PowerShell string replacement - regex

Question

I am trying to build a PowerShell script such that I give it an input file and regex, it replaces the matching content with the environment variable.
For example,
If the input file contains the following:
<?xml version="1.0" encoding="utf-8"?>
<Application xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" Name="fabric:/Services" xmlns="http://schemas.microsoft.com/2011/01/fabric">
<Parameters>
<Parameter Name="IntegrationManager_PartitionCount" Value="1" />
<Parameter Name="IntegrationManager_MinReplicaSetSize" Value="2" />
<Parameter Name="IntegrationManager_TargetReplicaSetSize" Value="#{INT_MGR_IC}" />
<Parameter Name="EventManager_InstanceCount" Value="#{EVT_MGR_IC}" />
<Parameter Name="Entities_InstanceCount" Value="#{ENT_IC}" />
<Parameter Name="Profile_InstanceCount" Value="#{PRF_IC}" />
<Parameter Name="Identity_InstanceCount" Value="#{IDNT_IC}" />
</Parameters>
</Application>
I would like to build a script that replaces #{INT_MGR_IC} with the value of the INT_MGR_IC environment variable and so on.
If you know of such script or can point me in the right direction, it would be a great help. Specifically, I am interested to know how to:
Extract and loop over keys from the file such as: #{INT_MGR_IC}, #{EVT_MGR_IC}, etc.
Once I have the key, how do I replace it with an associated environment variable. For example, #{INT_MGR_IC} with INT_MGR_IC env. variable.
Thanks a lot for looking into this :)
UPDATE 1
This is the RegEx I am planning to use: /#{(.+)}/g

Answer 1

Just load the file using the Get-Content cmdlet, iterate over each Parmeter, filter all parameter that Value starts with an #using Where-Object and change the value. Finally, use the Set-Content cmdlet to write it back:
$contentPath = 'Your_Path_Here'
$content = [xml] (Get-Content $contentPath)
$content.DocumentElement.Parameters.Parameter | Where Value -Match '^#' | ForEach-Object {
$_.Value = "REPLACE HERE"
}
$content | Set-Content $contentPath
In case you need to determine an environment variable, you could use [Environment]::GetEnvironmentVariable($_.Value).

Answer 2

Thanks a lot for everyone who helped. Especially, #jisaak :)
Here is the final script I built that solved the problem in the question. Hopefully its useful to someone!
$configPath = 'Cloud.xml'
$config = [xml] (Get-Content $configPath)
$config.DocumentElement.Parameters.Parameter | Where {$_.Value.StartsWith("#{")} | ForEach-Object {
$var = $_.Value.replace("#{","").replace("}","")
$val = (get-item env:$var).Value
Write-Host "Replacing $var with $val"
$_.Value = $val
}
$config.Save($configPath)