Check domain of the given URL

chkDomain.js

// https://www.npmjs.com/package/tr46
const tr46 = require("tr46");

// The function returns NULL if the given URL doesn't have example.jp domain, otherwise it returns URL converted to ASCII.
// No warranty, no license claim.
function chkDomain(url) {
  var regex = new RegExp("^(https://)([a-zA-Z0-9\\u0080-\\uDBFF\\uDFFF.-]+)\\.example\\.jp(:\\d{1,5})?(/|$)", "u");
  var mt = url.match(regex);

  if (!mt) return null;

  var prefix = mt[1].toLowerCase();
  var subDomain = mt[2];
  var port = mt[3] ?? '';
  var remaining = url.substring((prefix + subDomain).length);

  if (port.length > 0) {
    var portNum = Number(port);
    if (portNum > 65535) return null;
  }

  var opts = {checkBidi: true, checkHyphens: true, checkJoiners: true, useSTD3ASCIIRules: true, verifyDNSLength: true};
  var ascii = tr46.toASCII(subDomain, opts) ?? '';

  if (ascii.length === 0
      || subDomain.split('.').length !== ascii.split('.').length
      || !/^(?!\.)[a-z0-9.-]+(?<!\.)$/.test(ascii)) {
    return null;
  }
  return prefix + ascii + remaining;
}