<div id="block">
i am NOT HEADING h1 <h1>I AM HEADING h1</h1>
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s sadf sadf sd fsadf asdf
sdf sdaf sdf sdaf
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sdaf
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sdf s
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<button>transform now</button>
</div>document.querySelector('button')
.addEventListener('click', () => {
block = document.querySelector('#block')
html = block.innerHTML
html = html.replace(/(.*?)\<h1>/, (_, $1) => '<h1>' + $1 + '</h1><h1>')
block.innerHTML = html
})document.querySelector('button').addEventListener('click', () => {
const block = document.querySelector('#block');
block.innerHTML = block.innerHTML.replace(/(.*?)<h1>/, (_, $1) => {
const words = $1.trim().split(' ');
return `<h1>${words.slice(0, -2).join(' ')}</h1> ${words.slice(-2).join(' ')} <h1>`;
});
});