The current version 2.25 only supports the whole original filename with the <Filename> or <Filename With Ext> tag.
My suggestion is to extend it with 
Original filename is IMGP1234.jpg
- <Filename> is I
<Filename> is M
<Filename[1-4]> is IMGP
<Filename[5-8]> is 1234
<Filename[1-23]> is IMGP1234
<Filename With Ext[4-12]> is P1234.jpg
- <Ext> is .jpg
<Ext[2-4]> is jpg
- <FILENAME> converts to uppercase and
<filename> converts to lowercase
<Filename> doesn´t change the case
Please tell me what you think about this.
It would be great if XnView could to this job. So there would no more need to change to another program just for renaming files.