Special Formatting

In addition to regular text, HedgeDoc lets you format source code, emojis, and images using Markdown syntax. To format software code, you use three backticks (```) at the beginning and end of a code block. The code will be highlighted according to the language used. For emojis, you type : at the beginning of the line. An ! at the beginning of a line tells HedgeDoc to insert the code to embed an image in that position.

When embedding an image, its source location affects how HedgeDoc embeds the image. If the image is already online (on the same or another server), you just insert the image's URL in the Markdown code that HedgeDoc auto-inserts after the exclamation mark. If the image is on your computer, click on the Upload Image button in the toolbar and upload the image to the HedgeDoc server (or to an alternative location as described in the "Self-Hosting" section). Alternatively, you may drag and drop an image from your file manager directly to where you want it to appear in the note.

At time of writing, HedgeDoc 1.8.2 still lets you embed YouTube videos, as follows:

{%youtube ID_NUMBER_OF_YOUTUBE_VIDEO %}

However, this specific markup is officially deprecated. It should soon be replaced by embedding "plain links" (the meaning of which is currently unclear).

Metadata

Similar to most Markdown-capable applications, HedgeDoc supports adding metadata to files to set browser behavior. For the metadata to be recognized, it must be placed in a special section, called "frontmatter," at the beginning of each file.

You use YAML to format the frontmatter. The syntax to mark the start of the frontmatter is three dashes, and three dashes mark the end of the frontmatter. The YAML metadata goes inside, with one key/value pair on each line, separated by a colon, as follows:

---
title: real title
tags: tutorial, open source, writing
description: Another useful tutorial
robots: nofollow
---

HedgeDoc will use the title value as the HTML note's title, overriding the first level 1 heading (which would be used if there were no metadata). These tags can be used by any system (including search engines) that catalogs the notes by topic. The values of the description and robots keys will be added in the HTML note's header, to make search engines properly index the note and the links it contains. However, you can set robots to noindex if you don't want the page indexed. Set robots to nofollow if you do not want the pages linked from the current page to be indexed. In addition to these metadata tags, HedgeDoc recognizes many other tags. For more information, consult the documentation on HedgeDoc's website [2].

Diagrams and Mathematical Expressions

In addition to text, source code, and images, HedgeDoc can also format diagrams and charts, as well as complicated mathematical expressions. In my PlantUML tutorial [4], for example, I showed how to create a sequence diagram out of a text file as shown in Listing 1.

01 @startuml
02 Alice -> Bob  : Hello, is the store open?
03 Bob   -> Alice: Yes, it is. What do you want to buy?
04 Alice -> Bob  : I want to buy some jeans.
05 Alice <- Bob  : Please tell me the color.
06 Alice -> Bob  : Black
07 Alice <- Bob  : Now please tell me the size.
08 Alice -> Bob  : 42
09 @enduml

With HedgeDoc, you can obtain the same results by inserting sequence diagram text, but you use ```sequence at the beginning instead of @startuml and ``` at the end instead of @enduml, as shown in Listing 2.

```sequence
Alice->Bob: Hello, is the store open?
Bob->Alice: Yes, it is. What do you want to buy?
Alice->Bob: I want to buy some jeans.
Bob->Alice: Please tell me the color.
Alice->Bob: Black
Bob->Alice: Now please tell me the size.
Alice->Bob: 42
```

The sequence keyword following the backticks at the beginning of Listing 2 is what actually creates the diagram shown in Figure 7. If you left sequence out, HedgeDoc would treat the diagram text as source code. To learn about all the diagram types that HedgeDoc supports, see the many examples available online [5].

Figure 7: The Markdown syntax in the left pane generates the sequence diagram on the right.

HedgeDoc also supports mathematical expressions by using the MathJax library [6]. MathJax reads plain text descriptions in MathML, LaTeX, or AsciiMath formats and renders them as complex mathematical expressions. Depending on the situation, the expression is rendered using a combination of HTML code with special CSS stylesheets, web fonts, MathML code, or scalable vector graphics (SVG).

Because MathJax support is built into HedgeDoc, you can easily enter a mathematical expression in any of the source formats and immediately see how it looks. Figure 8 demonstrates the rendered expression from the LaTeX expressions shown in Listing 3.

$$\sum_{i=1}^n i^2 = \frac{n(n+1)(2n+1)}{6}$$
$$\int_{a}^b\int_{c}^d f(x,y)dxdy$$

Figure 8: The Markdown syntax (left) results in a beautiful formula (right).

The value HedgeDoc offers in rendering mathematical expressions can't be overestimated. Math students and teachers worldwide have been using LaTeX formulas for years, with thousands of expressions scattered inside countless LaTeX files. HedgeDoc offers an easy way to write and publish any paper based on these formulas.