Ruby confusion

block.rb

# Ruby's String#gsub can take a block argument.
'foo'.gsub(/f(.)/) { Regexp.last_match[1] }
=> "oo"


# Here's a method that takes a block argument and passes it to String#gsub
def do_thing(&block)
  'foo'.gsub(/f(.)/, &block)
end

# Here you can see that it works.
do_thing { 'hey' }
# => "heyo"

# But it doesn't work when we try to use Regexp.last_match :(
do_thing { Regexp.last_match[1] }
NoMethodError: undefined method `[]' for nil:NilClass
        from (irb):11:in `block in irb_binding'
        from (irb):8:in `gsub'
        from (irb):8:in `do_thing'
        from (irb):11
        from /Users/jcoyne/.rbenv/versions/2.4.1/bin/irb:11:in `<main>'