Positive Lookahead
HTML
<script src="https://rawgit.com/eu81273/jsfiddle-console/master/console.js"></script>JavaScript
const testString = "MangoJuice, VanillaShake, GrapeJuice";
const testRegExp = /[a-zA-Z]+(?=Juice)/g;
const matches = testString.match( testRegExp );
console.log( matches );