require "yaml"

YAML_FRONT_MATTER_REGEXP = /\A(---\s*\n.*?\n?)^((---|\.\.\.)\s*$\n?)/m

def update_data(data)
  data.merge("A" => 2)
end

if $stdin.read =~ YAML_FRONT_MATTER_REGEXP
  data, content = YAML.load($1), Regexp.last_match.post_match
  data = update_data(data)
  YAML.dump(data, $stdout)
  $stdout.puts("---", content)
end