How to extract a certain part of a string in powershell - powershell

Question

I want to extract the last 4 digits before ".txt" from this string:
09/14/2017 12:00:27 - mtbill_post_201709141058.txt 7577_Delivered: OK
Those represent the time at which that log was created and I want to display it as 10:58. I read from a file that has multiple lines similar to the one displayed.
Get-Content file.txt | ForEach-Object {
$splitUp = $_ -split "_"
$SC=$splitUp[2] -split "_"
Write-Host $SC
$len = $SC.Length
$folder2 = $SC.Substring($len - 12, 42)
}
I tried separating the string by "_" and then counting the characters in the obtained string and tried separating by the "Substring" command, but I receive the following error.
Exception calling "Substring" with "2" argument(s): "StartIndex cannot
be less than zero. Parameter name: startIndex"
At line:6 char:5
+ $folder2 = $SC.Substring($len - 12, 42)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
CategoryInfo : NotSpecified: (:) [], MethodInvocationException
FullyQualifiedErrorId : ArgumentOutOfRangeException

Answer 1

You can use a regex "lookahead".
What you are searching for is a set of four digits followed by ".txt":
$string = "09/14/2017 12:00:27 - mtbill_post_201709141058.txt 7577_Delivered: OK"
$regex = "\d{4}(?=\.txt)"
[regex]::matches($string, $regex).value

Answer 2

There's probably a more elegant solution:
$String = '09/14/2017 12:00:27 - mtbill_post_201709141058.txt 7577_Delivered: OK'
$String -Match '.*(?=\.txt)' | Out-Null
$Match = $Matches[0][-4..-1] -Join ''
$Time = [DateTime]::ParseExact($Match, 'HHmm',[CultureInfo]::InvariantCulture)
$Time.ToShortTimeString()
Uses RegEx to get all of the string before the .txt
Uses the Array Index to get the characters from 4th to last to the last character and joins them together as a single string.
Casts the value as a DateTime object using ParseExact to interpret it as 24 hour time code
Outputs the Short Date value of that DateTime object.

Answer 3

Just do it with Substring and IndexOf:
$string="09/14/2017 12:00:27 - mtbill_post_201709141058.txt 7577_Delivered: OK"
$string.Substring($string.IndexOf('.txt')-4, 4)