I want to make a parsing extension for Discourse which would change text like
say [Hello World!] to and many other images of this type. Do I need to make any changes to the existing Discourse files or can I just start writing code without making any changes to the existing files.
Hey and welcome to Meta @R4356th
No, making changes to Discourse files on your server (in this context) means that you’ll either be unable to update your site or that your changes will be wiped with the next update.
Those are the methods your can use to extend Discourse to match your needs.
For your particular feature, you’ll need a plugin. I recommend taking a look at the source code of the bbcode plugin for inspiration.
Thank you very much this should help.
Is it to write the parser in PHP?
You might want to look at Developer's guide to Markdown extensions