Simple class to handle SemVer (http://semver.org/).

semver.rb

class SemVer
  include Comparable

  attr_accessor :major, :minor, :patch, :prerelease, :build_metadata

  @@re = /^(0|[1-9]\d*)\.(0|[1-9]\d*)\.(0|[1-9]\d*)(?:-((?:0|[1-9A-Za-z-][0-9A-Za-z-]*)(?:\.(?:0|[1-9A-Za-z-][0-9A-Za-z-]*))*))?(?:\+((?:0|[1-9A-Za-z-][0-9A-Za-z-]*)(?:\.(?:0|[1-9A-Za-z-][0-9A-Za-z-]*))*))?$/

  def initialize(major, minor, patch, prerelease = "", build_metadata = "")
    @major = major.to_i
    @minor = minor.to_i
    @patch = patch.to_i
    @prerelease = prerelease.to_s
    @build_metadata = build_metadata.to_s
  end

  def valid?
    !(@@re !~ to_s)
  end

  def prerelease?
    @prerelease && !@prerelease.empty?
  end

  def self.parse(str)
    m = str.match(@@re)
    return nil unless m
    SemVer.new(*m[1..-1])
  end

  def to_s
    s = "#{@major}.#{@minor}.#{@patch}"
    s += "-#{@prerelease}" if prerelease?
    s += "+#{@build_metadata}" if @build_metadata && !@build_metadata.empty?
    s
  end

  def compare_major(other)
    return 1 if @major > other.major
    return -1 if @major < other.major
    return 0
  end

  def compare_minor(other)
    return 1 if @minor > other.minor
    return -1 if @minor < other.minor
    return 0
  end

  def compare_patch(other)
    return 1 if @patch > other.patch
    return -1 if @patch < other.patch
    return 0
  end

  def compare_major_minor_patch(other)
    c = compare_major(other)
    return c if c != 0
    c = compare_minor(other)
    return c if c != 0
    c = compare_patch(other)
    return c if c != 0
    return 0
  end

  def compare_prerelease(other)
    return 1 if !prerelease? && other.prerelease?
    return -1 if prerelease? && !other.prerelease?
    ids = @prerelease.split('.')
    other_ids = other.prerelease.split('.')
    ids_size = ids.size
    other_ids_size = other_ids.size
    min_size = ids_size < other_ids_size ? ids_size : other_ids_size
    for i in 0..min_size do
      next if ids[i] == other_ids[i]
      as_i = nil
      begin
        as_i = Integer(ids_[i])
      rescue
      end
      other_as_i = nil
      begin
        other_as_i = Integer(other_ids[i])
      rescue
      end
      return 1 if !as_i.nil? && other_as_i.nil?
      return -1 if as_i.nil? && !other_as_i.nil?
      return as_i <=> other_as_i if !as_i.nil? && !other_as_i.nil?
      return ids[i] <=> other_ids[i]
    end
    return 1 if ids_size > other_ids_size
    return -1 if ids_size < other_ids_size
    return 0
  end

  def <=>(other)
    c = compare_major_minor_patch(other)
    return c if c != 0
    c = compare_prerelease(other)
    return c if c != 0
    return 0
  end
end

semver_test.rb

require 'test/unit'
$:.unshift File.dirname(__FILE__) + '/.'
require 'semver'

class TestSemVer < Test::Unit::TestCase
  def test_parse
    s = '0.1.2'
    v = SemVer.parse(s)
    assert_true(v.valid?)
    assert_equal(0, v.major)
    assert_equal(1, v.minor)
    assert_equal(2, v.patch)
    assert_equal(s, v.to_s)

    s = '10.0.0-alpha.1.2+123'
    v = SemVer.parse(s)
    assert_true(v.valid?)
    assert_equal(10, v.major)
    assert_equal(0, v.minor)
    assert_equal(0, v.patch)
    assert_equal('alpha.1.2', v.prerelease)
    assert_equal('123', v.build_metadata)
    assert_equal(s, v.to_s)

    [
      '00.1.2',
      '0.1.2-$12',
      '0.1.2-alpha.12~3',
    ].each do |s|
      assert_nil(SemVer.parse(s))
    end
  end

  def test_compare
    [
      ['0.1.2', :==, '0.1.2'],
      ['0.1.2', :>=, '0.1.2'],
      ['0.1.2', :<, '0.1.3'],
      ['0.1.2', :<, '1.0.0'],
      ['1.0.0', :>, '0.100.100'],
      ['1.0.0-alpha', :<, '1.0.0-alpha.1'],
      ['1.0.0-alpha.1', :<, '1.0.0-alpha.beta'],
      ['1.0.0-alpha.1', :<, '1.0.0-beta'],
      ['1.0.0-beta', :<, '1.0.0-beta.2'],
      ['1.0.0-beta.2', :<, '1.0.0-beta.11'],
      ['1.0.0-beta.11', :<, '1.0.0-rc.1'],
      ['1.0.0-rc.1', :<, '1.0.0'],
    ].each do |c|
      assert_operator(SemVer.parse(c[0]), c[1], SemVer.parse(c[2]))
    end
  end
end