Powershell: Split-Path Not Working in -Replace

Question

I want to pre-process C code, replacing #include "../../abc/efg/hij.h" with "hij.h".

I use command

'#include "../../abc/efg/hij.h"' -replace '(#include\s+)(".*\w+\.h")',"$(Split-Path -Leaf `$2)"

To do the replacement. However, no matter how I tried (use single-/double- quote, $(), and other methods), the final string (I did not Set-Content yet, just want to see the result in standard output) is still "../../abc/efg/hij.h".

When I run Split-Path -Leaf "../../abc/efg/hij.h", the result is hij.h, which is correct.

Can some one please tell me why this happens? Thank you very much!

Separating paths can be done completely with regex, but I think the more interesting problem is how to manipulate the match groups using PS commands prior to replacement. — Ryan Bemrose
– Ryan Bemrose, Commented Dec 1, 2015 at 5:19

TessellatingHeckler · Accepted Answer · 2015-12-01 01:52:07Z

4

Seems weird that you are doing path parsing with regex, but then trying to use Split-Path to do it with a commandlet as well. Why not do the replace entirely with regexes?

Get rid of the bit you don't want by capturing it and replacing it with nothing:

PS C:\> '#include "../../abc/efg/hij.h"' -replace '(?<=#include\s+")(.*/)(?=\w+\.h")'
#include "hij.h"

or keep the bit you do want by capturing it and putting it into a new line:

PS C:\> '#include "../../abc/efg/hij.h"' -replace '#include\s+".*/(.*\.h)"','#include "$1"'
#include "hij.h"

answered Dec 1, 2015 at 1:52

TessellatingHeckler

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1 Comment

ivzhh Over a year ago

Yes, later I did like this. Just wondering why it happens. New to Powershell, and still confused. Your solution is the better way, thanks!

Ryan Bemrose · Accepted Answer · 2015-12-01 01:42:30Z

2

The problem you are seeing is that the -replace operator matches the regex and performs the replacement atomically. It provides no method to run any PowerShell code in between the two operations.

In your code, the scriptblock (Split-Path -Leaf $2) evaluates first, before the regex has been matched. The result of that scriptblock ($null, the empty string, or an exception depending on what value the variable $2 holds) is then passed to -replace, with observed result.

What you want is to be able to run a PowerShell command in between matching the regex and replacing it. One way to do this is to use -match to match the regular expression, and then build up your replacement string using the matches in the $matches variable.

$oldLine -match '#include\s+"(.*\w+\.h)"'
$newLine = '#include "{0}"' -f (split-path -leaf $matches[1])

Optionally you can do the replacement using the -replace operator, rather than building the whole line from scratch. However, because it evaluates the regex again, it can lead to performance problems for large input files.

answered Dec 1, 2015 at 1:42

Ryan Bemrose

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1 Comment

ivzhh Over a year ago

Your answer is very inspiring, thank you! I finally have something naive: get-content a.h | ForEach-Object {$_ -match '#include\s+"(.*\w+\.h)"' > $null; if ($matches[1]) { "#include "$(split-path -leaf $matches[1])"" } else { $_ } } | add-content b.h; Move-Item -Force b.h a.h

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