How to ignore some differences in diff command?

Question

diff has an option -I regexp, which ignores changes that just insert or delete lines that match the given regexp. I need an analogue of this for the case, when changes are between two lines (rather then insert or delete lines).

For instance, I want to ignore all differences like between "abXd" and "abYd", for given X and Y.

It seems diff has not such kind of ability. Is there any suitable alternative for diff?

Answer 1

You could filter the two files through sed to eliminate the lines you don't care about. The general pattern is /regex1/,/regex2/ d to delete anything between lines matching two regexes. For example:

diff <(sed '/abXd/,/abYd/d' file1) <(sed '/abXd/,/abYd/d' file2)

Answer 2

Improving upon the earlier solution by John Kugelman:

diff <(sed 's/ab[XY]d/abd/g' file1) <(sed 's/ab[XY]d/abd/g' file2)

is probably what you may be looking for! This version normalizes the specific change on each line without deleting the line itself. This allows diff to show any other differences that remain on the line.

Answer 3

Assuming X and Y are single characters, then -I 'ab[XY]d' works fine for me.

Answer 4

You could use sed to replace instances of the pattern with a standard string:

diff <(sed 's/ab[XY]d/ab__REPLACED__d/g' file1) <(sed 's/ab[XY]d/ab__REPLACED__d/g' file2)