$users = @("admin", "root1", "John_Doe", "Alice", "Bob42")

foreach($username in $users){

    $badChar = $false

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

    }

    $firstLetter = $username[0]
    $validFirstLetter = $firstLetter -match '[A-Za-z]'

    if(-not $badChar  -and $username.Length -le 20  -and $validFirstLetter ){
        Write-Host("$username is valid");
    }else{
        Write-Host("$username is Invalid");

    }
}