Using Powershell to replace string to end of line

Question

I need to replace a tag in an ini file for a number of files beginning with news_* using Powershell. I wrote a script to find the tag, but do not know how to replace everything from the tag to the end of the line.

for example if the ini file has:

[tag1] = A string
[tag2] = 1234567
[tag3] = Another string

And I want to replace all tag 2's with a blank string, I wrote:

$configFiles = Get-ChildItem . news* -rec
foreach ($file in $configFiles)
{
    (Get-Content $file.PSPath) |
    Foreach-Object { $_ -replace '[tag2] = ".*?"', '[tag2] = ' } |
    Set-Content $file.PSPath
}

Can you tell me what is wrong with this script?

Thanks!

Ansgar Wiechers · Accepted Answer · 2018-01-23 19:56:26Z

4

The string you want to match doesn't have any double quotes. Also, as TheIncorrigible1 pointed out in the comments, you need to escape the square brackets, otherwise that part of your expression would be matched as a character class (any of the characters t, a, g, or 2) rather than the literal substring "[tag2]".

Change

$_ -replace '[tag2] = ".*?"', '[tag2] = '

into

$_ -replace '^(\[tag2\] = ).*', '$1'

and the problem will disappear.

$1 is a backreference to the first capturing group (the submatch between the first set of parentheses in the expression). In this case you're replacing the entire line with just the (captured) part that you want to keep.

You could also do this without capturing group/backreference by using a positive lookbehind assertion:

$_ -replace '(?<=^\[tag2\] = ).*'

Lookaround assertions are used in the match, but what they're matching doesn't become part of the value that's returned. Basically a positive lookbehind assertion means "look for something that is preceded by this subexpression".

edited Jan 23, 2018 at 19:56

answered Jan 23, 2018 at 18:38

Ansgar Wiechers

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5 Comments

Maximilian Burszley Over a year ago

Don't forget he needs to escape his brackets otherwise he's searching a set.

HamletHub Over a year ago

Thanks! that did it after I escaped the brackets. What does the '$1' do? is that the equivalent of ""?

Maximilian Burszley Over a year ago

@HamletHub '$1' is equivalent to '\1' that you may recognize in other regex statements from other languages. It's a back-reference to the capture group (\[tag2\] = ) in the first replace argument.

Maximilian Burszley Over a year ago

@HamletHub btw. You don't want to use [tag1] when building an ini, just use tag1 = value otherwise you're declaring sections and syntactically wrong.

Ansgar Wiechers Over a year ago

@TheIncorrigible1 Indeed. I wasn't paying attention to the square brackets. Thanks for the heads up.

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