JSFiddle - React, Tailwind, and code Playground

JavaScript

var x = "foo foo".replace(/^[a-z\s]*$/, "XXX");
console.log(x);

var x = "foo|foo".replace(/^[a-z\s]*$/, "XXX");
console.log(x);

var x = "foo   foo".replace(/^[a-z\s]*$/, "XXX");
console.log(x);

var x = "foo 543foo".replace(/^[a-z\s]*$/, "XXX");
console.log(x);