Ignore case partially in Ruby

Try phosphorescence: Ignore case partially in Ruby

$ irb
irb(main):001:0> /(?i:a)bc(?i:d)/ =~ "AbcD"
=> 0
irb(main):002:0> $~
=> #<MatchData "AbcD">
irb(main):003:0> /(?i:a)bc(?i:d)/ =~ "abcd"
=> 0
irb(main):004:0> /(?i:a)bc(?i:d)/ =~ "Abcd"
=> 0
irb(main):005:0> /(?i:a)bc(?i:d)/ =~ "abcD"
=> 0
irb(main):006:0> /(?i:a)bc(?i:d)/ =~ "ABCD"
=> nil