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HTML

<pre id="lyricsText">
__Verse 1
At the starting of the week
At summit talks, you'll hear them speak
It's only Monday

Negotiations breaking down
See those leaders start to frown
It's sword and gun day

__Chorus 1
Tomorrow never comes until it's too late

__Verse 2
You could be sitting taking lunch
The news will hit you like a punch
It's only Tuesday

You never thought we'd go to war
After all the things we saw
It's April Fools' day

__Chorus 1 (R:2)
Tomorrow never comes until it's too late

__Verse 3
You hear a whistling overhead
Are you alive or are you dead?
It's only Thursday

You feel a shaking on the ground
A billion candles burn around
Is it your birthday?

__Chorus 1 (R:2)
Tomorrow never comes until it's too late

__Outro
Make tomorrow come, I think it's too late
</pre>

JavaScript

var lyrics = document.getElementById("lyricsText").textContent;
lyrics = (lyrics) ? lyrics:document.getElementById("lyricsText").innerText;
var headers = /__[A-Za-z0-9 ]*(?=\(R:\d\))?/g;
//headers = /__(.(?!\(R.{0,2}\)))*/g;

console.log(lyrics.match(headers));