<#
.SYNTAX       check-ipv6-address.ps1 [<address>]
.DESCRIPTION  checks the given IPv6 address for validity
.LINK         https://github.com/fleschutz/PowerShell
.NOTES        Author: Markus Fleschutz / License: CC0
#>

param($Address = "")

function IsIPv6AddressValid { param([string]$IP)
    $IPv4Regex = '(((25[0-5]|2[0-4][0-9]|[0-1]?[0-9]{1,2})\.){3}(25[0-5]|2[0-4][0-9]|[0-1]?[0-9]{1,2}))'
    $G = '[a-f\d]{1,4}'
    $Tail = @(":",
    "(:($G)?|$IPv4Regex)",
    ":($IPv4Regex|$G(:$G)?|)",
    "(:$IPv4Regex|:$G(:$IPv4Regex|(:$G){0,2})|:)",
    "((:$G){0,2}(:$IPv4Regex|(:$G){1,2})|:)",
    "((:$G){0,3}(:$IPv4Regex|(:$G){1,2})|:)",
    "((:$G){0,4}(:$IPv4Regex|(:$G){1,2})|:)")
    [string] $IPv6RegexString = $G
    $Tail | foreach { $IPv6RegexString = "${G}:($IPv6RegexString|$_)" }
    $IPv6RegexString = ":(:$G){0,5}((:$G){1,2}|:$IPv4Regex)|$IPv6RegexString"
    $IPv6RegexString = $IPv6RegexString -replace '\(' , '(?:' # make all groups non-capturing
    [regex] $IPv6Regex = $IPv6RegexString
    if ($IP -imatch "^$IPv6Regex$") {
    	return $true
    } else {
    	return $false
    }
}

try {
	if ($Address -eq "" ) {
		$Address = read-host "Enter IPv6 address to validate"
	}
	if (IsIPv6AddressValid $Address) {
		write-host -foregroundColor green "OK - IPv6 address $Address is valid"
		exit 0
	} else {
		write-warning "Invalid IPv6 address: $Address"
		exit 1
	}
} catch {
	write-error "⚠️ Error in line $($_.InvocationInfo.ScriptLineNumber): $($Error[0])"
	exit 1
}