require 'benchmark'

Benchmark.bm do |x|
  x.report do
    t = "Holy shit! I THINK I JUST SAW A ZUNE. This is a sign from the elder gods that means my trip shall bring me good fortune, right?"
    t = t.gsub(/(^|\s+)(?:I|me)([[:punct:]]|\s+|$)/i,' you ')
  end

  x.report do
    t = "Holy shit! I THINK I JUST SAW A ZUNE. This is a sign from the elder gods that means my trip shall bring me good fortune, right?"
    t.gsub!(/(^|\s+)(?:I|me)([[:punct:]]|\s+|$)/i,' you ')
  end
end

#    user     system      total        real
# 0.000000   0.000000   0.000000 (  0.000053)
# 0.000000   0.000000   0.000000 (  0.000040)