JSFiddle - React, Tailwind, and code Playground

JavaScript

var test_string = '<body><h1>Testing</h1><span class="price" id="product-price-831"> €*399,00 </span>';

var pattern = /\<span(.+)id="product-price-831"\>(.+)\<\/span\>/;

document.write(pattern.exec(test_string)[2]);