Regular expressions

compress_strings.rb

#!/usr/bin/env ruby

# if a character is followed by itself 1 or more times it is considered a duplicate
#
# the regular expression /(?<ct>(?<c>.)\k<c>{1,})|((?<d>.)\k<d>{0})/
# * capture group c captures a character
# * capture group ct captures the total string of duplicately matched charactesr
# * capture group d matches a character on its own
#
# scan therefore produces the following
#   => [["aaa", "a", nil], [nil, nil, "b"], ["ccc", "c", nil]]
#
# which we can then combine into a compressed string using map
#   => "a3bc3"
"aaabccc"
  .scan(/(?<ct>(?<c>.)\k<c>{1,})|((?<d>.)\k<d>{0})/)
  .map { |dups, dup, single| dups.nil? ? single : "#{dup}#{dups.length}" }
  .join