sub.rb

pry(main)> def hello(friends)
pry(main)*   puts "my friends i have come here to say hello"
pry(main)*   puts "hello is an unusual word, but is appropriate at a time like this"
pry(main)*   puts "when it has been so long since we last met"
pry(main)*   puts "before the dogs were put to sleep"
pry(main)* s/hello/goodbye
1: def goodbye(friends)
2:   puts "my friends i have come here to say goodbye"
3:   puts "goodbye is an unusual word, but is appropriate at a time like this"
4:   puts "when it has been so long since we last met"
5:   puts "before the dogs were put to sleep"
pry(main)* s/dogs/cats
1: def goodbye(friends)
2:   puts "my friends i have come here to say goodbye"
3:   puts "goodbye is an unusual word, but is appropriate at a time like this"
4:   puts "when it has been so long since we last met"
5:   puts "before the cats were put to sleep"
pry(main)* end
=> nil

command /s\/(.*?)\/(.*?)/ do |source, dest|
  eval_string.gsub!(/#{source}/) { dest }
  run "show-input"
end