test.rb

require 'benchmark'

chars = 'abcdeABCDE!@#$%1234 '
res_str = ''
n = 1000
100_000.times do
  res_str << chars[rand(chars.length)]
end

Benchmark.bm(15) do |x|
  x.report('map downcase'){n.times{res_str.scan(/\w/).map(&:downcase)}}
  x.report('scan downcase'){n.times{res_str.scan(/\w/){|w| w.downcase}}}
  x.report('one downcase'){n.times{res_str.downcase.scan(/\w/)}}
end

                      user     system      total        real
map downcase     26.920000   0.440000  27.360000 ( 27.362158)
scan downcase    25.990000   0.010000  26.000000 ( 25.998966)
one downcase     18.540000   0.350000  18.890000 ( 18.892575)