Ryan Wood

Product Builder, Rubyist, Entrepreneur of sorts
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12 Jun 2009
Slither: A Dsl For Parsing Fixed Width Text Files

Given the rage over XML and web services over the past decade or so, I was shocked at how common it is to still exchange data using a fixed-width file format over FTP. I guess it’s all about those mainframes and backward compatibility, but “WOW”.

At work I have had numerous projects (mostly in Ruby) that involve creating and/or reading these fixed-width text files. Occasionally, I have the luxury of a CSV file and can use FasterCSV to take care of business. To date, I haven’t had a single project that used XML.

Given the need, I decided to create an open source project. I had to work on it primarily on my own time, as our work environment is not what you would call “open source friendly”. So, slither was born.


Here’s a simple example. Assume you have the following file format:


Here’s a sample file (cars.txt).

200606011999BMW                 325                Smith      45500
200506011993Honda               Accord             Wood        7500
200601022006Lexus               350                Johnson    25365

Given this file, we would first need to create a slither definition.

Slither.define :cars do |d|
  d.section, :body do |body|
    # The trap is not very useful here since there is only one section
    body.trap { |line| line =~ /^\d{8}/ }
    body.column :date, 8
    body.column :year, 4
    body.column :make, 20, :align => :left
    body.column :model, 20, :align => :left
    body.column :name, 10, :align => :left
    body.column :price, 6


From there we could parse the above file like this:

# Arguments are the file path and the definition name
parsed_data = Slither.parse('cars.txt', :cars).inspect

It should return a hash with nested arrays keyed by section.

result = {
  :body => [
    { :date => '20060601', 
      :year => '1999', 
      :make => 'BWW', 
      :model => '325', 
      :name => 'Smith', 
      :price => '45500' },


To generate a file, simply do the opposite. Create a properly formatted hash and call generate:

# Generates the file as a string
puts Slither.generate(:cars, my_car_hash)

# Writes the file
Slither.write('cars.txt', :cars, my_car_hash)

Just the Beginning

There are many more things that Slither can do:

  • headers and footers
  • templated sections
  • typecasting
  • formatting
  • validation (not quite there yet)

So go ahead and install the gem or fork the Github project if you’re interested in hacking.

# Run the following if you haven't already:
gem sources -a http://gems.github.com
# Install the gem(s):
sudo gem install ryanwood-slither
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