ValidateScript unexpectedly returning false when condition is true - powershell

Question

I have a PowerShell function I'm writing to build and execute a variety of logman.exe commands for me so I don't have to reference the provider GUIDs and type up the command each time I want to capture from a different source. One of the parameters is the file name and I am performing some validation on the parameter. Originally I used -match '.+?\.etl$' to check that the file name had the .etl extension and additionally did some validation on the path. I later decided to remove the path validation but neglected to change the validation attribute to ValidatePattern.
What I discovered was that while it worked perfectly on the machine I was using to author and validate it, on my Server 2016 Core machine it seemed to misbehave when calling the function but that if I just ran the same check at the prompt it worked as expected.
The PowerShell:
[Parameter(ParameterSetName="Server", Mandatory=$true)]
[Parameter(ParameterSetName="Client", Mandatory=$true)]
[ValidateScript({$FileName -match '.+?\.etl$'}]
[string] $FileName = $null
The Output:
PS C:\Users\Administrator> Start-TBLogging -ServerLogName HTTPSYS -FileName ".\TestLog.etl"
PS C:\Users\Administrator> Start-TBLogging : Cannot validate argument on parameter 'FileName'. The "$FileName -match '.+?\.etl$'" validation script
for the argument with value ".\TestLog.etl" did not return a result of True. Determine why the validation script failed,
and then try the command again.
At line:1 char:50
+ Start-TBLogging -ServerLogName HTTPSYS -FileName ".\TestLog.etl"
+ ~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidData: (:) [Start-TBLogging], ParameterBindingValidationException
+ FullyQualifiedErrorId : ParameterArgumentValidationError,Start-TBLogging
Trying it manually worked:
PS C:\Users\Administrator> $FileName = ".\TestLog.etl"
PS C:\Users\Administrator> $FileName -match '.+?\.etl$'
True
After changing the function to use ValidatePattern it works just fine everywhere but I was wondering if anyone could shed light on the discontinuity.

Answer 1

As Joshua Shearer points out in a comment on a question, you must use automatic variable $_ (or its alias form, $PSItem), not the parameter variable to refer to the argument to validate inside [ValidateScript({ ... })].
Therefore, instead of:
# !! WRONG: The argument at hand has NOT yet been assigned to parameter
# variable $FileName; by design, that assignment
# doesn't happen until AFTER (successful) validation.
[ValidateScript({ $FileName -match '.+?\.etl$' }]
[string] $FileName
use:
# OK: $_ (or $PSItem) represents the argument to validate inside { ... }
[ValidateScript({ $_ -match '.+?\.etl$' })]
[string] $FileName
As briantist points out in another comment on the question, inside the script block $FileName will have the value, if any, from the caller's scope (or its ancestral scopes).