How do you extract the value of a regex backreference/match in Powershell

Question

I have a text file containing lines of data. I can use the following powershell script to extract the lines I'm interested in:

select-string -path *.txt -pattern "subject=([A-Z\.]+)," 

Some example data would be:

blah blah subject=THIS.IS.TEST.DATA, blah blah blah 

What I want is to be able to extract just the actual contents of the subject (i.e. the "THIS.IS.TEST.DATA" string). I tried this:

select-string -path *.txt -pattern "subject=([A-Z\.]+)," | %{ $_.Matches[0] } 

But the "Matches" property is always null. What am I doing wrong?

Answer 1

I don't know why your version doesn't work. It should work. Here is an uglier version that works.

$p = "subject=([A-Z\.]+)," select-string -path *.txt -pattern $p | % {$_ -match $p > $null; $matches[1]} 

Explanation:

-match is a regular expression matching operator:

>"foobar" -match "oo.ar" True 

The > $null just suppresses the True being written to the output. (Try removing it.) There is a cmdlet that does the same thing whose name I don't recall at the moment.

$matches is a magic variable that holds the result of the last -match operation.

Answer 2

In PowerShell V2 CTP3, the Matches property is implemented. So the following will work:

select-string -path *.txt -pattern "subject=([A-Z\.]+)," | %{ $_.Matches[0].Groups[1].Value }