modified xregexp.d.ts

// Matchception: Finding matches within matches, while passing forward and
// returning specific backreferences
html = '<a href="http://xregexp.com/api/">XRegExp</a>' +
       '<a href="http://www.google.com/">Google</a>';
XRegExp.matchChain(html, [
  {regex: /<a href="([^"]+)">/i, backref: 1},
  {regex: XRegExp('(?i)^https?://(?<domain>[^/?#]+)'), backref: 'domain'}
]);
// -> ['xregexp.com', 'www.google.com']
http://xregexp.com/api/#matchChain
I think need one more line.
`function matchChain(str: string, chain: { regex: RegExp; backref: number; }[]): string[];`
isn't it?