<# Get-ChildItem C:\test results:

file3.txt  <--- This is a directory
file1 - Copy.txt
file1.txt
file2 - Copy.txt
file2.txt
file4 - Copy.txt
file4.txt
[file].txt
#>

$gciParams = @{
    Path = Join-Path -Path 'C:\Test' -ChildPath '*'
    Filter = 'file*.*'
    File = $true
    Force = $true
}
<# (gci @gciParams).Name results:

file1 - Copy.txt
file1.txt
file2 - Copy.txt
file2.txt
file4 - Copy.txt
file4.txt
#>

$pattern = '.*[2-9].*'
$gciCommand = {Get-ChildItem @gciParams | Where-Object {$_.Name -match $pattern}}
<# (Invoke-Command $gciCommand).Name results:

file2 - Copy.txt
file2.txt
file4 - Copy.txt
file4.txt
#>

function Test-CompletionResult
{
    [CmdletBinding()]
    param (
        [Parameter(Mandatory, Position = 0, ParameterSetName = 'Num', ValueFromPipeline, ValueFromPipelineByPropertyName)]
        [ArgumentCompleter({
            param ($Command, $Parameter, $WordToComplete, $CommandAst, $FakeBoundParams)
            $objList = @()
            (Invoke-Command -ScriptBlock $gciCommand).foreach({
                $filePath = $_.FullName
                $fileNum  = ($_.Name | Select-String -Pattern '\d').Matches.Value
                $objList += [pscustomobject] @{
                    Path = $filePath
                    Num  = $fileNum
                }
            })
            ($objList | Where-Object {$_.Num -like "*$WordToComplete*"}).foreach({
                [System.Management.Automation.CompletionResult]::new($_.Num, $_.Num, 'ProviderItem', $_.Path)
            })
        })]        
        [int]$FileNum
    )
    process {Write-Output $FileNum}
}