class RuboCop::Cop::Style::GlobalVars

Note that backreferences like $1, $2, etc are not global variables.
users can allow additional variables via the AllowedVariables option.
Built-in global variables are allowed by default. Additionally
It does not report offenses for built-in global variables.
This cops looks for uses of global variables.

def allowed_var?(global_var) 

def allowed_var?(global_var)
  BUILT_IN_VARS.include?(global_var) || user_vars.include?(global_var)
end

def check(node) 

def check(node)
  global_var, = *node
  add_offense(node, :name) unless allowed_var?(global_var)
end

def on_gvar(node) 

def on_gvar(node)
  check(node)
end

def on_gvasgn(node) 

def on_gvasgn(node)
  check(node)
end

def user_vars 

def user_vars
  cop_config['AllowedVariables'].map(&:to_sym)
end