Format-MACAddress.ps1

<#
.SYNOPSIS
    Reformats a MAC Address depending on the use case.
.DESCRIPTION
    Takes a string or strings that should be reformated as a MAC address.
.PARAMETER Address
    The MAC Address to reformat as a string
.PARAMETER Separator
    What separator should be inserted, defaults to ':'
.EXAMPLE
    Format-MACAddress '01-23-45-67-89-AF
.EXAMPLE
    Add-CredentialToCache -Username user@domain.local
#>

[CmdletBinding(DefaultParameterSetName='Uppercase')]

param(
    [Parameter(Position=0, Mandatory, ValueFromPipeline)]
    [string[]]
    $Address,

    [Parameter(Position=1)]
    [string]
    $Separator = ':',

    [Parameter(ParameterSetName='Uppercase')]
    [switch]
    $Uppercase,

    [Parameter(ParameterSetName='Lowercase')]
    [switch]
    $Lowercase
)

process {

    $Address | ForEach-Object {

        $mac = $_ -replace '[^a-f0-9]', ''

        if ( ( $_ -match '[^a-f0-9\s:-]' ) -or ( ( $mac.Length -ne 12 ) -and ( $mac.Length -ne 28 ) ) ) {

            Write-Warning "May be an invalid MAC Address: $_"

        }

        $mac = $mac -replace '[^a-f0-9]', ''

        $mac = $mac -replace '(..(?!$))',"`$1$Separator"

        if ( $psCmdlet.ParameterSetName -eq 'Uppercase' ) { $mac.ToUpper() }
        else { $mac.ToLower() }

    }

}