CommonMark is a rationalized version of Markdown syntax, with a spec and BSD3-licensed reference implementations in C and JavaScript.
The C implementation provides both a library and a standalone program
cmark that converts CommonMark to HTML. It is written in standard C99
and has no library dependencies. (However, if you check it out from the
repository, you'll need re2c to generate
scanners.c from scanners.re. This is only a build dependency for
developers, since scanners.c can be provided in a released source
tarball.)
The parser is very fast, on par with
sundown. Some benchmarks (on
a 1.6 GHz Intel Core i5, measured using time, and parsing a
~500K book, the English version of Pro
Git by
Scott Chacon and Ben Straub):
| Implementation | Time |
|---|---|
| Markdown.pl | 3.990s |
| discount | 0.089s |
| sundown | 0.015s |
| cmark | 0.019s |
Usage: cmark [FILE*]
Options: --help, -h Print usage information
--ast Print AST instead of HTML
--version Print version
The JavaScript implementation is a single JavaScript file, with
no dependencies, that can be linked to in an HTML page. To build,
it, do make js/commonmark.js (this requires browserify, which you
can get using npm install -g browserify). You can also fetch
a pre-built copy from http://spec.commonmark.org/js/commonmark.js.
A command-line version (using node.js) is also provided
(js/bin/commonmark), and there is a "dingus" for playing with it
interactively. (make dingus will start this.)
Note: neither implementation attempts to sanitize link attributes or raw HTML. If you use these libraries in applications that accept untrusted user input, you must run the output through an HTML sanitizer to protect against XSS attacks.
The spec contains over 450 embedded examples which serve as conformance
tests. To run the tests for cmark, do make test. To run them for
another Markdown program, say myprog, do make test PROG=myprog. To
run the tests for commonmark.js, do make testjs.
To install the C program (and shared library), cmake is required:
mkdir build
cd build
cmake .. # optionally: -DCMAKE_INSTALL_PREFIX=path
make # executable will be created as build/src/cmake
make install
To run tests:
perl runtests.pl spec.txt build/src/cmark
To build the javascript library:
browserify --standalone commonmark js/lib/index.js -o js/commonmark.js
To run tests:
node js/test.js
The source of the spec is spec.txt. This is basically a Markdown
file, with code examples written in a shorthand form:
.
Markdown source
.
expected HTML output
.
To build an HTML version of the spec, do make spec.html. To build a
PDF version, do make spec.pdf. Both these commands require that
pandoc is installed, and creating a PDF requires a latex installation.
The spec is written from the point of view of the human writer, not the computer reader. It is not an algorithm---an English translation of a computer program---but a declarative description of what counts as a block quote, a code block, and each of the other structural elements that can make up a Markdown document.
Because John Gruber's canonical syntax description leaves many aspects of the syntax undetermined, writing a precise spec requires making a large number of decisions, many of them somewhat arbitrary. In making them, I have appealed to existing conventions and considerations of simplicity, readability, expressive power, and consistency. I have tried to ensure that "normal" documents in the many incompatible existing implementations of Markdown will render, as far as possible, as their authors intended. And I have tried to make the rules for different elements work together harmoniously. In places where different decisions could have been made (for example, the rules governing list indentation), I have explained the rationale for my choices. In a few cases, I have departed slightly from the canonical syntax description, in ways that I think further the goals of Markdown as stated in that description.
For the most part, I have limited myself to the basic elements described in Gruber's canonical syntax description, eschewing extensions like footnotes and definition lists. It is important to get the core right before considering such things. However, I have included a visible syntax for line breaks and fenced code blocks.
There are only a few places where this spec says things that contradict the canonical syntax description:
-
It allows all punctuation symbols to be backslash-escaped, not just the symbols with special meanings in Markdown. I found that it was just too hard to remember which symbols could be escaped.
-
It introduces an alternative syntax for hard line breaks, a backslash at the end of the line, supplementing the two-spaces-at-the-end-of-line rule. This is motivated by persistent complaints about the “invisible” nature of the two-space rule.
-
Link syntax has been made a bit more predictable (in a backwards-compatible way). For example,
Markdown.plallows single quotes around a title in inline links, but not in reference links. This kind of difference is really hard for users to remember, so the spec allows single quotes in both contexts. -
The rule for HTML blocks differs, though in most real cases it shouldn't make a difference. (See here for details.) The spec's proposal makes it easy to include Markdown inside HTML block-level tags, if you want to, but also allows you to exclude this. It is also makes parsing much easier, avoiding expensive backtracking.
-
It does not collapse adjacent bird-track blocks into a single blockquote:
> this is two > blockquotes > this is a single > > blockquote with two paragraphs -
Rules for content in lists differ in a few respects, though (as with HTML blocks), most lists in existing documents should render as intended. There is some discussion of the choice points and differences here. I think that the spec's proposal does better than any existing implementation in rendering lists the way a human writer or reader would intuitively understand them. (I could give numerous examples of perfectly natural looking lists that nearly every existing implementation flubs up.)
-
The spec stipulates that two blank lines break out of all list contexts. This is an attempt to deal with issues that often come up when someone wants to have two adjacent lists, or a list followed by an indented code block.
-
Changing bullet characters, or changing from bullets to numbers or vice versa, starts a new list. I think that is almost always going to be the writer's intent.
-
The number that begins an ordered list item may be followed by either
.or). Changing the delimiter style starts a new list. -
The start number of an ordered list is significant.
-
Fenced code blocks are supported, delimited by either backticks (
```) or tildes (~~~).
In all of this, I have been guided by eight years experience writing Markdown implementations in several languages, including the first Markdown parser not based on regular expression substitutions (pandoc) and the first markdown parsers based on PEG grammars (peg-markdown, lunamark). Maintaining these projects and responding to years of user feedback have given me a good sense of the complexities involved in parsing Markdown, and of the various design decisions that can be made. I have also explored differences between Markdown implementations extensively using babelmark 2. In the early phases of working out the spec, I benefited greatly from collaboration with David Greenspan, and from feedback from several industrial users of Markdown, including Jeff Atwood, Vincent Marti, and Neil Williams.
There is a forum for discussing CommonMark; you should use it instead of github issues for questions and possibly open-ended discussions. Use the github issue tracker only for simple, clear, actionable issues.