irb(main):002:0> "question"[/q(?=u)/]
=> "q"
irb(main):003:0> "qu>"[/qu(?=>)/]
=> "qu"
irb(main):004:0> "qu"[/qu(?=>)/]
=> nil
irb(main):006:0> "accumulator"[/(?=c)cumulator/]
=> "cumulator"
irb(main):007:0> "accumulator"[/a(?=c)cumulator/]
=> nil
irb(main):009:0> "abc"[/abc(?=c)/]
=> nil
irb(main):010:0> "abc"[/ab(?=c)/]
=> "ab"

irb(main):011:0> "abd"[/ab(?!c)/]
=> "ab"
irb(main):012:0> "abc"[/ab(?!c)/]
=> nil

irb(main):013:0> "abc"[/a(?<=b)c/] # lookbehind doesn't work in Ruby
SyntaxError: compile error
(irb):13: undefined (?...) sequence: /a(?<=b)c/
        from (irb):13
        from :0
irb(main):014:0> "abc"[/(?<=a)bc/] # lookbehind doesn't work in Ruby
SyntaxError: compile error
(irb):14: undefined (?...) sequence: /(?<=a)bc/
        from (irb):14
        from :0

irb(main):022:0> "how's the weather?"[/how.*(?=the)/]
=> "how's the wea"
irb(main):024:0> "how's the weather?"[/how.*(?=[the])/]
=> "how's the weath"
irb(main):025:0> "how's the weather?"[/how.*(?=t)/]
=> "how's the wea"
irb(main):037:0> "how's the weather?"[/how.+(?=\bt)/]
=> "how's "

irb(main):057:0> "how's the weather?"[/ho.*(?=[a-z])/]
=> "how's the weathe"
irb(main):061:0> "how's the weather?"[/ho.*(?=['])/]
=> "how"