Demonstrating how Ruby's lazy enumerators and enum_for work.

each_char_matching_pattern.rb

def each_char_matching_pattern(io, pattern)
  return enum_for(:each_char_matching_pattern, io, pattern) unless block_given?

  io.each_line.lazy.select do |line|
    line =~ pattern
  end.flat_map do |line|
    line.chars
  end.each do |line|
    yield line
    puts "\toffset: #{io.pos}"
  end
end

lazy_stream = each_char_matching_pattern(DATA, /foo/)

lazy_stream.each do |char|
  print char
end

__END__
foo bar
baz
bux
bar foo