How to extract data from the post?

What is the easier way to get key data from the post?

Or maybe it’s easier to get this information out of a tagged post when you’re creating a topic?

I mean, something like that.

New Topic: user on the forum creates a new topic and marks it with a t**ag that is key** e.g.: **classical music**
Then He create a post : 
About : [my keyword ] **classical music**:

> lassical music is art music produced or rooted in the traditions of Western culture, including both liturgical (religious) and secular music. While a more precise term is also used to refer to the period from 1750 to 1820 (the Classical period), this article is about the broad span of time from before the 6th century AD to the present day, which includes the Classical period and various other periods.[1] The central norms of this tradition became codified between 1550 and 1900, which is known as the common-practice period.

Now i want to extract this keyword from this topic and/or from the selected TAG in order to be able to something about it further . For example i wants to get interesting keywords from every subject in the post or as a tag about music with Words: classical music, , pop , hip hop or jazz

You might have a look at How to reverse engineer the Discourse API

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