Markdown parser written in Clojure/Script

Demo

You can try out the parser here.

Installation

A markdown parser that compiles to both Clojure and ClojureScript.

Note: markdown-clj versions prior to 0.9.68 requires Clojure 1.2+ to run, versions 0.9.68+ require Clojure 1.7.

NPM module

Usage Clojure

Markdown-clj can be invoked either by calling md-to-html or md-to-html-string functions.

The md-to-html function accepts an input containing Markdown markup and an output where the resulting HTML will be written. The input and output parameters will be passed to a reader and a writer respectively:

(ns foo
  (:use markdown.core))

(md-to-html "input.md" "output.html")

(md-to-html (input-stream "input.md") (output-stream "test.txt"))

The md-to-html-string function accepts a string with markdown content and returns a string with the resulting HTML:

(md-to-html-string "# This is a test\nsome code follows\n```clojure\n(defn foo [])\n```")

<h1> This is a test</h1>some code follows<pre><code class="clojure">&#40;defn foo &#91;&#93;&#41;
</code></pre>

Both md-to-html and md-to-html-string accept optional parameters:

Specifying :heading-anchors will create anchors for the heading tags, eg:

(markdown/md-to-html-string "###foo bar BAz" :heading-anchors true)

<h3 id=\"foo&#95;bar&#95;baz\">foo bar BAz</h3>

The code blocks default to a highlight.js compatible format of:

<pre><code class="clojure">some code</code></pre>

Specifying :code-style will override the default code class formatting for code blocks, eg:

(md-to-html-string "# This is a test\nsome code follows\n```clojure\n(defn foo [])\n```"
                   :code-style #(str "class=\"brush: " % "\""))

<h1> This is a test</h1>some code follows<pre><code class="brush: clojure">
&#40;defn foo &#91;&#93;&#41;
</code></pre>

reference style links

The parser defaults to using inline reference for performance reasons, to enable reference style links pass in the :reference-links? true option:

(md-to-html-string
  "This is [an example][id] reference-style link.

   [id]: http://example.com/ 'Optional Title Here'"
   :reference-links? true)

footnotes

To enable footnotes, pass the :footnotes? true option:

(md-to-html-string
  "Footnotes will appear automatically numbered with a link to the footnote at bottom of the page [^footnote1].

  [^footnote1]: The footnote will contain a back link to to the referring text."
  :footnotes? true)

Metadata

The metadata encoded using the syntax described by MultiMarkdown can be optionally extracted from the document.

The md-to-html function will attempt to parse the metadata when passed the :parse-meta? true option and return it as its output. Additionally, md-to-html-string-with-meta function can be used to parse string input. The function returns a map with two keys, :html containing the parsed HTML, and :metadata containing a map with the metadata included at the top of the document.

The value of each key in the metadata map will be a list of either 0, 1 or many strings. If a metadata value ends in two spaces then the string will end in a newline. If a line does not contain a header and has at least 4 spaces in front of it then it will be considered to be a member of the last key that was found.

(let [input    (new StringReader text)
      output   (new StringWriter)
      metadata (md-to-html input output :parse-meta? true)
      html     (.toString output)]
  {:metadata metadata :html html})

(md-to-html-string-with-meta
  "Author: Rexella van Imp
    Kim Jong-un
Date: October 31, 2015

   # Hello!")

{:metadata {:author ["Rexella van Imp"
                     "Kim Jong-un"],
            :date ["October 31, 2015"]},
 :html "<h1>Hello!</h1>"}

Selectively inhibiting the Parser

If you pass :inhibit-separator "some-string", then any text within occurrences of some-string will be output verbatim, eg:

(md-to-html-string "For all %$a_0, a_1, ..., a_n in R$% there is _at least one_ %$b_n in R$% such that..."
                   :inhibit-separator "%")

For all $a_0, a_1, ..., a_n in R$ there is <i>at least one</i> $b_n in R$ such that...

This may be useful to use markdown-clj along with other parsers of languages with conflicting syntax (e.g. asciimath2jax).

If you need to output the separator itself, enter it twice without any text inside. Eg:

(md-to-html-string "This is one of those 20%% vs 80%% cases."
                   :inhibit-separator "%")

This is one of those 20% vs 80% cases.

Some caveats:

• Like other tags, this only works within a single line.

• If you remove the default transformers with :replacement-transformers (which see below), inhibiting will stop working.

• Currently, dashes (-- and ---) can't be suppressed this way.

Customizing the Parser

Additional transformers can be specified using the :custom-transformers key. A transformer function must accept two arguments. First argument is the string representing the current line and the second is the map representing the current state.

The default state keys are:

• :code - inside a code section
• :codeblock - inside a code block
• :eof - end of file
• :heading - in a heading
• :hr - in a horizontal line
• :lists - inside a list
• :blockquote - inside a blockquote
• :paragraph - in a paragraph
• :last-line-empty? - was last line an empty line?

For example, if we wanted to add a transformer that would capitalize all text we could do the following:

(defn capitalize [text state]
  [(.toUpperCase text) state])

(markdown/md-to-html-string "#foo" :custom-transformers [capitalize])

<H1>FOO</H1>

Alternatively, you could provide a custom set of transformers to replace the default transformers using the :replacement-transformers key.

(markdown/md-to-html-string "#foo" :replacement-transformers [capitalize])

This can also be used to add preprocessor transformers. For example, if we wanted to sanitize any image links and escape HTML we could do the following:

(use 'markdown.transformers 'markdown.core)

(defn escape-images [text state]
  [(clojure.string/replace text #"(!\[.*?\]\()(.+?)(\))" "") state])

(defn escape-html
    "Change special characters into HTML character entities."
    [text state]
    [(if-not (or (:code state) (:codeblock state))
       (clojure.string/escape
         text
         {\& "&"
          \< "&lt;"
          \> "&gt;"
          \" "&quot;"
          \' "&#39;"})
       text) state])
       
(markdown/md-to-html-string
  "<h1>escaped</h1>foo ![Alt text](/path/to/img.jpg \"Optional Title\") bar [text](http://test)"
  :replacement-transformers (into [escape-images escape-html] transformer-vector))

"<p>&lt;h1&gt;escaped&lt;/h1&gt;foo  bar <a href='http://test'>text</a></p>"

Usage ClojureScript

The ClojureScript portion works the same as above except that the entry function is called md->html. It accepts a string followed by the options as its input, and returns the resulting HTML string:

(ns myscript
  (:require [markdown.core :refer [md->html]]))

(.log js/console
  (md->html "##This is a heading\nwith a paragraph following it"))

(.log js/console
  (md->html "# This is a test\nsome code follows\n```clojure\n(defn foo [])\n```"
               :code-style #(str "class=\"" % "\"")))

(md->html-with-meta "# This is a test\nsome code follows\n```clojure\n(defn foo [])\n```")

Usage JavaScript

console.log(markdown.core.mdToHtml("##This is a heading\nwith a paragraph following it"));

Supported syntax

Control characters can be escaped using \

\\ backslash
\` backtick
\* asterisk
\_ underscore
\{ curly braces
\}
\[ square brackets
\]
\( parentheses
\)
\# hash mark
\+ plus sign
\- minus sign (hyphen)
\. dot
\! exclamation mark
\^ caret / circumflex accent

Basic Elements

Blockquote, Strong, Bold, Bold-Italic, Emphasis, Italics, Heading, Line, Linebreak, Paragraph, Strikethrough

Links

Image, Link

Automatic Links

This is a shortcut style for creating “automatic” links for URLs and email addresses:

<http://example.com/>

will be turned this into:

<a href="http://example.com/">http://example.com/</a>

Automatic links for email addresses work similarly, except that they are hex encoded:

<address@example.com&>

will be turned into:

<a href=\"&#x61&#x64&#x64&#x72&#x65&#x73&#x73&#x40&#x65&#x78&#x61&#x6d&#x70&#x6c&#x65&#x2e&#x63&#x6f&#x6d\">&#x61&#x64&#x64&#x72&#x65&#x73&#x73&#x40&#x65&#x78&#x61&#x6d&#x70&#x6c&#x65&#x2e&#x63&#x6f&#x6d</a>

Lists

Ordered List, Unordered List

Code

Code Block, Indented Code, Inline Code

---

Heading

the number of hashes indicates the level of the heading

# Heading

##Sub-heading

### Sub-sub-heading

headings can also be defined using = and - for h1 and h2 respectively

Heading 1
=========

Heading 2
---------

Line

***

* * *

*****

- - -

______

Linebreak

If a line ends with two or more spaces a <br /> tag will be inserted at the end.

Emphasis

*foo*

Italics

_foo_

Strong

**foo**

Bold

__foo__

Bold-Italic

***bold italic***

Blockquote

> prefixes regular blockquote paragraphs. >- prefixes a blockquote footer that can be used for author attribution.

>This is a blockquote
with some content

>this is another blockquote

> Everyone thinks of changing the world,
but no one thinks of changing himself.
>- Leo Tolstoy

Paragraph

This is a paragraph, it's
split into separate lines.

This is another paragraph.

Unordered List

indenting an item makes it into a sublist of the item above it, ordered and unordered lists can be nested within one another. List items can be split over multiple lines.

* Foo
* Bar
 * Baz

* foo
* bar

   * baz
     1. foo
     2. bar
        more content
        ## subheading
        ***
        **strong text** in the list

   * fuzz

      * blah
      * blue
* brass

Ordered List

1. Foo
2. Bar
3. Baz

Inline Code

Any special characters in code will be escaped with their corresponding HTML codes.

Here's some code `x + y = z` that's inlined.

Code block

Using three backquotes indicates a start of a code block, the next three backquotes ends the code block section. Optionally, the language name can be put after the backquotes to produce a tag compatible with highlight.js, eg:

```clojure

(defn foo [bar] "baz")

```

Indented Code

indenting by at least 4 spaces creates a code block

some
code
here

note: XML is escaped in code sections

Strikethrough

~~foo~~

Superscript

a^2 + b^2 = c^2

Link

[github](http://github.com)

Reference Link

This is [an example][id] reference-style link.

[id]: http://example.com/  "Optional Title Here"

note: reference links require the :reference-links? option to be set to true

Footnote

"Footnotes will appear automatically numbered with a link to the footnote at bottom of the page [^footnote1].
[^footnote1]: The footnote will contain a back link to to the referring text."

note: to enable footnotes, the :footnotes? option must be set to true.

Image

![Alt text](http://server/path/to/img.jpg)
![Alt text](/path/to/img.jpg "Optional Title")

Image Reference

This is ![an example][id] reference-style image descriptor.

[id]: http://example.com/  "Optional Title Here"

note: reference links require the :reference-links? option to be set to true

Image Link

[![Continuous Integration status](https://secure.travis-ci.org/yogthos/markdown-clj.png)](http://travis-ci.org/yogthos/markdown-clj)

Table

You can create tables by assembling a list of words and dividing them with hyphens - (for the first row), and then separating each column with a pipe |:

| First Header  | Second Header |
| ------------- | ------------- |
| Content Cell  | Content Cell  |
| Content Cell  | Content Cell  |

By including colons : within the header row, you can define text to be left-aligned, right-aligned, or center-aligned:

| Left-Aligned  | Center Aligned    | Right Aligned |
| :------------ | :---------------: | ------------: |
| col 3 is      |  some wordy text  | $1600         |
| col 2 is      |  centered         |   $12         |
| zebra stripes |  are neat         |    $1         |

A colon on the left-most side indicates a left-aligned column; a colon on the right-most side indicates a right-aligned column; a colon on both sides indicates a center-aligned column.

Limitations

The parser reads the content line by line, this means that tag content is not allowed to span multiple lines.

License

Copyright © 2015 Dmitri Sotnikov

Distributed under the Eclipse Public License, the same as Clojure.