Using variables inside PowerShell replace - powershell

Question

I'm trying to add some new settings to a tomcat server.xml file datasource. I can match the last setting in the datasource, which has a password that I need to capture, but when I try to replace it, I'm not see any changes.
$serverXml = "C:\server.xml"
$xml = Get-Content $serverXml
$password = (($xml -match " password=""(.*)""").Replace(' password="', "").Replace('" />', ''))[0]
$oldString = #"
username="cf.user"
password="$password" />
"#
$newString = #"
username="cf.user"
password="$password"
testWhileIdle="true"
testOnBorrow="true"
testOnReturn="false"
validationQuery="select 1"
validationInterval="30000"
minEvictableIdleTimeMillis="30000" />
"#
$xml = $xml.replace($oldString, $newString)
Set-Content -Path $serverXml -Value $xml
I'm able to match the $password fine, but when I'm using it as a variable to pass into $oldString and $newString in the replace, its not matching anymore. Even $xml -match $oldString doesn't return anything, but totally should as far as I can tell.

Answer 1

Do not edit XML via string replacements. Use the gratuitous XML parser PowerShell provides you with.
Load the config file like this:
[xml]$xml = Get-Content $serverXml
or like this:
$xml = New-Object Xml.XmlDocument
$xml.Load($serverXml)
The latter is a bit more failsafe, because it will (for instance) check that the encoding of the file actually matches the encoding specified in the preamble.
Select nodes via XPath expressions:
$node = $xml.SelectSingleNode('/Path/To/Node')
Change existing attributes like this:
$node.Attributes['password'] = 'newpassword'
Add new attributes like this:
$attr = $xml.CreateAttribute('testWhileIdle')
$attr.Value = 'true'
[void]$node.Attributes.Append($attr)
Then save the modified XML back to the file:
$xml.Save($serverXml)