regexp_compilation.rb

require 'pathname'
SEPARATOR_PAT = Pathname::SEPARATOR_PAT

def chop_basename(path)
    base = File.basename(path)
    if /\A#{SEPARATOR_PAT}?\z/ =~ base
        return nil
    else
        return path[0, path.rindex(base)], base
    end
end

RE = Regexp.new("/\A#{SEPARATOR_PAT}?\z/")
def chop_basename2(path)
    base = File.basename(path)
    if RE =~ base
        return nil
    else
        return path[0, path.rindex(base)], base
    end
end



TIMES=100000

start = Time.now
TIMES.times do
    chop_basename('aewsome_file.rb')
end
puts Time.now - start
start = Time.now

start = Time.now
TIMES.times do
    chop_basename2('aewsome_file.rb')
end
puts Time.now - start
start = Time.now

RE shouldn't have / at the beginning and end.

$ ruby -e 'SEPARATOR_PATH="/"; RE=/\A#{SEPARATOR_PATH}?\z/; puts "#{RE}"'
(?-mix:\A\/?\z)

$ ruby -e 'SEPARATOR_PATH="/"; RE=Regexp.new("/\A#{SEPARATOR_PATH}?\z/"); puts "#{RE}"'
(?-mix:\/A\/?z\/)

I don't know if that will impact your benchmarks necessarily, but you definitely aren't measuring the same thing. :(