How to get the captured groups from Select-String?

Question

I'm trying to extract text from a set of files on Windows using the Powershell (version 4):

PS > Select-String -AllMatches -Pattern <mypattern-with(capture)> -Path file.jsp | Format-Table

So far, so good. That gives a nice set of MatchInfo objects:

IgnoreCase                    LineNumber Line                          Filename                      Pattern                       Matches
----------                    ---------- ----                          --------                      -------                       -------
    True                            30   ...                           file.jsp                      ...                           {...}

Next, I see that the captures are in the matches member, so I take them out:

PS > Select-String -AllMatches -Pattern <mypattern-with(capture)> -Path file.jsp | ForEach-Object -MemberName Matches | Format-Table

Which gives:

Groups        Success Captures                 Index     Length Value
------        ------- --------                 -----     ------ -----
{...}         True    {...}                    49        47     ...

or as list with | Format-List:

Groups   : {matched text, captured group}
Success  : True
Captures : {matched text}
Index    : 39
Length   : 33
Value    : matched text

Here's where I stop, I have no idea how to go further and obtain a list of captured group elements.

I've tried adding another | ForEach-Object -MemberName Groups, but it seems to return the same as the above.

The closest I get is with | Select-Object -Property Groups, which indeed gives me what I'd expect (a list of sets):

Groups
------
{matched text, captured group}
{matched text, captured group}
...

But then I'm unable to extract the captured group from each of them, I tried with | Select-Object -Index 1 I get only one of those sets.

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Update: a possible solution

It seems that by adding | ForEach-Object { $_.Groups.Groups[1].Value } I got what I was looking for, but I don't understand why - so I can't be sure I would be able to get the right result when extending this method to whole sets of files.

Why is it working?

As a side note, this | ForEach-Object { $_.Groups[1].Value } (i.e. without the second .Groups) gives the same result.

I'd like to add that, upon further attempts, it seems the command can be shortened by removing the piped | Select-Object -Property Groups.

Answer 1

Have a look at the following

$a = "http://192.168.3.114:8080/compierews/" | Select-String -Pattern '^http://(.*):8080/(.*)/$' 

$a is now a MatchInfo ($a.gettype()) it contain a Matches property.

PS ps:\> $a.Matches
Groups   : {http://192.168.3.114:8080/compierews/, 192.168.3.114, compierews}
Success  : True
Captures : {http://192.168.3.114:8080/compierews/}
Index    : 0
Length   : 37
Value    : http://192.168.3.114:8080/compierews/

in the groups member you'll find what you are looking for so you can write :

"http://192.168.3.114:8080/compierews/" | Select-String -Pattern '^http://(.*):8080/(.*)/$'  | % {"IP is $($_.matches.groups[1]) and path is $($_.matches.groups[2])"}

IP is 192.168.3.114 and path is compierews

Answer 2

This script will grab a regex's specified capture group from a file's content and output its matches to console.

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$file is the file you want to load
$cg is capture group you want to grab
$regex is the regular expression pattern

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Example file and its content to load:

C:\some\file.txt

This is the especially special text in the file.

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Example Use: .\get_regex_capture.ps1 -file "C:\some\file.txt" -cg 1 -regex '\b(special\W\w+)'

Output: special text

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get_regex_capture.ps1

Param(
    $file=$file,
    [int]$cg=[int]$cg,
    $regex=$regex
)
[int]$capture_group = $cg
$file_content = [string]::Join("`r`n", (Get-Content -Raw "$file"));
Select-String -InputObject $file_content -Pattern $regex -AllMatches | % { $_.Matches.Captures } | % { echo $_.Groups[$capture_group].Value }