simple_bench.rb

require 'benchmark'

N = 1_000_000
REGEX = /^abc/
STR = 'abc are letters'.freeze

Benchmark.bm(13) do |x|
  x.report('start_with?') do
    N.times do
      STR.start_with?('abc')
    end
  end

  x.report('regex') do
    N.times do
      STR =~ REGEX
    end
  end
end

                    user     system      total        real
start_with?     0.115529   0.000000   0.115529 (  0.116792)
regex           0.187361   0.000909   0.188270 (  0.189854)