run.rb

require 'fileutils'

Dir['test/**.jpg'].each do |f|
	dirname = File.dirname(f)
	filename = File.basename(f)
	new_filename = filename.gsub(/IMG_([0-9]+)/, "reU-")
	puts "#{dirname}/#{filename} => #{dirname}/#{new_filename}"
	FileUtils.mv "#{dirname}/#{filename}", "#{dirname}/#{new_filename}"
end

test.rb

Dir['test/**.jpg'].each do |f|
	filename = File.basename(f)
	new_filename = filename.gsub(/IMG_([0-9]+)/, "reU-")
	puts "#{filename} => #{new_filename}"
end