In reply to https://www.reddit.com/r/PowerShell/comments/4kjhvo/need_some_help_with_restricting_a_prompt_for/

InputValidation.ps1

Add-Type -TypeDefinition @"
    public enum PcType {
        DESKTOP,
        LAPTOP
    }
"@

$PcChoices = [System.Enum]::GetValues([type]"PcType")
# '^(DESKTOP|LAPTOP)\d{6}$' , but this lets us add/modify enum choices later with no code changes
$RegExMatch = "^({0})\d{{6}}$" -f ( ( $PcChoices | ForEach-Object { $_.ToString() } ) -join "|")

do {
    if ( $i ) { Write-Host "`nYou entered invalid data. Please try again.`n" }
        Write-Host "Valid choices:"
    for ($i = 1; $i -le $PcChoices.Count; $i++) { 
        Write-Host ("{0}.`t{1}" -f  $i, $PcChoices[$i-1])
    }
    $Type = Read-Host "Enter PC Type: "

    $Code = Read-Host "Enter 6 digit code"

    $ComputerName = "{0}{1}" -f $PcChoices[$Type - 1], ([int]$Code).ToString("000000")

} until ($ComputerName -cmatch $RegExMatch)

$ComputerName