a = "hello there"
   a[1]                   #=> "e"
   a[1,3]                 #=> "ell"
   a[1..3]                #=> "ell"
   a[-3,2]                #=> "er"
   a[-4..-2]              #=> "her"
   a[12..-1]              #=> nil
   a[-2..-4]              #=> ""
   a[/[aeiou](.)\1/]      #=> "ell"
   a[/[aeiou](.)\1/, 0]   #=> "ell"
   a[/[aeiou](.)\1/, 1]   #=> "l"
   a[/[aeiou](.)\1/, 2]   #=> nil
   a["lo"]                #=> "lo"
   a["bye"]               #=> nil