snippet.ps1

function ValidateUsername {
    param(
        [Parameter(Position = 0)]
        [string]$Username
    )

    $onlyLetters = "[a-zA-Z]"
    $blacklist = @("admin", "administrator")

    if($Username -notmatch $onlyLetters){
        return $false
    }

    if($blacklist -contains $Username){
        return $false
    }

    return $true
}
ValidateUsername "admin"