This is by far the most powerful, but also most complicated, feature of the Advanced Rename dialog. Using VBScript, JScript or other ActiveX scripting languages, you can write a rename function that gives you complete control over the outcome of the rename operation.
Click the Edit button in the actions section of the Rename dialog to display the script editor.
The script editor will pop-out to the right of the dialog. The drop-down at
the top of the script editor lets you choose the script language to use.
Note that a script can be defined (and have effect) even if the editor isn't visible. As shown in the screenshot above, the words Script defined will be displayed to indicate that a script is active. The drop-down attached to the Hide button gives you a clear command which lets you clear out the script contents.
See the Rename Scripts page in the Scripting section for more information about writing rename scripts.