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Ruby is a reflective, object-oriented programming language. It combines syntax inspired by Ada and Perl with Smalltalk-like object-oriented features, and also shares some features with Python, Lisp, Dylan and CLU. Ruby is a single-pass interpreted language. Its main implementation is Free software distributed under an open-source license.
Matz's primary design consideration is to make programmers happy by reducing the menial work they must do, following the principles of good user interface design.  He stresses that systems design needs to emphasize human, rather than computer, needs :
- Often people, especially computer engineers, focus on the machines. They think, "By doing this, the machine will run faster. By doing this, the machine will run more effectively. By doing this, the machine will something something something." They are focusing on machines. But in fact we need to focus on humans, on how humans care about doing programming or operating the application of the machines. We are the masters. They are the slaves.
Ruby is said to follow the principle of least surprise (POLS), meaning that the language typically behaves intuitively or as the programmer assumes it should. The phrase did not originate with Matz and, generally speaking, Ruby may more closely follow a paradigm best termed as "Matz's Least Surprise", though many programmers have found it to be close to their own mental model as well.
Matz defined it this way in an interview: "Everyone has an individual background. Someone may come from Python, someone else may come from Perl, and they may be surprised by different aspects of the language. Then they come up to me and say, 'I was surprised by this feature of the language, so therefore Ruby violates the principle of least surprise.' Wait. Wait. The principle of least surprise is not for you only. The principle of least surprise means principle of least my surprise. And it means the principle of least surprise after you learn Ruby very well. For example, I was a C++ programmer before I started designing Ruby. I programmed in C++ exclusively for two or three years. And after two years of C++ programming, it still surprised me."
Ruby is object-oriented: every bit of data is an object, even classes and types many other languages designate primitive such as integers. Every function is a method. Named values (variables) always designate references to objects, not the objects themselves. Ruby supports inheritance with dynamic dispatch, mixins and singleton methods (belonging to, and defined for, a single instance rather than being defined on the class). Though Ruby does not support multiple inheritance, classes can import modules as mixins. Procedural syntax is supported, but everything done in Ruby procedurally (that is, outside of the scope of a particular object) is actually done to an Object instance named 'main'. Since this class is parent to every other class, the changes become visible to all classes and objects.
Ruby has been described as a multi-paradigm programming language: it allows you to program procedurally (defining functions/variables outside classes makes them part of the root, 'self' Object), with object orientation (everything is an object) or functionally (it has anonymous functions, closures, and continuations; statements all have values, and functions return the last evaluation). It has support for introspection, reflection and meta-programming, as well as support for threads. Ruby features dynamic typing, and supports parametric polymorphism.
According to the Ruby FAQ, "If you like Perl, you will like Ruby and be right at home with its syntax. If you like Smalltalk, you will like Ruby and be right at home with its semantics. If you like Python, you may or may not be put off by the huge difference in design philosophy between Python and Ruby/Perl." 
Ruby currently lacks full support for Unicode, though it has partial support for UTF-8.
The Ruby official distribution also includes "irb", an interactive command-line interpreter which can be used to test code quickly. A session with this interactive program might be:
irb(main):001:0> "Hello, World"
=> "Hello, World"
The syntax of Ruby is broadly similar to Perl and Python. Class and method definitions are signaled by keywords. In contrast to Perl, variables are not obligatorially prefixed with a sigil. (When used, the sigil changes the semantics of scope of the variable.) The most striking difference from C and Perl is that keywords are typically used to define logical code blocks, without brackets. Line breaks are significant and taken as the end of a statement; a semicolon may be equivalently used. Indentation is not significant (unlike Python).
See the Examples section for samples of code demonstrating Ruby syntax.