How to delete initial parts of filename

[rah]

I did not see in the support page anywhere you could delete the initial part of a file:
E.G.
   index.part1235222 -> part1235222
   index.part1250 -> part1250

Using the [N] doesn't seem to work: [N,7-100]
Perhaps I can use regular expressions

[Mathias (Author)]

You can do it in two ways

"[N,7]"  then it starts at position 7.
Or in the Search and Replace part you can enter "index." in the find field and blank in the replace with field