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Problem

I am working on a script that has a user provide a specific IP address and I want to mask this IP in some fashion so that it isn't stored in the logs. My problem is, that I can easily do this when I know what the first three values of the IP typically are; however, I want to avoid storing/hard coding those values into the code to if at all possible. I also want to be able to replace the values even if the first three are unknown to me.

Examples:

10.11.12.50 would display as XX.XX.XX.50
10.12.11.23 would also display as XX.XX.XX.23

I have looked up partial string replacements, but none of the questions or problems that I found came close to doing this. I have tried doing things like:

# This ended up replacing all of the numbers
$tempString = $str -replace '[0-9]', 'X'

I know that I am partway there, but I aiming to only replace only the first 3 sets of digits so, basically every digit that is before a '.', but I haven't been able to achieve this.

Question

Is what I'm trying to do possible to achieve with PowerShell? Is there a best practice way of achieving this?

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2
  • Use a regular expression replacement pattern Commented Sep 5, 2017 at 13:33
  • Related post - Replace substring in PowerShell Commented Apr 11, 2018 at 9:46

2 Answers 2

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3

Here's an example of how you can accomplish this:

Get-Content 'File.txt' |
  ForEach-Object { $_ = $_ -replace '\d{1,3}\.\d{1,3}\.\d{1,3}','xx.xx.xx' }

This example matches a digit 1-3 times, a literal period, and continues that pattern so it'll capture anything from 0-999.0-999.0-999 and replace with xx.xx.xx

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5 Comments

PERFECT!! Thank you. That gave me the control I desired. I tried to choose this as the answer, but it told me that it was too soon to pick an answer
++, but I assume you didn't mean to put the $_ = there (assigning to $_ should generally be avoided, and in this case it would also prevent output).
@mklement0 It's intentional and fine to do if you're going to re-use $_ in the current scriptblock. I don't know his use-case, but it looks like he just wants to obfuscate it in his console output, so my example is partially applicable in his exact use-case.
@TheIncorrigible1: Understood, but note that changes are coming that will produce an editor warning when you assign to $_ (making $_ read-only in the engine was ultimately rejected to avoid break existing code).
@mklement0 very interesting read. Good decision overall, kind of bites for legacy code.
2

TheIncorrigible1's helpful answer is an exact way of solving the problem (replacement only happens if 3 consecutive .-separated groups of 1-3 digits are matched.)

A looser, but shorter solution that replaces everything but the last .-prefixed digit group:

PS> '10.11.12.50' -replace '.+(?=\.\d+$)', 'XX.XX.XX'
XX.XX.XX.50

(?=\.\d+$) is a (positive) lookahead assertion ((?=...)) that matches the enclosed subexpression (a literal . followed by 1 or more digits (\d) at the end of the string ($)), but doesn't capture it as part of the overall match.

The net effect is that only what .+ captured - everything before the lookahead assertion's match - is replaced with 'XX.XX.XX'.

Applied to the above example input string, 10.11.12.50:

  • (?=\.\d+$) matches the .-prefixed digit group at the end, .50.

  • .+ matches everything before .50, which is 10.11.12.

  • Since the (?=...) part isn't captured, it is therefore not included in what is replaced, so it is only substring 10.11.12 that is replaced, namely with XX.XX.XX, yielding XX.XX.XX.50 as a result.

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1 Comment

@Brandon: Correct; I've updated the answer again to explain it with a concrete example.

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