A long-lived project that still receives updates
A ruby library for working with Machine Readable Cataloging


~> 1.0


>= 1.0.1, < 2
>= 0
>= 0
 Project Readme

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marc is a ruby library for reading and writing MAchine Readable Cataloging (MARC). More information about MARC can be found at



reader ="myfile.mrc")
reader.each do |record|
  first_245 = record["245"] #=> #<MARC::DataField...>
  first_245.to_s #=> "245 04 $a The Texas ranger $h [sound recording] / $c Sung by Beale D. Taylor. "
  first_245.value #=> "The Texas ranger[sound recording] /Sung by Beale D. Taylor." #=> ["a", "h", "c"]
  first_245["a"] #=> "The Texas ranger"
  # A record is an enumerable over its fields and thus can use things like
  # #each, #select, #find, etc.
  subject_fields ={|f| f.tag =~ /\A6/}
  # Get author fields by supplying a list of tags
  record.fields.each_by_tag(["100", "110", "111"]) do |field|
    puts field.value

Reading / Writing MARC21 binary data

require 'marc'

# marc21 binary format uses MARC::Reader and MARC::Writer

reader ='marc.dat')
reader.each do |record|
  title = record["245"].value
  puts title

If you know you have another encoding, you can specify it

reader ="marc.dat", external_encoding: "MARC-8")

While generally used with files, you can also give a reader an IO object (usually an already-opened file or a StringIO object)

marc_data ="marc.dat")
reader =

Similarly, you can write to either a file or an IO-like object

writer ="myfile.dat")
# writer ="myfile.dat.gz"))

myrecords.each do |rec|

Reading/Writing marc-in-json

marc-in-json is a simple hash-based serialization format for MARC, often used with the jsonl (aka jsonlines or newline-delimited-json) file format which puts one json structure on each line.

reader ="myfile.jsonl")
writer ="my_other_file.jsonl")
reader.each do |record|

Reading/Writing MARC-XML

MARC-XML is an XML-based serialiation format for MARC records. It is, generally speaking, a lot slower than using MARC21 or marc-in-json.

There are two XML parsers supported going forwards within the ruby-marc code base: REXML (the first, and for a long time only, ruby XML parser based on regular expressions) and Nokogiri. Both are compatible with both MRI ("normal") ruby and JRuby.

The Nokogiri parser is about 6x faster than using REXML. See performance numbers, below.

At one time, it was difficult to install Nokogiri under MRI and impossible under JRuby. Because of this historical blip, nokogiri is not automatically included when doing require "marc" in your code. If you want to use the Nokogiri-based parser, you must include it explicitly.

require "nokogiri"
require "marc"

reader ="myfile.xml", parser: "nokogiri")

The parser argument works as follows:

  • if not included, REXML is used
  • if "rexml" or "nokogiri", the appropriate parser will be used
  • if "magic", the Nokogiri parser will be used if Nokogiri has been loaded; otherwise it will fall back to using REXML.
# Use the best available
reader ="my_file.xml", parser: "magic")

"Self-closing" writers

Much like one can open a file and have it automatically close at the end of a block in standard ruby, the various writers will do the same.

# separate writer and #close
reader ="my_marc.mrc")
writer ="my_marc.xml")
reader.each do |record|

# "self-closing" equivalent
reader ="my_marc.mrc")"my_marc.xml") do |w|
  reader.each do |record|
# no need to close the writer here

Serializing a single record

The MARC::Record class has utility functions to serialize to the various formats. These are generally thin wrappers around the encode class methods (e.g., MARC::Writer.encode, MARC::XMLWriter.encode, etc.)

  • record.to_marc will production a marc21 binary string
  • record.to_json_string returns a string containing the JSON document for the marc-in-json serialization
    • This just json-ifies record.to_hash, which returns a hash compatible with the marc-in-json format.
  • record.to_xml_string returns the actual XML string, with the following options:
    • include_namespace: true (default: true) will include the MARC namespace attributes
    • fast_but_unsafe: true (default: false) will use the much faster MARC::UnsafeXMLWriter code, which produces the XML by string concatenation. See that class for more information, but in general, if your MARC isn't wildly invalid, it works fine and is roughly 15x faster. The default (REXML) simply does record.to_xml.to_s

Note that * record.to_xml, for historical reasons, returns an REXML document of the XML serialization and not an XML string as one might expect.

Benchmarking reading MARC in various formats

A simple benchmark run on a single thread on a 2017-era x64 Macintosh gives the numbers below.

With mri 3.1.0  and  jruby	

Format    Implementation  Ruby  r/sec  x Slower compared to fastest  
jsonl     stdlib JSON     mri   6512  1.0  
jsonl     O j             mri   6199  1.0  
marc21    MARC::Reader    mri   2889  2.3  
marc-xml  Nokogiri        mri   1451  4.6  
marc-xml  REXML           mri   239   28.0  

marc21    MARC::Reader    jruby  5455  1.2  
jsonl     stdlib JSON     jruby  5437  1.2  
marc-xml  Nokogiri        jruby  1631  4.1  
marc-xml  REXML           jruby  253   26.5  

Note especially that if you're using MARC-XML, Nokogiri will read in records 4-5 times faster.

Character Encoding issues

The Marc binary (ISO 2709) Reader (MARC::Reader) has some features for helping you deal with character encodings in ruby 1.9. It is always recommended to supply an explicit :external_encoding option to MARC::Reader; either any valid ruby encoding, or the string "MARC-8".
MARC-8 input will by default be transcoded to a UTF-8 internal representation.

MARC::Reader does not currently have any facilities for guessing encoding from MARC21 leader byte 9, that is ignored.

Consult the MARC::Reader class docs for a more complete discussion and range of options.

The MARC binary Writer (MARC::Writer) does not have any encoding-related features -- it's up to you the developer to make sure you create MARC::Records with consistent and expected char encodings, although MARC::Writer will write out a legal ISO 2709 either way, it just might have corrupted encodings.

When parsing MARCXML with Nokogiri as your XML parser implementation up to and including version 1.0.2 of this gem, if the XML was badly formed, parsing would stop and no error would be reported to your code.

If you are using a version > 1.0.2 of ruby-marc with MRI + Nokogiri, XML syntax errors will be thrown (and you may need to adjust your code to account for this). JRuby users: If you are using a version later than 1.0.2 and using Nokogiri as an XML parser with JRuby as your ruby implementation, XML syntax errors will still be ignored unless you have Nokogiri version 1.10.2 or later.

JRubySTAXReader caveats

NOTE: The JRubyStaxReader is deprecated. Nokogiri should be used instead.

  • Under Java 9+, MARC::JRubySTAXReader requires adding the following to JAVA_OPTS in order to work around Java module system restrictions:

    --add-opens java.xml/
  • MARC::JRubySTAXReader is deprecated and will be removed in a future version of ruby-marc. Please use MARC::JREXMLReader or MARC::NokogiriReader instead.


Source code at:

Find generated API docs at:

Run automated tests in source with rake test.

Developers, release new version of gem to rubygems with rake release (bundler-supplied task). Note that one nice thing this will do is automatically tag the version in git, very important for later figuring out what's going on.


gem install marc

Or if you're using bundler, add to your Gemfile

gem 'marc'


Kevin Clarke Bill Dueber William Groppe Ross Singer Ed Summers