This topic has been superseded by Developing Discourse Plugins - Part 1 - Create a basic plugin
So, you want to manipulate the text in the composer but have no idea how? Let met guide you through the creation of a basic plugin that will turn the text into Pirate Speak.
Update August 2018: you can create a complete discourse plugin skeleton in your plugins directory with Rails plugin generator
First of all, you need to create a plugin.rb file that will be your entry point.
# name: pirate-speak
# about: plugin that transform the text into pirate speak
# version: 0.1
# authors: Régis Hanol
register_asset "javascripts/pirate_speak.js", :server_side
I highly recommend that you add at least the following pieces of meta data:
- name: the name of your plugin
- about: a one line description of your plugin
- version: the version of your plugin
Then you can add the code of your plugin. Here, we’re only registering a javascript asset that will also be used on the server side. Why? Because we use the same javascript code for baking and cooking (ie. post processing) the posts on the client-side and on the server-side. If you want your text-manipulation plugin to work properly, you have to indicate that this file must be used when post-processing the posts on the server, hence the :server_side argument.
Let’s have a look at the file we’ve just registered:
function piratize (text) {
return text.replace(/\b(am|are|is)\b/ig, "be")
.replace(/ing\b/ig, "in'")
.replace(/v/ig, "'");
}
Discourse.Dialect.postProcessText(function (text) {
text = [].concat(text);
for (var i = 0; i < text.length; i++) {
if (text[i].length > 0 && text[i][0] !== "<") {
text[i] = piratize(text[i]);
}
}
return text;
});
First, there’s our core function: piratize. It takes a string as argument and uses regular expressions to transform the text into a basic pirate speak. You are more than welcome to improve that function and submit pull requests 
But the most important part is the call to the Discourse.Dialect.postProcessText function. This tells Discourse to call our anonymous function whenever the text in the composer/post needs to be post-processed. The text argument might be a string or an array of string. That’s why we use text = [].concat(text) at the beginning to convert that string into an array if needs be. Then we go through the array and call our piratize function for each element in the array except when the element is an html tag (ie. it starts with a <). Finally, we return the text array so that it can be used in the rest of the post-processing pipeline.
The finished Pirate Speak plugin source code is available on GitHub if you want to have a look.
Another great example is @sam’s ALL CAPS plugin which converts the text to UPPERCASE.
Also, if you want advanced text manipulation plugins, you might want to check out
- MathJax plugin which adds support for math notations using MathJax library.
- Spoiler Alert plugin which allows anyone to prevent spoilers.
