JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

JavaScript

var str = `
// THIS IS START
..
// THIS IS END`;
console.log(str.replace(/\/\/ THIS IS START\n(.+)\/\/ THIS IS END/gs, "$1"));