Explaining Ruby regex's /o modifier

regex-o.rb

require "benchmark"

def letters
  puts "letters() called"
  sleep 0.5
  "A-Za-z"
end

words = %w[the quick brown fox jumped over the lazy dog]

Benchmark.bm do |bm|
  bm.report("without /o:\n") do
    words.each do |word|
      puts "Matches!" if word.match(/\A[#{letters}]+\z/)
    end
  end

  bm.report("with /o:\n") do
    words.each do |word|
      puts "Matches!" if word.match(/\A[#{letters}]+\z/o)
    end
  end
end

An explanation:

The o modifier causes the interpolation to happen once; the result of the interpolated regex is, for all intents and purposes, memoised.

So if you're matching a regular expression in a loop, and interpolating something that's the result of an expensive calculation (here simulated with a sleep), using the o modifier will greatly speed up your code.

Admittedly, those are uncommon circumstances. But it's one to keep in mind.