const regex = new RegExp('/upload/[^/]+/', 'gm')
const str = `<div class="slide"><a class="slide__card fancybox" rel="gallery1" href="/upload/f2d/f2d520b5d31920e1227c4414cd4f214a.jpg"><img src="/upload/f2d/700_525_1/f2d520b5d31920e1227c4414cd4f214a.jpg" /><span class="zoom-cell"></span></a></div><div class="slide"><a class="slide__card fancybox" rel="gallery1" href="/upload/3ff/3ff30ba3ec6e5d5b1b753b3749ff601c.jpg"><img src="/upload/3ff/700_525_1/3ff30ba3ec6e5d5b1b753b3749ff601c.jpg" /><span class="zoom-cell"></span></a></div><div class="slide"><a class="slide__card fancybox" rel="gallery1" href="/upload/0aa/0aa859109670c4aac06eaee7eec33d6f.jpg"><img src="/upload/0aa/700_525_1/0aa859109670c4aac06eaee7eec33d6f.jpg" /><span class="zoom-cell"></span></a></div><div class="slide"><a class="slide__card fancybox" rel="gallery1" href="/upload/b56/b56537718cabbaa75e0b56056ae8a1db.jpg"><img src="/upload/b56/700_525_1/b56537718cabbaa75e0b56056ae8a1db.jpg" /><span class="zoom-cell"></span></a></div><div class="slide"><a class="slide__card fancybox" rel="gallery1" href="/upload/e99/e99800f9ece27c61fb0c395aed4423d1.jpg"><img src="/upload/e99/700_525_1/e99800f9ece27c61fb0c395aed4423d1.jpg" /><span class="zoom-cell"></span></a></div>`;
const subst = `/upload/`;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions