In case you couldn't already tell, I rather like playing around with regex in PowerShell. There's something very enjoyable about poking it in just the right way to get it to do exactly what you want. I've already talked a bit about how we can use it to create PSCustomObjects.

Working with -replace

-replace is a very handy operator to have quick access to in PowerShell. In its most basic usage, it allows you to swap out bits and pieces of text, or to remove unwanted pieces from a string.

PS> $string = 'Glass half empty, glass half full.'
PS> $string -replace 'glass','cup'
cup half empty, cup half full.
PS> $string -replace '[^a-z]'
Glasshalfemptyglasshalffull

Using Match Groups

With the use of parentheses to create a match group, -replace becomes even more powerful. Stripping out everything from a string except the one or two things we want is extremely simple.

PS> $string = 'You lost your mind in the sound.'
PS> $string -replace '.*(\w+)\.','$1'
sound

There are two things to note in the above example. First of all, that pattern would match the entire string, so it would normally just remove the string. Second, we don't get the characters $1 back in our result.

While that syntax might look familiar to you from PowerShell's variables, don't be fooled. This isn't PowerShell, at least not where it counts here. PowerShell isn't processing that $ symbol, and we don't want it to; that's why I used single quotes to create a literal string.

Instead, this string is given to the underlying regex method, which does recognise it as a variable. In this context, that variable refers to "the first match group". Why is it 1-indexed, you might ask, when literally everything else in PowerShell is 0-indexed? Because $0 is quietly already taken.

You might recall from my previous post on regex that PowerShell has a $matches variable, where $matches[0] refers to "the complete matched string". It's the same thing here.

PS> 'hello world' -replace '(\w+) \w+','$1 $0'
hello hello world

Note that the match group is respected, pulling the first word, and then the entire match is retrieved again and reinserted. As you can imagine, this can be made incredibly complex and strange very quickly. With a bit of tinkering and a careful eye, you can do some wonderful stuff with -replace!

Named Match Groups

Bit of a theme with me, isn't it? I love anything that can lend a little extra verbosity to a complicated regex pattern, a bit of self-documentation, if you will.

I was tinkering around with the above syntaxes for -replace (not $0, though, I hadn't realised that part until I started writing this post!) the other week, and got to wondering. Is there a syntax for inserting named match groups in a replace string? It stands to reason that there ought to be, doesn't it? Nobody I asked knew a way, but then again, nobody could say for sure that it couldn't be done, either.

So, I got to tinkering. My first attempt looked a bit like this:

PS> $string = 'Hello darkness, my old friend.'
PS> $string -replace '(hello) (?<Item>Darkness)','$Item says $1'
$Item says Hello, my old friend.

Hmm. Okay, not quite there. As expected, the unnamed group is noticed, but the named group is ignored. A little bit more fiddling with the syntax, and I landed on the correct way to do it quite by accident.

PS> $string -replace '(hello) (?<Item>Darkness)','${Item} says $1'
darkness says Hello, my old friend.

Yep. That worked, very nicely.

So just remember: the temptation to obscure your code with regex is always there. But you have the tools available to make it that crucial bit more self-documenting, provided you name the bits and pieces effectively. A well-named match string makes it entirely unnecessary to have those four extra lines of comments attempting to explain what the heck is going on.

Summary

When using -replace on a string, the following tokens hold special meaning in the replacement string.

Keep your code clear and clean. It'll save you a great many headaches, especially when dealing with regex!