Bad Ruby! No performance for you!

gistfile1.txt

# $~ and $_ are frame-local, but they're set across frames and you don't know when

def foo(a, b)
  p $~ # => nil
  a[b] # call to String#[], b is a regexp
  p $~ # => MatchData
end

foo("hello", /hel/)

# $_ may also be consumed without direct reference

def foo2
  gets # will set $_ in caller scope
  print # will access $_ in caller scope
end

foo2 # prints out whatever string you type in

def foo3(a)
  gets
  ~a
  p $~ # => MatchData
end

foo3(/hel/) # will match against string you type in

# Blocks can be used as bindings when turned into a Proc

def bar
  a = 1
  baz { puts a } # => 1
  puts a # => 2
end

def baz(&block)
  eval "a = 2", &block
end

bar