Ruby 1.9 regex, named capture groups to local variables

regex_a.rb

/(?<area_code>\d{3}) (?<local_number>\d{3}-\d{4})/ =~ "123 456-7890"
local_variables     # => [:area_code, :local_number]
area_code           # => "123"
local_number        # => "456-7890"

regex_b.rb

regex = /(?<area_code>\d{3}) (?<local_number>\d{3}-\d{4})/
regex =~ "123 456-7890"
local_variables     # => [:regex]

# also doesn't work if you switch string and regex in eval
"123 456-7890" =~ /(?<area_code>\d{3}) (?<local_number>\d{3}-\d{4})/ 
local_variables # => []