352 lines
12 KiB
Markdown
352 lines
12 KiB
Markdown
# Markdown Parser and HTML Renderer for Go
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[](https://pkg.go.dev/github.com/gomarkdown/markdown)
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Package `github.com/gomarkdown/markdown` is a Go library for parsing Markdown text and rendering as HTML.
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It's very fast and supports common extensions.
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Try code examples online: https://replit.com/@kjk1?path=folder/gomarkdown
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## API Docs:
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- https://pkg.go.dev/github.com/gomarkdown/markdown : top level package
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- https://pkg.go.dev/github.com/gomarkdown/markdown/ast : defines abstract syntax tree of parsed markdown document
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- https://pkg.go.dev/github.com/gomarkdown/markdown/parser : parser
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- https://pkg.go.dev/github.com/gomarkdown/markdown/html : html renderer
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## Users
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Some tools using this package: https://pkg.go.dev/github.com/gomarkdown/markdown?tab=importedby
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## Usage
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To convert markdown text to HTML using reasonable defaults:
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```go
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md := []byte("## markdown document")
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output := markdown.ToHTML(md, nil, nil)
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```
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Try it online: https://replit.com/@kjk1/gomarkdown-basic
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## Customizing markdown parser
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Markdown format is loosely specified and there are multiple extensions invented after original specification was created.
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The parser supports several [extensions](https://pkg.go.dev/github.com/gomarkdown/markdown/parser#Extensions).
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Default parser uses most common `parser.CommonExtensions` but you can easily use parser with custom extension:
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```go
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import (
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"github.com/gomarkdown/markdown"
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"github.com/gomarkdown/markdown/parser"
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)
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extensions := parser.CommonExtensions | parser.AutoHeadingIDs
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parser := parser.NewWithExtensions(extensions)
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md := []byte("markdown text")
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html := markdown.ToHTML(md, parser, nil)
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```
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Try it online: https://replit.com/@kjk1/gomarkdown-customized-html-renderer
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## Customizing HTML renderer
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Similarly, HTML renderer can be configured with different [options](https://pkg.go.dev/github.com/gomarkdown/markdown/html#RendererOptions)
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Here's how to use a custom renderer:
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```go
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import (
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"github.com/gomarkdown/markdown"
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"github.com/gomarkdown/markdown/html"
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)
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htmlFlags := html.CommonFlags | html.HrefTargetBlank
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opts := html.RendererOptions{Flags: htmlFlags}
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renderer := html.NewRenderer(opts)
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md := []byte("markdown text")
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html := markdown.ToHTML(md, nil, renderer)
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```
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Try it online: https://replit.com/@kjk1/gomarkdown-customized-html-renderer
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HTML renderer also supports reusing most of the logic and overriding rendering of only specific nodes.
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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).
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The function is called for each node in AST, you can implement custom rendering logic and tell HTML renderer to skip rendering this node.
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Here's the simplest example that drops all code blocks from the output:
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````go
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import (
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"github.com/gomarkdown/markdown"
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"github.com/gomarkdown/markdown/ast"
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"github.com/gomarkdown/markdown/html"
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)
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// return (ast.GoToNext, true) to tell html renderer to skip rendering this node
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// (because you've rendered it)
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func renderHookDropCodeBlock(w io.Writer, node ast.Node, entering bool) (ast.WalkStatus, bool) {
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// skip all nodes that are not CodeBlock nodes
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if _, ok := node.(*ast.CodeBlock); !ok {
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return ast.GoToNext, false
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}
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// custom rendering logic for ast.CodeBlock. By doing nothing it won't be
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// present in the output
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return ast.GoToNext, true
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}
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opts := html.RendererOptions{
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Flags: html.CommonFlags,
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RenderNodeHook: renderHookDropCodeBlock,
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}
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renderer := html.NewRenderer(opts)
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md := "test\n```\nthis code block will be dropped from output\n```\ntext"
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html := markdown.ToHTML([]byte(md), nil, renderer)
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````
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## Sanitize untrusted content
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We don't protect against malicious content. When dealing with user-provided
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markdown, run renderer HTML through HTML sanitizer such as [Bluemonday](https://github.com/microcosm-cc/bluemonday).
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Here's an example of simple usage with Bluemonday:
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```go
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import (
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"github.com/microcosm-cc/bluemonday"
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"github.com/gomarkdown/markdown"
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)
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// ...
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maybeUnsafeHTML := markdown.ToHTML(md, nil, nil)
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html := bluemonday.UGCPolicy().SanitizeBytes(maybeUnsafeHTML)
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```
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## Windows / Mac newlines
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The library only supports Unix newlines. If you have markdown text with possibly
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Windows / Mac newlines, normalize newlines before calling this library using
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`d = markdown.NormalizeNewlines(d)`
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## mdtohtml command-line tool
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https://github.com/gomarkdown/mdtohtml is a command-line markdown to html
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converter built using this library.
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You can also use it as an example of how to use the library.
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You can install it with:
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go get -u github.com/gomarkdown/mdtohtml
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To run: `mdtohtml input-file [output-file]`
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## Features
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- **Compatibility**. The Markdown v1.0.3 test suite passes with
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the `--tidy` option. Without `--tidy`, the differences are
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mostly in whitespace and entity escaping, where this package is
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more consistent and cleaner.
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- **Common extensions**, including table support, fenced code
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blocks, autolinks, strikethroughs, non-strict emphasis, etc.
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- **Safety**. Markdown is paranoid when parsing, making it safe
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to feed untrusted user input without fear of bad things
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happening. The test suite stress tests this and there are no
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known inputs that make it crash. If you find one, please let me
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know and send me the input that does it.
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NOTE: "safety" in this context means _runtime safety only_. In order to
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protect yourself against JavaScript injection in untrusted content, see
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[this example](https://github.com/gomarkdown/markdown#sanitize-untrusted-content).
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- **Fast**. It is fast enough to render on-demand in
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most web applications without having to cache the output.
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- **Thread safety**. You can run multiple parsers in different
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goroutines without ill effect. There is no dependence on global
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shared state.
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- **Minimal dependencies**. Only depends on standard library packages in Go.
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- **Standards compliant**. Output successfully validates using the
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W3C validation tool for HTML 4.01 and XHTML 1.0 Transitional.
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## Extensions
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In addition to the standard markdown syntax, this package
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implements the following extensions:
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- **Intra-word emphasis supression**. The `_` character is
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commonly used inside words when discussing code, so having
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markdown interpret it as an emphasis command is usually the
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wrong thing. We let you treat all emphasis markers as
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normal characters when they occur inside a word.
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- **Tables**. Tables can be created by drawing them in the input
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using a simple syntax:
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```
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Name | Age
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--------|------
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Bob | 27
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Alice | 23
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```
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Table footers are supported as well and can be added with equal signs (`=`):
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```
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Name | Age
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--------|------
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Bob | 27
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Alice | 23
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========|======
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Total | 50
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```
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A cell spanning multiple columns (colspan) is supported, just repeat the pipe symbol:
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```
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Name | Age
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--------|------
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Bob ||
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Alice | 23
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========|======
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Total | 23
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```
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- **Fenced code blocks**. In addition to the normal 4-space
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indentation to mark code blocks, you can explicitly mark them
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and supply a language (to make syntax highlighting simple). Just
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mark it like this:
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```go
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func getTrue() bool {
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return true
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}
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```
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You can use 3 or more backticks to mark the beginning of the
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block, and the same number to mark the end of the block.
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- **Definition lists**. A simple definition list is made of a single-line
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term followed by a colon and the definition for that term.
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Cat
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: Fluffy animal everyone likes
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Internet
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: Vector of transmission for pictures of cats
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Terms must be separated from the previous definition by a blank line.
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- **Footnotes**. A marker in the text that will become a superscript number;
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a footnote definition that will be placed in a list of footnotes at the
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end of the document. A footnote looks like this:
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This is a footnote.[^1]
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[^1]: the footnote text.
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- **Autolinking**. We can find URLs that have not been
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explicitly marked as links and turn them into links.
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- **Strikethrough**. Use two tildes (`~~`) to mark text that
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should be crossed out.
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- **Hard line breaks**. With this extension enabled newlines in the input
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translates into line breaks in the output. This extension is off by default.
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- **Non blocking space**. With this extension enabled spaces preceeded by a backslash
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in the input translates non-blocking spaces in the output. This extension is off by default.
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- **Smart quotes**. Smartypants-style punctuation substitution is
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supported, turning normal double- and single-quote marks into
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curly quotes, etc.
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- **LaTeX-style dash parsing** is an additional option, where `--`
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is translated into `–`, and `---` is translated into
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`—`. This differs from most smartypants processors, which
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turn a single hyphen into an ndash and a double hyphen into an
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mdash.
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- **Smart fractions**, where anything that looks like a fraction
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is translated into suitable HTML (instead of just a few special
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cases like most smartypant processors). For example, `4/5`
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becomes `<sup>4</sup>⁄<sub>5</sub>`, which renders as
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<sup>4</sup>⁄<sub>5</sub>.
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- **MathJaX Support** is an additional feature which is supported by
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many markdown editor. It translates inline math equations quoted by `$`
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and displays math blocks quoted by `$$` into MathJax compatible format.
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Hyphens (`_`) won't break LaTeX render within a math element any more.
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```
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$$
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\left[ \begin{array}{a} a^l_1 \\ ⋮ \\ a^l_{d_l} \end{array}\right]
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= \sigma(
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\left[ \begin{matrix}
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w^l_{1,1} & ⋯ & w^l_{1,d_{l-1}} \\
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⋮ & ⋱ & ⋮ \\
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w^l_{d_l,1} & ⋯ & w^l_{d_l,d_{l-1}} \\
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\end{matrix}\right] ·
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\left[ \begin{array}{x} a^{l-1}_1 \\ ⋮ \\ ⋮ \\ a^{l-1}_{d_{l-1}} \end{array}\right] +
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\left[ \begin{array}{b} b^l_1 \\ ⋮ \\ b^l_{d_l} \end{array}\right])
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$$
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```
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- **Ordered list start number**. With this extension enabled an ordered list will start with
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the number that was used to start it.
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- **Super and subscript**. With this extension enabled sequences between ^ will indicate
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superscript and ~ will become a subscript. For example: H~2~O is a liquid, 2^10^ is 1024.
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- **Block level attributes** allow setting attributes (ID, classes and key/value pairs) on block
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level elements. The attribute must be enclosed with braces and be put on a line before the
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element.
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```
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{#id3 .myclass fontsize="tiny"}
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# Header 1
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```
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Will convert into `<h1 id="id3" class="myclass" fontsize="tiny">Header 1</h1>`.
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- **Mmark support**, see <https://mmark.miek.nl/post/syntax/> for all new syntax elements this adds.
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## Todo
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- port https://github.com/russross/blackfriday/issues/348
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- port [LaTeX output](https://github.com/Ambrevar/Blackfriday-LaTeX):
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renders output as LaTeX.
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- port https://github.com/shurcooL/github_flavored_markdown to markdown
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- port [markdownfmt](https://github.com/shurcooL/markdownfmt): like gofmt,
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but for markdown.
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- More unit testing
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- Improve unicode support. It does not understand all unicode
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rules (about what constitutes a letter, a punctuation symbol,
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etc.), so it may fail to detect word boundaries correctly in
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some instances. It is safe on all utf-8 input.
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## History
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markdown is a fork of v2 of https://github.com/russross/blackfriday that is:
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- actively maintained (sadly in Feb 2018 blackfriday was inactive for 5 months with many bugs and pull requests accumulated)
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- refactored API (split into ast/parser/html sub-packages)
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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).
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## License
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[Simplified BSD License](LICENSE.txt)
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