PowerShell cmdlet parameter value tab completion

Question

How do you implement the parameter tab completion for PowerShell functions or cmdlets like Get-Service and Get-Process in PowerShell 3.0?

I realise ValidateSet works

Answer 1

Classically, I used regex.

for example,

function TabExpansion {

    param($line, $lastWord) 

    if ( $line -match '(-(\w+))\s+([^-]*$)' )
    {
    ### Resolve Command name & parameter name
        $_param = $matches[2] + '*'
        $_opt = $Matches[3].Split(" ,")[-1] + '*'
        $_base = $Matches[3].Substring(0,$Matches[3].Length-$Matches[3].Split(" ,")[-1].length)

        $_cmdlet = [regex]::Split($line, '[|;=]')[-1]

        if ($_cmdlet -match '\{([^\{\}]*)$')
        {
            $_cmdlet = $matches[1]
        }

        if ($_cmdlet -match '\(([^()]*)$')
        {
            $_cmdlet = $matches[1]
        }

        $_cmdlet = $_cmdlet.Trim().Split()[0]

        $_cmdlet = @(Get-Command -type 'Cmdlet,Alias,Function,Filter,ExternalScript' $_cmdlet)[0]

        while ($_cmdlet.CommandType -eq 'alias')
        {
            $_cmdlet = @(Get-Command -type 'Cmdlet,Alias,Function,Filter,ExternalScript' $_cmdlet.Definition)[0]
        }

    ### Currently target is Get-Alias & "-Name" parameter

        if ( "Get-Alias" -eq $_cmdlet.Name -and "Name" -like $_param )
        {
           Get-Alias -Name $_opt | % { $_.Name } | sort | % { $_base + ($_ -replace '\s','` ') }
           break;
        }
    }
}

Reference http://gallery.technet.microsoft.com/scriptcenter/005d8bc7-5163-4a25-ad0d-25cffa90faf5

---

Posh-git renames TabExpansion to TabExpansionBackup in GitTabExpansion.ps1.
And posh-git's redifined TabExpansion calls original TabExpansion(TabExpansionBackup) when completions don't match with git commands.
So all you have to do is redefine TabExpansionBackup.

(cat .\GitTabExpansion.ps1 | select -last 18)
============================== GitTabExpansion.ps1 ==============================

if (Test-Path Function:\TabExpansion) {
    Rename-Item Function:\TabExpansion TabExpansionBackup
}

function TabExpansion($line, $lastWord) {
    $lastBlock = [regex]::Split($line, '[|;]')[-1].TrimStart()

    switch -regex ($lastBlock) {
        # Execute git tab completion for all git-related commands
        "^$(Get-AliasPattern git) (.*)" { GitTabExpansion $lastBlock }
        "^$(Get-AliasPattern tgit) (.*)" { GitTabExpansion $lastBlock }

        # Fall back on existing tab expansion
        default { if (Test-Path Function:\TabExpansionBackup) { TabExpansionBackup $line $lastWord } }
    }
}

===============================================================================

Redefine TabExpansionBackup(original TabExpansion)

function TabExpansionBackup {
    ...

    ### Resolve Command name & parameter name

    ...

    ### Currently target is Get-Alias & "-Name" parameter

    ...
}