacking a bulleted list or block quote onto the end of a paragraph, i.e. without a blank line before it
Allow only * for bulleted lists, not + or - (previously created confusion with diff-style text sloppily not included in a code block)
Disable ordered list syntax: it automatically renumbers, which can be really confusing when sending a numbered list across multiple messages.

Code

Enable fenced code block extension, with syntax highlighting
Disable line-numbering within fenced code blocks -- the <table> output confused our web client code.

Other

Disable headings, both # foo and == foo == syntax: they don't make much sense for chat messages.
Disabled images.
Allow embedding any avatar as a tiny (list bullet size) image. This is used primarily by version control integrations.
We added the ~~~ quote block quote syntax.

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