Exporting ranges of rows from large text files - powershell

Question

I have about 5GBs of logdata I need to filter down and find matching rows and then include +/- 75 rows from the matching row. If the format of the data is important it is in broken XML which is missing some tags.
My code to find the rows with matches:
$ExampleFile = [System.IO.File]::ReadLines("C:\temp\filestomove\ExampleLog.txt")
$AccountNumber = "*123456789*"
$LineCount = 0
$RowsToExport = #()
foreach($line in $ExampleFile){
if($line -like "*$AccountNumber*"){
$RowsToExport += $LineCount
}
$LineCount += 1
}
Above code does the job fairly quickly, it manages about a MB of log per second. Which is a speed I can live with since it's a one time job.
What I am struggling with, is exporting the matched rows in a way that is not very slow.
My Current code for that looks something like this:
foreach($row in $RowsToExport){
$IndexRangeHigh = [int]$row + 75
$IndexRangeLow = [int]$row - 75
$test | select -Index ($IndexRangeLow..$IndexRangeHigh) | out-file C:\temp\Example.txt -append
}
That takes a really long time, I have my doubts about using select -index as I suspect it is very slow.
Measure-command on above gave me the following result on a 50MB test file:
TotalDays : 0,00354806909375
TotalHours : 0,08515365825
TotalMinutes : 5,109219495
TotalSeconds : 306,5531697
TotalMilliseconds : 306553,1697
While reading the file and matching the rows only took me 55 seconds.
To sum everything up to a question:
How can I export a range of rows from a large variable? Is there other method I can use to select rows from the $ExampleFile variable instead of using select -index ($ExampleRangeLow..$ExampleRangeHigh)?

Answer 1

PowerShell has a cmdlet (Select-String) that allows extracting text befor and/or after a match.
Select-String -Path 'C:\path\to\your.log' -Pattern '123456789' -Context 75
The output of Select-String is an object with several properties, so additional code is required if you need the matching lines in text form:
... | ForEach-Object {
$pre = $_.Context.PreContext | Out-String
$post = $_.Context.PostContext | Out-String
"{0}{1}`n{2}" -f $pre, $_.Line, $post
}