write-file like function help needed

["keithv" <kvetter@gmail.com>]

Hi,

I need help writing a function that works like write-file but
first modifies the file name by locating and incrementing
any numbers in the file name. For example, if the buffer
name is "test1.txt", repeatedly invoking this function
would save files with the names "test2.txt", "test3.txt",
"test4.txt", etc.

The goal being to keep all versions of a given file.

My elisp is rusty--I know how I would do the
number locate and increment on the buffer text
but not for an arbitrary string.

Or better yet, is there a package out there
to do something similar?

Thanks in advance,
Keith