JSFiddle - React, Tailwind, and code Playground

HTML

<textarea name="" id="a" cols="30" rows="10">div
  div
    Текст
  div
div</textarea>
<textarea name="" id="b" cols="30" rows="10"></textarea>

JavaScript

var input = a.value

var isOdd = false
b.innerHTML = input.replace(/div/g, (str, index)=>{
  isOdd = !isOdd
  if (isOdd) {
    return `<${str}>`
  } else {
    return `</${str}>`
  }
})