Group unique filenames by expression?

I have a directory full of unique names, all in the same format - Scriptname_datecodestuff.txt
How can I group these based on 'scriptname' ?
What I need to accomplish is I only want to have 10 'versions' of a file in this directory, so if their are more than 10, I need to delete the oldest of that group

Example files in the dir
localhost_11.02.2010-0645.txt
localhost_11.02.2010-0655.txt
localhost_11.02.2010-0705.txt
otherhost_11.02.2010-0645.txt
otherhost_11.02.2010-0650.txt
otherhost_11.02.2010-0650.txt

With those listtings, the 2 groups would be localhost with a count of 3, and otherhost with a count of 3.

I have something semi working, but its not elogant, and suffers some bad performance..

$Group =$File.Name.SubString(0,(($File.Name.Length)-20))
ElseIf (($Directory | Where {$_.Name -Match $Group} | %{$_.Name.SubString(0,(($_.Name.Length)-20))} | Group | %{$_.count}) -gt 10) {
		$Directory | Where {$_.Name -Match $Group} | Sort -Property LastWriteTime | Select -First 1 | Remove-Item

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