Modifying Registry Keys - Add to if not already present

Hello PowerShell Users,

Pre-face: I am very new to PowerShell scripting and until this point have only really used a handful of select commands.

I am currently playing with PSADT for app deployments and as a post-installation task, I am trying to write a script to check an existing multistring registry key and if certain values are missing, add them.

I feel like I am missing something obvious or really over-thinking it but my google-fu is not powerful enough.

Currently, I am hitting this error:

parsing "HKEY_LOCAL_MACHINE\SOFTWARE\PISystem
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\PISystem" - Malformed \p{X} character escape.
At line:15 char:1
+ $entryExists = $CurrentValue -match $ExpectedEntries
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : OperationStopped: (:) [], ArgumentException
    + FullyQualifiedErrorId : System.ArgumentException

Script:

The value of $CurrentValue in this instance is a multistring registry value of "HKEY_LOCAL_MACHINE\SOFTWARE\PISystem
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\PISystem
"

Below is the full script:
# Set path to registry key
$RegistryPath = 'HKLM:\SOFTWARE\Microsoft\Office\ClickToRun\REGISTRY\MACHINE\Software\Microsoft\AppV\Subsystem\VirtualRegistry'
$Name = 'PassThroughPaths'

# Get current value for registry key
$CurrentValue = Get-ItemPropertyValue -Path $RegistryPath -Name $Name

# Setup the desired entry
$ExpectedEntries = @"
HKEY_LOCAL_MACHINE\SOFTWARE\PISystem
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\PISystem
"@

# Check if the desired entry already exists
$entryExists = $CurrentValue -match $ExpectedEntries

if (-not $entryExists) {
    # Append the entry to the file
    $testValue = $CurrentValue + $ExpectedEntries
} else {
    # Entry already exists
    $testValue = $CurrentValue
}

$testValue
# Now $content contains the updated data
# You can save it back to the registry or use it as needed
Set-ItemProperty -Path $RegistryPath -Name $Name -Value $testValue

any help would be very welcome