powershell How to capture 'left side' of the -replace regex?

First time posting.

i have the following code to replace the suffix of an email and its working fine

replace all characters after @ sign with @testdomain.com

$a = 'john.doe@domain.com'
$b = $a -replace "[?=@].*", '@testdomain.com'
$b
john.doe@testdomain.com

what i would like to do, is to capture the actual left side ‘source’ regex expression to a variable, which would be @domain.com so that i know what i;m replacing and i don;t know how to do it.

Sorry if this had been posted before.

Thank you

>Solution :

So, I’m not sure if this is possible using only the -replace operator and without the use of -match which would store the capture group on the $Matches automatic variable.

This is how you could do it using the regex class directly:

$a = 'john.doe@domain.com'
$Capture = @{}

$b = [regex]::Replace($a, "[?=@].*", {
    param($s)
    
    '@testdomain.com'
    $Capture.Value = $s.Value
})

$b # => john.doe@testdomain.com
$Capture.Value # => @domain.com