I had a directory full of compressed printer driver files for Windows. The file abc.de_ is expanded into abc.def (and the fact that _ is replaced by f is governed by a file with an .INF exension). Normally this is all dealt with behind the scenes automatically, but I had to do it manually.
The following snippet will look for lines that contain the abc.de_/abc.def mapping and expands those files.
Additional error checking is left as an exercise to the reader :)