I use Emacs to write code occasionally, mostly Fortran. A common thing I need to do is search and replace everywhere in a region confined to a rectangle within the text. An example is altering an existing code from Fortran 77 format to Fortran 90, specifically changing all of the Cs in column one (indicating comments) to !s. In Emacs terminology, this is a rectangle of width 1 character and as high as the number of lines in the code.
I have had difficulty figuring out how to do this...