code: function dnToDomain (dn) {
    if (!dn || typeof dn !== 'string') { return null }
    var matches = domainDnRegex.exec(dn)
    if (matches) {
      return matches[1].replace(dnPartRegex, '$1.').toLowerCase()
    } else {
      return null
    }
  }
docs: Extracts a lowercased domain from a distinguished name for comparison
  purposes.
  
  e.g. "CN=User,OU=MyBusiness,DC=example,DC=com" will return "example.com."