reverse.rb

input = "abc cba"                                                                          # => "abc cba"
input = "cB!a This is a valid\n input, you have to read the specification carefully! abc"  # => "cB!a This is a valid\n input, you have to read the specification carefully! abc"

input = input.downcase  # => "cb!a this is a valid\n input, you have to read the specification carefully! abc"

input = input.gsub(/\W/) { |c|  # => "cb!a this is a valid\n input, you have to read the specification carefully! abc"
  c =~ /\s/ ? " " : ""          # => "", " ", " ", " ", " ", " ", " ", "", " ", " ", " ", " ", " ", " ", " ", "", " "
}                               # => "cba this is a valid  input you have to read the specification carefully abc"

arr = input.split(" ")  # => ["cba", "this", "is", "a", "valid", "input", "you", "have", "to", "read", "the", "specification", "carefully", "abc"]

result = []  # => []

while !arr.empty?                               # => true, true, true, true, true, true, true, true, true, true, true, true, true, true, false
  v = arr.shift                                 # => "cba", "this", "is", "a", "valid", "input", "you", "have", "to", "read", "the", "specification", "carefully", "abc"
  if arr.include?(v.reverse) && v != v.reverse  # => true, false, false, false, false, false, false, false, false, false, false, false, false, false
    result << v                                 # => ["cba"]
    result << v.reverse                         # => ["cba", "abc"]
    # arr.delete v.reverse                        # => "abc"
  end                                           # => ["cba", "abc"], nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil
end                                             # => nil

puts result.join(",")  # => nil

# >> cba,abc