$users = Get-LocalUser

foreach($username in -split $users){

    $badChar = $false
    
    foreach ($char in $username.ToCharArray()) {
        if($char -notmatch "[A-Za-z]"){
            $badChar = $true
            break
        }
    }

    $fistChar = $username[0]
    $validFirstLetter = $fistChar -match "[A-Za-z]"

    if ($badChar -or $username.Length -ge 20 -or -not $validFirstLetter ){
        Write-Host "the userName " + $username + " is invalid"
    }

}