diffutils: Case Folding

 
 1.5 Suppressing Case Differences
 ================================
 
 GNU 'diff' can treat lower case letters as equivalent to their upper
 case counterparts, so that, for example, it considers 'Funky Stuff',
 'funky STUFF', and 'fUNKy stuFf' to all be the same.  To request this,
 use the '-i' or '--ignore-case' option.