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Markdown Parser and HTML Renderer for Go

GoDoc codecov

Package github.com/gomarkdown/markdown is a very fast Go library for parsing Markdown documents and rendering them to HTML.

It's fast and supports common extensions.

API Docs:

Users

Some tools using this package:

Usage

To convert markdown text to HTML using reasonable defaults:

md := []byte("## markdown document")
output := markdown.ToHTML(md, nil, nil)

Customizing markdown parser

Markdown format is loosely specified and there are multiple extensions invented after original specification was created.

The parser supports several extensions.

Default parser uses most common parser.CommonExtensions but you can easily use parser with custom extension:

import (
    "github.com/gomarkdown/markdown"
    "github.com/gomarkdown/markdown/parser"
)

extensions := parser.CommonExtensions | parser.AutoHeadingIDs
parser := parser.NewWithExtensions(extensions)

md := []byte("markdown text")
html := markdown.ToHTML(md, parser, nil)

Customizing HTML renderer

Similarly, HTML renderer can be configured with different options

Here's how to use a custom renderer:

import (
    "github.com/gomarkdown/markdown"
    "github.com/gomarkdown/markdown/html"
)

htmlFlags := html.CommonFlags | html.HrefTargetBlank
opts := html.RendererOptions{Flags: htmlFlags}
renderer := html.NewRenderer(opts)

md := []byte("markdown text")
html := markdown.ToHTML(md, nil, renderer)

HTML renderer also supports reusing most of the logic and overriding rendering of only specifc nodes.

You can provide RenderNodeFunc in RendererOptions.

The function is called for each node in AST, you can implement custom rendering logic and tell HTML renderer to skip rendering this node.

Here's the simplest example that drops all code blocks from the output:

import (
    "github.com/gomarkdown/markdown"
    "github.com/gomarkdown/markdown/ast"
    "github.com/gomarkdown/markdown/html"
)

// return (ast.GoToNext, true) to tell html renderer to skip rendering this node
// (because you've rendered it)
func renderHookDropCodeBlock(w io.Writer, node ast.Node, entering bool) (ast.WalkStatus, bool) {
    // skip all nodes that are not CodeBlock nodes
	if _, ok := node.(*ast.CodeBlock); !ok {
		return ast.GoToNext, false
    }
    // custom rendering logic for ast.CodeBlock. By doing nothing it won't be
    // present in the output
	return ast.GoToNext, true
}

opts := html.RendererOptions{
    Flags: html.CommonFlags,
    RenderNodeHook: renderHookDropCodeBlock,
}
renderer := html.NewRenderer(opts)
md := "test\n```\nthis code block will be dropped from output\n```\ntext"
html := markdown.ToHTML([]byte(md), nil, renderer)

Sanitize untrusted content

We don't protect against malicious content. When dealing with user-provided markdown, run renderer HTML through HTML sanitizer such as Bluemonday.

Here's an example of simple usage with Bluemonday:

import (
    "github.com/microcosm-cc/bluemonday"
    "github.com/gomarkdown/markdown"
)

// ...
maybeUnsafeHTML := markdown.ToHTML(md, nil, nil)
html := bluemonday.UGCPolicy().SanitizeBytes(maybeUnsafeHTML)

Windows / Mac newlines

The library only supports Unix newlines. If you have markdown text with possibly Windows / Mac newlines, normalize newlines before caling this librar using d = markdown.NormalizeNewlines(d)

mdtohtml command-line tool

https://github.com/gomarkdown/mdtohtml is a command-line markdown to html converter built using this library.

You can also use it as an example of how to use the library.

You can install it with:

go get -u github.com/gomarkdown/mdtohtml

To run: mdtohtml input-file [output-file]

Features

  • Compatibility. The Markdown v1.0.3 test suite passes with the --tidy option. Without --tidy, the differences are mostly in whitespace and entity escaping, where this package is more consistent and cleaner.

  • Common extensions, including table support, fenced code blocks, autolinks, strikethroughs, non-strict emphasis, etc.

  • Safety. Markdown is paranoid when parsing, making it safe to feed untrusted user input without fear of bad things happening. The test suite stress tests this and there are no known inputs that make it crash. If you find one, please let me know and send me the input that does it.

    NOTE: "safety" in this context means runtime safety only. In order to protect yourself against JavaScript injection in untrusted content, see this example.

  • Fast. It is fast enough to render on-demand in most web applications without having to cache the output.

  • Thread safety. You can run multiple parsers in different goroutines without ill effect. There is no dependence on global shared state.

  • Minimal dependencies. Only depends on standard library packages in Go.

  • Standards compliant. Output successfully validates using the W3C validation tool for HTML 4.01 and XHTML 1.0 Transitional.

Extensions

In addition to the standard markdown syntax, this package implements the following extensions:

  • Intra-word emphasis supression. The _ character is commonly used inside words when discussing code, so having markdown interpret it as an emphasis command is usually the wrong thing. We let you treat all emphasis markers as normal characters when they occur inside a word.

  • Tables. Tables can be created by drawing them in the input using a simple syntax:

    Name    | Age
--------|------
Bob     | 27
Alice   | 23

    Table footers are supported as well and can be added with equal signs (=):

    Name    | Age
--------|------
Bob     | 27
Alice   | 23
========|======
Total   | 50

    A cell spanning multiple columns (colspan) is supported, just repeat the pipe symbol:

    Name    | Age
--------|------
Bob     ||
Alice   | 23
========|======
Total   | 23

  • Fenced code blocks. In addition to the normal 4-space indentation to mark code blocks, you can explicitly mark them and supply a language (to make syntax highlighting simple). Just mark it like this:

    ```go
func getTrue() bool {
    return true
}
```

    You can use 3 or more backticks to mark the beginning of the block, and the same number to mark the end of the block.

  • Definition lists. A simple definition list is made of a single-line term followed by a colon and the definition for that term.

    Cat
: Fluffy animal everyone likes

Internet
: Vector of transmission for pictures of cats

    Terms must be separated from the previous definition by a blank line.

  • Footnotes. A marker in the text that will become a superscript number; a footnote definition that will be placed in a list of footnotes at the end of the document. A footnote looks like this:

    This is a footnote.[^1]

[^1]: the footnote text.

  • Autolinking. We can find URLs that have not been explicitly marked as links and turn them into links.

  • Strikethrough. Use two tildes (~~) to mark text that should be crossed out.

  • Hard line breaks. With this extension enabled newlines in the input translates into line breaks in the output. This extension is off by default.

  • Non blocking space. With this extension enabled spaces preceeded by a backslash in the input translates non-blocking spaces in the output. This extension is off by default.

  • Smart quotes. Smartypants-style punctuation substitution is supported, turning normal double- and single-quote marks into curly quotes, etc.

  • LaTeX-style dash parsing is an additional option, where -- is translated into –, and --- is translated into —. This differs from most smartypants processors, which turn a single hyphen into an ndash and a double hyphen into an mdash.

  • Smart fractions, where anything that looks like a fraction is translated into suitable HTML (instead of just a few special cases like most smartypant processors). For example, 4/5 becomes <sup>4</sup>&frasl;<sub>5</sub>, which renders as 4⁄5.

  • MathJaX Support is an additional feature which is supported by many markdown editor. It translates inline math equations quoted by $ and displays math blocks quoted by $$ into MathJax compatible format. Hyphens (_) won't break LaTeX render within a math element any more.

    $$
\left[ \begin{array}{a} a^l_1 \\ ⋮ \\ a^l_{d_l} \end{array}\right]
= \sigma(
 \left[ \begin{matrix}
 	w^l_{1,1} & ⋯  & w^l_{1,d_{l-1}} \\
 	⋮ & ⋱  & ⋮  \\
 	w^l_{d_l,1} & ⋯  & w^l_{d_l,d_{l-1}} \\
 \end{matrix}\right]  ·
 \left[ \begin{array}{x} a^{l-1}_1 \\ ⋮ \\ ⋮ \\ a^{l-1}_{d_{l-1}} \end{array}\right] +
 \left[ \begin{array}{b} b^l_1 \\ ⋮ \\ b^l_{d_l} \end{array}\right])
 $$

  • Ordered list start number. With this extension enabled an ordered list will start with the number that was used to start it.

  • Super and subscript. With this extension enabled sequences between ^ will indicate superscript and ~ will become a subscript. For example: H~2~O is a liquid, 2^10^ is 1024.

  • Block level attributes allow setting attributes (ID, classes and key/value pairs) on block level elements. The attribute must be enclosed with braces and be put on a line before the element.

    {#id3 .myclass fontsize="tiny"}
# Header 1

    Will convert into <h1 id="id3" class="myclass" fontsize="tiny">Header 1</h1>.

  • Mmark support, see https://mmark.miek.nl/post/syntax/ for all new syntax elements this adds.

Todo

History

markdown is a fork of v2 of https://github.com/russross/blackfriday that is:

  • actively maintained (sadly in Feb 2018 blackfriday was inactive for 5 months with many bugs and pull requests accumulated)
  • refactored API (split into ast/parser/html sub-packages)

Blackfriday itself was based on C implementation sundown which in turn was based on libsoldout.

License

Simplified BSD License