Cannot remove text between two strings with ReadLines - regex

Question

test.txt contents:
foo
[HKEY_USERS\S-1-5-18\Software\Microsoft]
bar
delete me!
[HKEY_other_key]
end-------------
Online regex matches the text to be removed correctly (starting from string delete until string [HKEY), but code written in PowerShell doesn't remove anything when I run it in PowerShell ISE:
$file = [System.IO.File]::ReadLines("test.txt")
$pattern = $("(?sm)^delete.*?(?=^\[HKEY)")
$file -replace $pattern, "" # returns original test.txt including line "delete me!" which should be removed
It seems to be a problem with ReadLines because when I use alternative Get-Content:
$file = Get-Content -Path test.txt -Raw
it removes the unwanted line correctly, but I don't want to use Get-Content.

Answer 1

[System.IO.File]::ReadAllLines(..) reads all lines of the file into a string array and you're using a multi-line regex pattern.
Get-Content -Raw same as [System.IO.File]::ReadAllText(..), reads all the text in the file into a string.
[System.IO.File]::ReadAllText("$pwd\test.txt") -replace "(?sm)^delete.*?(?=^\[HKEY)"
Results in:
foo
[HKEY_USERS\S-1-5-18\Software\Microsoft]
bar
[HKEY_other_key]
end-------------
In case you do need to read the file line-by-line due to, for example, high memory consumption, switch -File is an excellent built-in PowerShell alternative:
switch -Regex -File('test.txt') {
'^delete' { # if starts with `delete`
$skip = $true # set this var to `$true
continue # go to next line
}
'^\[HKEY' { # if starts with `[HKEY`
$skip = $false # set this var to `$false`
$_ # output this line
continue # go to next line
}
{ $skip } { continue } # if this var is `$true`, go next line
Default { $_ } # if none of the previous conditions were met, ouput this line
}