18.1.6 Ignoring Certain Changes

It would be nice to have a feature for specifying two strings, one in from-file and one in to-file, which should be considered to match. Thus, if the two strings are `foo' and `bar', then if two lines differ only in that `foo' in file 1 corresponds to `bar' in file 2, the lines are treated as identical.

It is not clear how general this feature can or should be, or what syntax should be used for it.

A partial substitute is to filter one or both files before comparing, e.g.:

sed 's/foo/bar/g' file1 | diff - file2

However, this outputs the filtered text, not the original.