Subdomains validation, inspired by Matthew Hutchinson.

subdomain_validator.rb

# Each subdivision can go down to 127 levels deep, and each DNS label can contain up to 63 characters,
# as long as the whole domain name does not exceed a total length of 255 characters.
class SubdomainValidator < ActiveModel::EachValidator
  def validate_each(object, attribute, value)
    return unless value

    reserved_names = %w[admin beta blog ftp imap mail pop pop3 sftp smtp ssl www]
    reserved_names += options[:reserved] if options[:reserved]
    
    object.errors[attribute] << 'cannot be a reserved name' if reserved_names.include?(value.downcase)
    object.errors[attribute] << 'must have between 3 and 63 characters' unless (3..63) === value.length
    object.errors[attribute] << 'cannot start or end with a hyphen' unless value =~ /\A[^-].*[^-]\z/i
    object.errors[attribute] << 'must be alphanumeric; A-Z, 0-9 or hyphen' unless value =~ /\A[a-z0-9\-]*\z/i
  end
end

subdomain_validator_spec.rb

require 'spec_helper'

describe SubdomainValidator do
  it "valids subdomain" do
    foo = Foo.new(subdomain: 'bar')
    foo.valid?
    expect(foo.errors[:subdomain]).to be_empty
  end
  it "does not valid subdomain using a reserved name" do
    foo = Foo.new(subdomain: 'WWW')
    foo.valid?
    expect(foo.errors[:subdomain]).to include 'cannot be a reserved name'
  end
  it "does not valid subdomain length less than 3 characters" do
    foo = Foo.new(subdomain: 'ab')
    foo.valid?
    expect(foo.errors[:subdomain]).to include 'must have between 3 and 63 characters'
  end
  it "does not valid subdomain length greater than 63 characters" do
    foo = Foo.new(subdomain: ('a' * 64))
    foo.valid?
    expect(foo.errors[:subdomain]).to include 'must have between 3 and 63 characters'
  end
  it "does not valid hyphen at the beginning of the subdomain" do
    foo = Foo.new(subdomain: '-bar')
    foo.valid?
    expect(foo.errors[:subdomain]).to include 'cannot start or end with a hyphen'
  end
  it "does not valid hyphen at the end of the subdomain" do
    foo = Foo.new(subdomain: 'bar-')
    foo.valid?
    expect(foo.errors[:subdomain]).to include 'cannot start or end with a hyphen'
  end
  it "does not valid subdomain with invalid name" do
    foo = Foo.new(subdomain: 'bar.beta')
    foo.valid?
    expect(foo.errors[:subdomain]).to include 'must be alphanumeric; A-Z, 0-9 or hyphen'
  end
end