Rename numbered files to remove gaps

$ToNatural = { [regex]::Replace($_, '\d+', { $args[0].Value.PadLeft(20) }) }
Get-ChildItem ./ | Sort-Object $ToNatural |
Foreach-Object { $lastFileName = '0' } {
        $fileName = [System.IO.Path]::GetFileNameWithoutExtension($_)
        $expectedName = [string]([int]$lastFileName + 1)
        if ($expectedName -ne $fileName) {
                Rename-Item -Path $_ -NewName ($expectedName + [System.IO.Path]::GetExtension($_))
        }
        $lastFileName = $expectedName
}