I wish to request an enhancement to diff which would add a command line option which takes a regular _expression_ and suppresses differences none of whose lines match the regular _expression_. Multiple instances of this option can be used to specify multiple regular expressions, which cases differences will be suppressed if none of the lines of the difference match any of the regular expressions. (Such a feature would be more or less the complement of the existing --ignore-matching-lines=regexp (-I regexp) command line option to diff.)
Please let me know if there already is way to accomplish this using existing options to GNU diff and/or by the use of other GNU Diffutils.
The grepdiff tool (from patchutils) might be able to do what you want, perhaps in conjunction with other tools: