A complete example of a custom Blackfriday v2 renderer with pygmentizing CodeBlock handler

main.go

package main

import (
	"fmt"
	"io"
	"os/exec"

	bf "gopkg.in/russross/blackfriday.v2"
)

var md = "This is some sample code.\n\n```\n" +
	`func main() {
	fmt.Println("Hi")
}
` + "```"

func renderWithPygments(w io.Writer, text []byte) error {
	cmd := exec.Command("pygmentize", "-l", "go", "-f", "html")
	stdin, err := cmd.StdinPipe()
	if err != nil {
		return err
	}
	stdout, err := cmd.StdoutPipe()
	if err != nil {
		return err
	}
	defer stdout.Close()
	go func() {
		defer stdin.Close()
		stdin.Write(text)
	}()
	if err := cmd.Start(); err != nil {
		return err
	}
	io.Copy(w, stdout)
	return cmd.Wait()
}

type pygmentedRenderer struct {
	defR *bf.HTMLRenderer
}

func (r *pygmentedRenderer) RenderNode(w io.Writer, node *bf.Node, entering bool) bf.WalkStatus {
	switch node.Type {
	case bf.CodeBlock:
		if err := renderWithPygments(w, node.Literal); err != nil {
			return r.defR.RenderNode(w, node, entering)
		}
		return bf.SkipChildren
	default:
		return r.defR.RenderNode(w, node, entering)
	}
}

func (r *pygmentedRenderer) RenderHeader(w io.Writer, ast *bf.Node) {
	r.defR.RenderHeader(w, ast)
}

func (r *pygmentedRenderer) RenderFooter(w io.Writer, ast *bf.Node) {
	r.defR.RenderFooter(w, ast)
}

func main() {
	r := pygmentedRenderer{
		defR: bf.NewHTMLRenderer(bf.HTMLRendererParameters{
			Flags: bf.CommonHTMLFlags,
		}),
	}
	html := bf.Run([]byte(md), bf.WithRenderer(&r))
	fmt.Println(string(html))
}