string_sub_bm.rb

require 'benchmark'

N = 10_000_000

Benchmark.bmbm do |bm|
  bm.report('String') do
    N.times do
      a = 'foobarfoobarhmm'
      a.sub('foo', 'BAR')
    end
  end

  bm.report('Regexp') do
    N.times do
      a = 'foobarfoobarhmm'
      a.sub(/foo/, 'BAR')
    end
  end
end

Interesting result. It is surprise for me that RefExp version can be faster. I was using string for simple cases always.