file.rb

str = '1/2'
regex = /(?<group1>\d+)\/(?<group1>\d+)/

str.gsub!(regex) do |match|
  match['group1'] = '3'
  match['group2'] = '4'
  match
end

stacktrace

IndexError: string not matched
	from (irb):9:in `[]='
	from (irb):9:in `block in irb_binding'
	from (irb):8:in `gsub!'
	from (irb):8