Powershell, how many replacements did you make? - replace

Question

I need to know how many replacements are made by Powershell when using either the -replace operator or Replace() method. Or, if that's not possible, if it made any replacements at all.
For example, in Perl, because the substitution operation returns the number of replacements made, and zero is false while non-zero is true in a boolean context, one can write:
$greeting = "Hello, Earthlings";
if ($greeting ~= s/Earthlings/Martians/) { print "Mars greeting ready." }
However with Powershell the operator and method return the new string. It appears that the operator provides some additional information, if one knows how to ask for it (e.g., captured groups are stored in a new variable it creates in the current scope), but I can't find out how to get a count or success value.
I could just compare the before and after values, but that seems entirely inefficient.

Answer 1

You're right, I don't think you can squeeze anything extra out of -replace. However, you can find the number of matches using Regex.Matches(). For example
> $greeting = "Hello, Earthlings"
> $needle = "l"
> $([regex]::matches($greeting, $needle)).Length # cast explicitly to an array
3
You can then use the -replace operator which uses the same matching engine.
After looking a little deeper, there's an overload of Replace which takes a MatchEvaluator delegate which is called each time a match is made. So, if we use that as an accumulator, it can count the number of replacements in one go.
> $count = 0
> $matchEvaluator = [System.Text.RegularExpressions.MatchEvaluator]{$count ++}
> [regex]::Replace("Hello, Earthlings","l",$matchEvaluator)
> $count
Heo, Earthings
3

Answer 2

Here a complete functional example which preserves the replacement behavior and count the number of matches
$Script:Count = 0
$Result = [regex]::Replace($InputText, $Regex, [System.Text.RegularExpressions.MatchEvaluator] {
param($Match)
$Script:Count++
return $Match.Result($Replacement)
})

Answer 3

None of the above answers are actually do replacement and working in recent PS versions:
James Kolpack - show how to count a removed regex (not replaced);
Kino101 - incomplete answer, variables not defined;
Annarfych - outdated answer, in recent PS version the evaluator count variable need to be global
Here is how you can do a replace and count it:
$String = "Hello World"
$Regex = "l|o" #search for 'l' or 'o'
$ReplaceWith = "?"
$Count = 0
$Result = [regex]::Replace($String, $Regex, { param($found); $Global:Count++; return $found.Result($ReplaceWith) })
$Result
$Count
Result in Powershell 5.1:
He??? W?r?d
5

Answer 4

Version of the script that actually does replace things and not null them:
$greeting = "Hello, earthlings. Mars greeting ready"
$counter = 0
$search = '\s'
$replace = ''
$evaluator = [System.Text.RegularExpressions.MatchEvaluator] {
param($found)
$counter++
Write-Output ([regex]::Replace($found, [regex] $search, $replace))
}
[regex]::Replace($greeting, [regex] $search, $evaluator);
$counter
->
> Hello,earthlings.Marsgreetingready
> 4