Ruby.scala is a simple parser written in the language Scala by using 
parser combinators for ISO Ruby, ISO/IEC 30170:2012. It is written by 
Dr. Ikuo Nakata, the chairman of the working group for the standardization 
of the language Ruby, with the help by Dr. Kota Mizushima, a deputation of 
Japan Scala Users Group and working at Ubiregi Inc.

Ruby.scala directly follows the syntax of ISO Ruby, except the following:

1. The left recursions are eliminated.

2. A local-variable-identifier which begins with the letter 'm' is 
 considered as a method name. Any other local-variable-identifiers are 
 considered as local variable names.

3. The nested brackets in an array-literal are not permitted.

4. The production rules for multiple-right-hand-side, class-definition,
 method-definition and singleton-method-definition are corrected as follows:

 11.4.2.4
 multiple-right-hand-side :: 
   operator-expression-list [no line-terminator here] , splatting-right-hand-side 
 | operator-expression [no line-terminator here] , operator-expression-list
 | splatting-right-hand-side

 13.2.2
 class-definition :: 
   class class-path ( [no line-terminator here] < superclass )? 
     separator class-body end 

 13.3.1
 method-definition :: 
   def defined-method-name 
     ( [no line-terminator here] method-parameter-part | separator )
     method-body end

 13.4.3
 singleton-method-definition :: 
   def singleton-object ( . | :: ) defined-method-name 
     ( [no line-terminator here] method-parameter-part | separator )
     method-body end

5. "method-identifier block" in the right hand of production of primary-method-
 invocation (11.3.1) is changed to "method-identifier block?".

6. Expressions which end with a do-block are not permitted as the expression in 
 while-expression, until-expression or for-expression.

Usage:

  $ scalac Ruby.scala

  $ scala -cp . RubyParser test.rb

Example 1:
 ------ test.rb --------
 class X 
   @@a = 'abc' 
   def mprint_a 
     mputs @@a 
   end 
   def mset_a(value) 
     @@a = value 
   end 
 end
 -----------------------

 ------- output --------
 class X 
 var-assign-ex(@@a='abc')
 def mprint_a;
 method-inv-without-par(command(mputs @@a)) end-def
 def mset_a(value)
 var-assign-ex(@@a=value) end-def
 end-class
 ----------------------

Example 2:
 ------ test.rb --------
<<ABC + <<XXX
efg
ABC
pqr
XXX
----------------------

------- output --------
(Heredoc("efg
") + Heredoc("pqr
"))
----------------------

The output is customizable by modifying each Parser object
corresponding to a production rule.
The current output is a string, and the string can be easily modified 
by changing the output strings of relevant Parser objects.