DB<7> $U=qr/U/                # always upper case


  DB<8> $i=qr/i${U}i/i          # surrounding case insensitive


  DB<9> p $i
(?^ui:i(?^u:U)i)

  DB<10> p 'iui' =~ $i


  DB<11> p 'iUi' =~ $i
1

  DB<12> p 'IUI' =~ $i
1

  DB<13> p 'IuI' =~ $i


  DB<14> p join "\n", grep { $_ =~ $i } <{i,I}{u,U}{i,I}>
iUi
iUI
IUi
IUI