module Regexp::Expression

def self.parsed(exp)
  case exp
  when String
    Regexp::Parser.parse(exp)
  when Regexp
    Regexp::Parser.parse(exp.source)
  when Regexp::Expression
    exp
  else
    raise ArgumentError, 'Expression.parsed accepts a String, Regexp, or '\
                         'a Regexp::Expression as a value for exp, but it '\
                         "was given #{exp.class.name}."
  end
end