Markdown Parser and HTML Renderer for Go

Package github.com/gomarkdown/markdown is a Go library for parsing Markdown text and rendering as HTML.

It's very fast and supports common extensions.

Tutorial: https://blog.kowalczyk.info/article/cxn3/advanced-markdown-processing-in-go.html

Code examples:

https://tools.arslexis.io/goplayground/#txO7hJ-ibeU : basic markdown => HTML
https://tools.arslexis.io/goplayground/#yFRIWRiu-KL : customize HTML renderer
https://tools.arslexis.io/goplayground/#2yV5-HDKBUV : modify AST
https://tools.arslexis.io/goplayground/#9fqKwRbuJ04 : customize parser
https://tools.arslexis.io/goplayground/#Bk0zTvrzUDR : syntax highlight

Those examples are also in examples directory.

API Docs:

https://pkg.go.dev/github.com/gomarkdown/markdown : top level package
https://pkg.go.dev/github.com/gomarkdown/markdown/ast : defines abstract syntax tree of parsed markdown document
https://pkg.go.dev/github.com/gomarkdown/markdown/parser : parser
https://pkg.go.dev/github.com/gomarkdown/markdown/html : html renderer

Usage

To convert markdown text to HTML using reasonable defaults:

package main

import (
    "os"

    "github.com/gomarkdown/markdown"
    "github.com/gomarkdown/markdown/ast"
    "github.com/gomarkdown/markdown/html"
    "github.com/gomarkdown/markdown/parser"

    "fmt"
)

var mds = `# header

Sample text.

[link](http://example.com)
`

func mdToHTML(md []byte) []byte {
    // create markdown parser with extensions
    extensions := parser.CommonExtensions | parser.AutoHeadingIDs | parser.NoEmptyLineBeforeBlock
    p := parser.NewWithExtensions(extensions)
    doc := p.Parse(md)

    // create HTML renderer with extensions
    htmlFlags := html.CommonFlags | html.HrefTargetBlank
    opts := html.RendererOptions{Flags: htmlFlags}
    renderer := html.NewRenderer(opts)

    return markdown.Render(doc, renderer)
}

func main() {
    md := []byte(mds)
    html := mdToHTML(md)

    fmt.Printf("--- Markdown:\n%s\n\n--- HTML:\n%s\n", md, html)
}

Try it online: https://onlinetool.io/goplayground/#txO7hJ-ibeU

For more documentation read this guide

Comparing to other markdown parsers: https://babelmark.github.io/

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)

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:
```
  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 preceded 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.