class XSemVer::SemVer

def self.parse(version_string, format = nil, allow_missing = true)
  format ||= TAG_FORMAT
  regex_str = Regexp.escape format
  # Convert all the format characters to named capture groups
  regex_str = regex_str.gsub('%M', '(?<major>\d+)').
    gsub('%m', '(?<minor>\d+)').
    gsub('%p', '(?<patch>\d+)').
    gsub('%s', '(?:-(?<special>[A-Za-z][0-9A-Za-z\.]+))?')
  regex = Regexp.new(regex_str)
  match = regex.match version_string
  if match
      major = minor = patch = nil
      special = ''
      # Extract out the version parts
      major = match[:major].to_i if match.names.include? 'major'
      minor = match[:minor].to_i if match.names.include? 'minor'
      patch = match[:patch].to_i if match.names.include? 'patch'
      special = match[:special] || '' if match.names.include? 'special'
      # Failed parse if major, minor, or patch wasn't found
      # and allow_missing is false
      return nil if !allow_missing and [major, minor, patch].any? {|x| x.nil? }
      # Otherwise, allow them to default to zero
      major ||= 0
      minor ||= 0
      patch ||= 0
      SemVer.new major, minor, patch, special
  end
end