JSFiddle - React, Tailwind, and code Playground

JavaScript

// Function to escape regular expressions special characters
// From: http://stackoverflow.com/questions/3115150/how-to-escape-regular-expression-special-characters-using-javascript
RegExp.escape = function(text) {
  return text.replace(/[-[\]{}()*+?.,\\^$|#\s]/g, "\\$&");
};

var text1 = 'ThE foX iS brown';
var text2 = 'the fox is brown the fox is brown THE FOX IS BROWN';

// Create the regular expressions with "i"gnore-case flag
var re = new RegExp(RegExp.escape(text1), "i");

var output = text2.replace(re, text1);

alert(output);