# accept the username only if
# it contain letters, the username is inferior at 20
# start with a letter, no digit, no space

$username = Read-Host("Please enter your username");
$badChar = $false;

if($username -eq ''){
    Write-Host("The username can't be empty");
    exit;
}

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("Accepted");
}else{
    Write-Host("Invalid");

}