website/pkg/markdown/markdown.go

// Package markdown provides markdown render functions.
package markdown

import (
	"regexp"
	"strings"

	"github.com/microcosm-cc/bluemonday"
	"github.com/shurcooL/github_flavored_markdown"
)

var (
	RegexpHTMLTag      = regexp.MustCompile(`<(.|\n)+?>`)
	RegexpMarkdownLink = regexp.MustCompile(`\[([^\[\]]*)\]\((.*?)\)`)
)

// Render markdown from untrusted sources.
func Render(input string) string {
	// Render Markdown to HTML.
	html := github_flavored_markdown.Markdown([]byte(input))

	// Sanitize the HTML from any nasties.
	p := bluemonday.UGCPolicy()
	safened := p.SanitizeBytes(html)
	return string(safened)
}

// Quotify a message putting it into a Markdown "> quotes" block.
func Quotify(input string) string {
	var lines = []string{}
	for _, line := range strings.Split(input, "\n") {
		lines = append(lines, "> "+line)
	}
	return strings.Join(lines, "\n")
}

/*
DeMarkify strips some Markdown syntax from plain text snippets.

It is especially useful on Forum views that show plain text contents of posts, and especially
for linked at-mentions of the form `[@username](/go/comment?id=1234)` so that those can be
simplified down to just the `@username` text contents.

This function will strip and simplify:

- Markdown hyperlinks as spelled out above
- Literal HTML tags such as <a href="">
*/
func DeMarkify(input string) string {
	// Strip all standard HTML tags.
	input = RegexpHTMLTag.ReplaceAllString(input, "")

	// Replace Markdown hyperlinks.
	matches := RegexpMarkdownLink.FindAllStringSubmatch(input, -1)
	for _, m := range matches {
		input = strings.ReplaceAll(input, m[0], m[1])
	}

	return input
}