Sometimes you need to quickly do a mass rename of a large number of files in a directory.

Rename a file like this: 
ABC_20150321124112_1801.xml 
to a filename like this:
XYZ_201503211241.xml

Sample Code

cls
cd "C:\TempRename" 
Get-ChildItem -Filter *.xml | Foreach-Object{   
   $NewName = $_.Name -replace "ABC_(.*?)\d{2}_\d{4}.xml", "XYZ_`$1.xml"
   Write-Host $NewName
   Rename-Item -Path $_.FullName -NewName $NewName
}

Step by Step Explanation

1. The CD shouldn’t be needed, but if you are running Powershell in a different directory in ISE, it can be helpful.
2. Get-ChildItem returns all files in the current directory with the mask *.xml
3. Then “ForEach” matching file, do what is in the curly brackets of the Foreach-Object loop. If you know what you are doing you can pipe without the Foreach, but I like to break it down, so I can add the debug Write-Host statements and run a simulation run (by commenting out the actual Rename-Item statement) before the final rename.
4. The -replace is the keyword that tells us that we are doing a RegEx replace. Here I’m changing a date like this: YYYYMMDDHHMMSS_xxxx to YYYYMMDDHHMM. (This was a requirement of the a customer. The downside is you could have duplicate files on the rename if more than one file was created in the same minute; but that was not our issue.)
I’m using the () to capture the YYYYMMDDHHMM string and then the `$1 substitutes that string back into the new filename. The grave-accent mark is the escape character to tell Powershell that I don’t want to insert a variable by the name $1 (which would have a value of null or empty-string, because I don’t have such a variable. $1 is used only with the Powershell replace, it’s not a real Powershell variable.
5. Write-Host shows the new filename.
6. Do the actual rename. Just comment out this line with # at the beginning to do a simulation run and verify the names.