regexps.rb

# The regexp is re-compiled for every line in the file, which is SLOW
# If you are processing a lot of data, inline regexps are to be avoided!
class SlowWorker
  def initialize(custom)
    @custom = custom
  end
  
  def call(file)
    File.open(file,'r') do |file|
      file.each_line do |line|
        puts if line =~ /my arbitrary #{@custom} regexp/
      end
    end
  end
end

# The regexp is compiled once, which is FAST.
# The regexp is always the same, so a constant makes sense for storing it.
class FastWorker
  REGEXP = /my arbitrary regexp/
  
  def call(file)
    File.open(file,'r') do |file|
      file.each_line do |line|
        puts if line.match(REGEXP)
      end
    end
  end
end

# If your regexp depends on some custom values, create it once when initializing the class.
class CustomFastWorker
  def initialize(value)
    @regexp = /my arbitrary #{value}/
  end
  
  def call(file)
    File.open(file,'r') do |file|
      file.each_line do |line|
        puts if line.match(@regexp)
      end
    end
  end
end