Get the root domain from a given subdomain; i.e. the first level under a public suffix / "top level domain" (TLD). So given the string "test.example.co.uk" this would return "example.co.uk", whilst "example.com" would return itself unchanged..

ConvertTo-RootDomain.ps1

function ConvertTo-RootDomain {
  [CmdletBinding()]
  Param (
    [Parameter(Mandatory)]
    [string]$Domain
  )
  if ($null -eq $Global:CachedPublicDomainSuffixList) {
    $publicDomainSuffixList = Invoke-WebRequest -Method GET -Uri 'https://publicsuffix.org/list/public_suffix_list.dat' -ContentType 'text/plain' -ErrorAction Stop
    $tempSet = [System.Collections.Generic.HashSet[string]]::new()
    foreach ($suffix in @($publicDomainSuffixList.Content -split '\s*[\r\n]+')) {
      if ($suffix -notlike '//*') {
        $tempSet.Add($suffix) | Out-Null
      }
    }
    $Global:CachedPublicDomainSuffixList = $tempSet
  }
  $domainPieces = $Domain -split '\.'
  for ($i=1; $i -lt $domainPieces.Count; $i++) {
    if ($Global:CachedPublicDomainSuffixList.Contains(($domainPieces[$i..$domainPieces.Count] -join '.'))) {
      return ($domainPieces[($i-1)..$domainPieces.Count] -join '.')
    }
  }
  return $Domain
}