How to define class level constant regular expressions in Ruby to be used by external classes

Question

I'm writing some tests to verify the behaviour of some regular expressions to be used within a Ruby console application. I'm trying to define constant class level fields on a class not meant to be instantiated (just supposed to have constant RE values defined on it. I'm having trouble defining this properly using Ruby idiom (I have C++/C# background).

First I tried to define a class constant

class Expressions
  # error is on following line (undefined method DATE)
  Expressions.DATE = /(?<Year>\d{4})-(?<Month>\d{2})-(?<Day>\d{2})/

end

class MyTest < Test::Unit::TestCase
  def setup
    @expression = Expressions::DATE
  end

  def test
    assert "1970-01-01" =~ @expression
  end
end

this just produces error: undefined method `DATE=' for Expressions:Class (NoMethodError)

Next I tried class attributes:

class Expressions
  @@Expressions.DATE = /(?<Year>\d{4})-(?<Month>\d{2})-(?<Day>\d{2})/ 
end

class MyTest < Test::Unit::TestCase
  def setup
    # NameError: uninitialized constant Expressions::DATE here:
    @expression = Expressions::DATE
  end

  def test
    assert "1970-01-01" =~ @expression
  end
end

This produces an NameError: uninitialized constant Expressions::DATE error.

I know I could just define attributes on a class to be used as an instance, but this is inefficient and not a correct solution to the problem (just a hack). (In C++ I would use a static const, done)

So I'm stuck really. I need to know what is the correct way in Ruby to define constant regular expressions that need to be used in other classes. I'm having a problem with the definition, initialisation and its use,

thanks.

Answer 1

A constant in Ruby is like a variable, except that its value is supposed to remain constant for the duration of a program. The Ruby interpreter does not actually enforce the constancy of constants, but it does issue a warning if a program changes the value of a constant. Lexically, the names of constants look like the names of local variables, except that they begin with a capital letter. By convention, most constants are written in all uppercase with underscores to separate words, LIKE_THIS. Ruby class and module names are also constants, but they are conventionally written using initial capital letters and camel case, LikeThis.

The Ruby Programming Language: David Flanagan; Yukihiro Matsumoto.

This should work:

class Expressions
  DATE = /.../
end

class MyTest < Test::Unit::TestCase
  def setup
    @expression = Expressions::DATE
  end
  # ...
end