# Markdown Parser and HTML Renderer for Go
[](https://pkg.go.dev/github.com/gomarkdown/markdown)
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.
Try code examples online: https://replit.com/@kjk1?path=folder/gomarkdown
## 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
## Users
Some tools using this package: https://pkg.go.dev/github.com/gomarkdown/markdown?tab=importedby
## Usage
To convert markdown text to HTML using reasonable defaults:
```go
md := []byte("## markdown document")
output := markdown.ToHTML(md, nil, nil)
```
Try it online: https://replit.com/@kjk1/gomarkdown-basic
## Customizing markdown parser
Markdown format is loosely specified and there are multiple extensions invented after original specification was created.
The parser supports several [extensions ](https://pkg.go.dev/github.com/gomarkdown/markdown/parser#Extensions ).
Default parser uses most common `parser.CommonExtensions` but you can easily use parser with custom extension:
```go
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)
```
Try it online: https://replit.com/@kjk1/gomarkdown-customized-html-renderer
## Customizing HTML renderer
Similarly, HTML renderer can be configured with different [options ](https://pkg.go.dev/github.com/gomarkdown/markdown/html#RendererOptions )
Here's how to use a custom renderer:
```go
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)
```
Try it online: https://replit.com/@kjk1/gomarkdown-customized-html-renderer
HTML renderer also supports reusing most of the logic and overriding rendering of only specific nodes.
You can provide [RenderNodeFunc ](https://pkg.go.dev/github.com/gomarkdown/markdown/html#RenderNodeFunc ) in [RendererOptions ](https://pkg.go.dev/github.com/gomarkdown/markdown/html#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:
````go
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 ](https://github.com/microcosm-cc/bluemonday ).
Here's an example of simple usage with Bluemonday:
```go
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 calling this library 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 ](https://github.com/gomarkdown/markdown#sanitize-untrusted-content ).
- **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>⁄<sub>5</sub>` , which renders as
< sup > 4< / sup > ⁄ < sub > 5< / sub > .
- **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
- port https://github.com/russross/blackfriday/issues/348
- port [LaTeX output ](https://github.com/Ambrevar/Blackfriday-LaTeX ):
renders output as LaTeX.
- port https://github.com/shurcooL/github_flavored_markdown to markdown
- port [markdownfmt ](https://github.com/shurcooL/markdownfmt ): like gofmt,
but for markdown.
- More unit testing
- Improve unicode support. It does not understand all unicode
rules (about what constitutes a letter, a punctuation symbol,
etc.), so it may fail to detect word boundaries correctly in
some instances. It is safe on all utf-8 input.
## 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 ](https://github.com/vmg/sundown ) which in turn was based on [libsoldout ](http://fossil.instinctive.eu/libsoldout/home ).
## License
[Simplified BSD License ](LICENSE.txt )
