JSFiddle - React, Tailwind, and code Playground

JavaScript

var css = '.myClass{color:red;}' + '\n' +
  '.myClass .myOtherClass{color:green;}' + '\n' +
  '#something .somethingElse{color:blue;}' + '\n' +
  '.something #myIdhere{color:purple;}' + '\n' +
  '#myId {color:black}' + '\n' +
  '.myClass1, .myClass2{colour:green}' + '\n' +
  '.menu_tab span{colour:purple}' + '\n' +
  '.myPseudo:after{}' + '\n' +
  '.-moz-special, .-ms-special{}' + '\n' +
  //'/*.commentedOut{}*/' + '\n' +
  '.4not2work{}' + '\n'

var classNames = css.match(/(\.-?[_a-zA-Z]+[_a-zA-Z0-9-]*)/g)
var lastClass = classNames[classNames.length - 1]

console.log(lastClass)