Export email to PST based on specified subject for all mailboxes using -ContentFilter Entered via a Read-Host variable

Question

I am attempting to recreate what we have done previously using exmerge under Exchange 2003 where we can archive out specific emails based on subject to pull out 0 days spam etc.  What I want to do first is to copy the mails to an individual .pst per
user (in case we need to pull any emails back)
I have manged to do this with using the following command (in this example extracting a subject containing the sub-phrase junktest):
foreach ($mbx in (Get-Mailbox | Where {$_.ExchangeVersion.ExchangeBuild.Major -eq 14})) {New-MailboxExportRequest -Mailbox $mbx -ContentFilter {(subject -like "*junktest*")} -FilePath "\\ssf-adm-ex01\mailboxexport\$($mbx.Alias).pst"}
I would like now to simplify the use of this so I am attempting to create a script where the user can input the emails Subject rather than having to edit the code each time.
However I cannot get this to work. Using the following script it just exports emails with a subject line of $subjectstring rather than the variables content
$subjectstring = Read-Host 'Please enter the Subject String'
foreach ($mbx in (Get-Mailbox | Where {$_.ExchangeVersion.ExchangeBuild.Major -eq 14})) {New-MailboxExportRequest -Mailbox $mbx -ContentFilter {(subject -like "$subjectstring")} -FilePath "\\ssf-adm-ex01\mailboxexport\$($mbx.Alias).pst"}
using the following just exports all of the emails from the server
$subjectstring = Read-Host 'Please enter the Subject String'
foreach ($mbx in (Get-Mailbox | Where {$_.ExchangeVersion.ExchangeBuild.Major -eq 14})) {New-MailboxExportRequest -Mailbox $mbx -ContentFilter {(subject -like $subjectstring)} -FilePath "\\ssf-adm-ex01\mailboxexport\$($mbx.Alias).pst"}
I have tried all sorts of  combinations however I cannot get this to work correctly.
Can anyone help a powershll newbie out?  Thanks in advance.

Answer 1

Try:
-ContentFilter "subject -like '$subjectstring'"
No braces.  See what that does for you.
This works like a dream.
Thank you for your help,  it is appreciated.