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Ever needed to validate a domain name? This gem will validate any normalized domain name.
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Ever needed to validate a domain name? This gem will validate any domain name represented in ASCII.

The scope of this gem is deliberately focused on validating domain names. It simply answers the question: "Is this a real domain name?" Using this command, you can make a realistic assessment about whether you want to store a domain name or URL in your database. This gem will tell you 1) that a domain is or is not valid, and 2) if it's not valid, what the errors are.

There are other gems and libraries that parse domain names into their various components, or parse URLS, or properly handle Unicode domain names, etc. Use them; many of them are very good at their well-defined roles. But none of the ones that I came across were very good at simply telling me whether a domain names was valid, or WHY it was invalid if it failed the validations.

How It Works

To validate a domain name:

v =
if v.validate('')
  # Do something

What about error messages? If a domain isn't valid, it's often desirable to find out why the domain ewasn't valid. To do this, simply pass an array into the "validate" message as the optional second argument.

errs = []
v =
unless v.validate('keenertech.123', errs)
  puts("Errors: #{errs.inspect}")

This generates the following output:

Errors: ["The top-level domain (the extension) cannot be numerical"]

This gem should make it easy to validate domain names.

About Domain Names

Domain names provide a unique, memorizable name to represent numerically addressable Internet resources. They also provide a level of abstraction that allows the underlying Internet address to be changed while still referencing a resource by its domain name. The domain name space is managed by the Internet Corporation for Assigned Names and Numbers (ICANN).

The right-most label of a domain name is referred to as the top-level domain, or TLD. A limited set of top-level domain names, and two-character country codes, have been standardized. The Internet Assigned Numbers Authority (IANA) maintains an annotated list of top-level domains, as well as a list of "special use," or reserved, top-level domain names.

Domain names follow some very detailed rules:

  • The maximum length of a domain name is 253 characters.

  • A domain name is divided into "labels" separated by periods. The maximum number of labels is 127.

  • The maximum length of any label within a domain name is 63 characters.

  • No label, including TLDs, can begin or end with a dash.

  • Top-level domain names cannot be all numeric.

  • The right-most label must be either a recognized TLD or a 2-letter country code. The only exception is for international domain names, for which TLD checking is currently bypassed.

  • Top-level domain names cannot be all numeric.

  • Domain names may not begin with a period.

Internationalized Domain Names

What about internationalized domain names? ICANN approved the Internationalized Domain Name (IDNA) system in 2003. This standard allows for Unicode domain names to be encoded into ASCII using Punycode. Essentially, a label may contain "xn--" as a prefix, followed by the Punycode representation of a Unicode string, resulting in domain names such as Note that there are also some approved Unicode TLDs.

The process of rendering an internationalized domain name in ASCII via Punycode is called normalization. This gem will validate a normalized domain name, but not a Unicode domain name. Note, however, that it currently does not validate normalized TLDs against ICANN's list of valid TLDs.

It's also unclear whether the "xn--" prefix should count against the label size limit of 63 characters. In the absence of specific guidelines, and because I've never actually seen an overly long label, I have chosen to apply the limit irregardless of the presence of the "xn--" prefix within a label.


This is a Ruby gem with no run-time dependencies on anything else. It's only been tested under Ruby 1.9.3, but it should be compatible with all versions of Ruby more recent than Ruby 1.8.6.


Installation doesn't get much simpler than this:

gem install domain_name_validator

Road Map

More types of checks will be added as they are identified. Support for validating top-level domains (TLD's) is also in the works (it's a bit more complex than you might imagine). This will also bring in the capability to validate effective TLD's as well, i.e. - strings that are treated as top-level domains by registrars around the world.

Alternative Gems

If this domain_name_validator gem does not suit your needs, here are a few recommended gems that may provide you with the additional power (and complexity) that is deliberately absent from this highly focused gem:

  • domain_name - A full-featured gem for parsing/manipulating domain names.
  • ip_address - For everything you need to do with Ipv4 and Ipv6 addresses.


David Keener

He's a long-time Rubyist, with extensive experience both in the Internet startup world and government contracting. He is one of the founders of the RubyNation, DevIgnition and NationJS conferences. He speaks often at technical conferences, and blogs regularly on Internet-related subjects at


Many thanks for the support of General Dynamics and the Department of Homeland Security (DHS). And more specifically on input from Andrew Finch, Josh Lentz, Jonathan Quigg and Dave Roberts.


Please help me make this gem as useful as possible for its admittedly limited purpose. If ICANN's rules change, a clearer rules interpretation becomes available, or new valid TLDs are added, please help out by notifying me of potentially useful changes, or by submitting a pull request via GitHub.

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