Here is a listing from my irb session, as you can see I'm learning character sets with regular expressions. The characters ('a' and 'e') within the square brackets are treated as operands which are XORed between the characters 'h' and 'y'.
irb(main):155:0> "hello"[/[ae]/] => "e" irb(main):156:0> "hello"[/h[ae]y/] => nil irb(main):157:0> "hay"[/h[ae]y/] => "hay" irb(main):158:0> "howdy"[/h[ae]y/] => nil irb(main):159:0> "hardy"[/h[ae]y/] => nil irb(main):160:0> "hardy"[/h[ae]/] => "ha" irb(main):161:0> "hardy"[/h[ae]y/] => nil irb(main):162:0> "hoy"[/h[ae]y/] => nil irb(main):165:0> "they"[/h[ae]y/] => "hey" irb(main):166:0> "they would"[/h[ae]y/] => "hey"
