Powershell pipes and regular expressions, order of evaluation

Question

This piece of code actually works, and I'm curious whether I got lucky(!?) and found a bug in the Power Shell language (this is pseudo code-ish, but it illustrates my question):

$regex = "prefix([0-9]+)"
$collection = Get-Collection | Where-Object {$_.Name -match $regex} `
                             | ForEach-Object { [int] $Matches[1] }

Input is basically objects whose property Name may be on the format "prefix[Integer]". If that is the case, I want to extract that integer and insert it into a new sequence.

This seemingly works, but it feels like I'm exploiting an implementation detail in the Power Shell language.

How would you solve this problem?

Answer 1

The issue is that each pipeline expression is a closure.

Where-Object is only going to send the Item that matched down the pipeline, not the context.

The simplest to understand method is to do:

Get-Collection | ForEach-Object { if($_.Name -match $regex) { [int] $Matches[1] } }

Answer 2

You're not exploiting anything here, it looks perfectly valid to me.

1. You get a collection of objects
2. You filter out the ones which match your regex using Where-Object
3. You pass the filtered objects to a ForEach-Object loop which simply states the regex match in the pipeline as an integer.

Perfectly normal code. What's the problem?