Rename file - delete all characters AFTER 2nd underscore - powershell

Question

I need to replace the time\date stamp that's included in the filename after 2nd underscore (needs to be in the same format yyyyMMddHHmmss)
example file: 123456_123456_20190716163001.xml
sometimes the file in question gets created with an additional character which invalidates the file, in this case I need to replace this with the current timestamp.
example: 123456_123456_current Timestamp here.xml
The file should never exceed 32 characters(including extension)
I found a script but it deletes everything after the 1st underscore not the 2nd and I'm struggling to find a way to replace the text with the current timestamp.
Get-ChildItem c:\test -Filter 123456_123456*.xml | Foreach-Object -Process {
$NewName = [Regex]::Match($_.Name,"^[^_]*").Value + '.xml' $_ | Rename-Item -NewName $NewName
}
timestamp after 2nd underscore to be updated to the current timestamp if original file exceeds 32 characters
123456_123456_current Timestamp here.xml

Answer 1

this takes advantage of the way a [fileinfo] object is structured. the .BaseName is easy to get to & use .Split() on. then one can use -join to put it back into one basename & finally add the extension onto the basename.
# fake reading in a file info object
# in real life, use Get-ChildItem or Get-Item
$FileObject = [System.IO.FileInfo]'123456_123456_current Timestamp here.xml'
$NewName = -join (($FileObject.BaseName.Split('_')[0,1] -join '_'), $FileObject.Extension)
$NewName
output = 123456_123456.xml

Answer 2

Sticking with the regex theme, you can do the following:
$CurrentTime = Get-Date -Format 'yyyyMMddHHmmss'
$RegexReplace = "(.*?_.*?_).*(\..*)"
Get-ChildItem c:\test -Filter 123456_123456*.xml |
Rename-Item -NewName {$_.Name -replace $RegexReplace,"`${1}$CurrentTime`${2}"}
If duplicate file names are a concern, you can build in an increment to $CurrentTime.
$CurrentTime = Get-Date -Format 'yyyyMMddHHmmss'
$RegexReplace = "(.*?_.*?_).*(\..*)"
Get-ChildItem c:\test -Filter 123456_123456*.xml |
Rename-Item -NewName {
$NewName = $_.Name -replace $RegexReplace,"`${1}$CurrentTime`${2}"
if (test-path $NewName) {
$CurrentTime = [double]$CurrentTime + 1
$NewName = $_.Name -replace $RegexReplace,"`${1}$CurrentTime`${2}"
}
$NewName
}
Explanation:
$RegexReplace contains the regex expression that will need to be matched for the ideal rename operation to happen. The regex mechanisms are explained below:
.*?_.*?_: Matches a minimal number of characters (lazy matching) followed by an underscore and then another minimal number of characters followed by an underscore.
.*: Greedily matches any characters
\.: Literally matches the dot character (.).
(): The parentheses here represent capture groups with the first set being 1 and the second set being 2. These are later referenced as ${1} and ${2} in the -replace operation.
Since Rename-Item -NewName supports delayed script binding, we can just pipe Get-ChildItem output directly to it. The current pipeline object is $_.
The -replace operation uses the variable $CurrentTime, which must be expanded in order for a successful outcome. For that reason, we use double quotes around the replacement. Since we do not want capture groups ${1} and ${2} expanded, we backtick escape them.