Using $1, $2, etc. global variables inside method definition

Question

Given the following two pieces of code:

def hello(z)
  \"hello\".gsub(/(o)/, &z)
end
z = proc {|m| p $1}
hello(z)
# prints: nil

---

Answer 1

Things like $1, $2 acts like LOCAL VARIABLES, despite its leading $. You can try the code below to prove this:

def foo
  /(hell)o/ =~ 'hello'
  $1
end

def bar
  $1
end

foo #=> "hell"
bar #=> nil

Your problem is because the proc z is defined outside the method hello, so z accesses the $1 in the context of main, but gsub sets the $1 in the context of method hello.