const str = 'qwe//qwe';
const replacement = 'hello, world!!';const newStr = str.replace(/(qwe\/).*?(\/qwe)/, (m, g1, g2) => g1 + replacement + g2);
// или
const newStr = str.replace(/((qwe)\/).*?(\/\2)/, `$1${replacement}$3`);
// или
const newStr = str.replace(/(?<=(qwe)\/).*?(?=\/\1)/, replacement);