I know this thread is a bit old but, I was looking for something similar and could not find it. Here's what I came up with. I create a string object using the .Net String class to expose all the methods normally found if using C#
[System.String]$myString
$myString = "237801_201011221155.xml"
$startPos = $myString.LastIndexOf("_") + 1 # Do not include the "_" character
$subString = $myString.Substring($startPos,$myString.Length - $startPos)
Result: 201011221155.xml