Powershell: Leave item alone if regex doesn't match

Question

I have a list of pdf files (from daily processing), some with date stamps of various formatting, some without.

Example:

$f = @("testLtr06-09-02.pdf", "otherletter.pdf","WelcomeLtr043009.pdf")

I am trying to remove the datestamp by stripping out dashes, then replacing any consecutive group of numbers (4 or more, I may change this to 6) with the string "DATESTAMP".

So far I have this:

$d =  $f | foreach {$_ -replace "-", ""} | foreach { $_ -replace ([regex]::Matches($_ , "\d{4,}")), "DATESTAMP"}
echo $d

The output:

testLtrDATESTAMP.pdf
DATESTAMPoDATESTAMPtDATESTAMPhDATESTAMPeDATESTAMPrDATESTAMPlDATESTAMPeDATESTAMPtDATESTAMPtDATESTAMPeDATESTAMPrDATESTAMP.DATESTAMPpDATESTAMPdDATESTAMPfDATESTAMP
WelcomeLtrDATESTAMP.pdf

It works fine if the file has a datestamp but it seems to be freaking out the -replace and inserting DATESTAMP after every character. Is there a way to fix this? I tried to change it to a foreach loop but I couldn't figure out how to get true/false from regex.

Thanks in advance.

Answer 1

$_ -replace ([regex]::Matches($_ , "\d{4,}")), "DATESTAMP"

Means in $_ replace every finding of ([regex]::Matches($_ , "\d{4,}")) with "DATESTAMP".

As in a filename with no timestamp (or at least 4 consecutive numbers) there is no match, it returns "" (an empty string).

Thus every empty string gets replaced with DATESTAMP. And such a empty string "" sits at the start of the string and after every other character.

Thats why you get this long string with every character surrounded by DATESTAMP.

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To check if there even exists a \d{4,} in your string you should able to use

[regex]::IsMatch($_, "\d{4,}")

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I'm no Powershell user but this line alone should do the job. But I'm not sure about being able to use the if in a pipeline and wether or not the assignment and the echo $d are needed

$f | foreach-object {$_ -replace "-", ""} | foreach-object {if ($_ -match "\d{4,}") { $_ -replace "\d{4,}", "DATESTAMP"} else { $_ }}

Answer 2

You can simply do:

PS > $f -replace "(\d{2}-){2}\d{2}|\d{4,}","DATESTAMP"
testLtrDATESTAMP.pdf
otherletter.pdf
WelcomeLtrDATESTAMP.pdf