const regex = new RegExp('%(\\w)', 'g')
const str = `%252525253E%252553Csvg%25252525252525252Fonload%252525252525252525252525252525252525252525252525252525253Dalert(document.domain)%252525252`;
const subst = `\1`;
// 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