Question

Rather than

str.downcase!
str.gsub!(/\W/, "")

it seems that I should be able to use multiple destructive String methods in succession:

str.downcase!.gsub!(/W/, "")

Sometimes this works, but sometimes it causes an error.

irb(main):001:0> str = "Foobar!"
"Foobar!"
irb(main):002:0> str.downcase!.gsub!(/\W/, "")
"foobar"
irb(main):003:0> str
"foobar"
irb(main):004:0> str.downcase!.gsub!(/\W/, "")
NoMethodError: undefined method `gsub!' for nil:NilClass
    from (irb):4
    from /usr/bin/irb:12:in `<main>'

Why is this?

Answer 1

Many destructive methods, including gsub, return nil if they don’t change anything. So it’s usually not a good idea to chain them. Instead use

str.downcase!
str.gsub!(/\W/, "")

or

str = str.downcase.gsub(/\W/, "")