How can I ignore some differences in the 'diff' command?

diff has an option, -I regexp, which ignores changes that just insert or delete lines that match the given regular expression. I need an analogue of this for the case, when changes are between two lines (rather than 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 doesn't have any such kind of ability. Is there a suitable alternative for diff?