const regex = new RegExp('^((?P<scheme>[^:/?#]+):)?(//(?P<authority>[^/?#]*))?(?P<path>[^?#]*)(\\?(?P<query_string>[^#]*))?(#(?P<fragment>.*))?', '')
const str = ``;
const subst = `\g<scheme>`;
// 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