$unameFirst = "Jon"
$unameLast = "Jones"
$unameMiddle = ""

$unamefirst = $unameFirst.ToLower()
if ($unameMiddle) {
    $unameMiddle = $unameMiddle.ToLower()
    $unameMiddle = $unameMiddle.split(".")[0]
    }
$unameLast = $unameLast.ToLower()

if ($unameMiddle) {
$possibleNames = 1..$unameFirst.Length | ForEach-Object {
    -join ($unameFirst.Substring(0,$_),$unameMiddle,$unameLast)
    }
}
else {
    $possibleNames = 1..$unameFirst.Length | ForEach-Object {
        -join ($unameFirst.Substring(0,$_),$unameLast)
        }
}

#iterate through the possible names until we find one that's not in use
foreach ($possibleName in $possibleNames) {
    Write-Host "Trying username $possibleName"
    try {
    $usernameToCheck = Get-ADUser $possibleName
    }
    catch [Microsoft.ActiveDirectory.Management.ADIdentityNotFoundException] {
    }
        if (!$usernameToCheck) {
            write-host "Found available username: $possibleName"
            break
        }
        else { Write-Host "Username $possibleName taken"}
}