Case-Sensitive Sarch and replace

[Thomas Philips]

Using glob(), I obtain a list of filenames with some characters in
upper case, others in lower case. I want to rename the files by
replacing one substring in each filename with another, but with two
twists.

1. The search must be case insensitive
2. After portion of the filename that does not match the search string
must not have its case changed.

For example, if fn="AlphaMin.txt", searchstring="min" and
replacestring= "Max", I want the file to be renamed "AlphaMax.txt" and
not "alphaMax.txt" or "alphamax.txt"

I can easily get alphaMax.txt by using
fn.lower().replace(searchstring.lower(),replacestring), but then the
portion of fn that is not being replaced is lowercased.

It's not hard to write a function that repeatedly finds
searchstring.lower() in fn.lower(), and then uses slices to replace
the appropriate portions of fn, but there must be a simpler and
cleaner way to acheive this goal. Suggestions?

Thomas Philips